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Why have the eye and CNS have immune privilege? Why does the body not develop tolerance against their tissue and instead risk their damage in case an accidental immune cell infiltration?
… in immune privileged sites, tissue grafts can survive for extended periods of time without rejection occurring
The immune privilege is happening for brain, CNS, eye, testis and the uterus. All these tissues (or structures) share that they cannot be regenerated once they get damaged by an overshooting immune reaction. Infections in the eye followed by a strong immune response often lead to the loss of the eye, in rodents loss if immune tolerance against the fetus leads to abortion.
So basically this is a protective mode for these tissues to protect them from a stong immune reaction. See the references for some more details (especially on how this works).
- See no evil, hear no evil, do no evil: the lessons of immune privilege
- What is immune privilege (not)?
- Immune privilege or privileged immunity?
- Immune Privilege and the Philosophy of Immunology
Each immune privileged tissue has a unique function. The eye must protect the light path and signals that stimulate the retina, and photoreceptors to preserve vision. The testis has to protect the sperm as they proceed to the epididymis where they mature. The maternal reproductive tract has to protect its eggs both before and after fertilization and thus has developed specialized mechanisms to modify the body's response to foreign antigens. These unique challenges require different solutions and lead to unique immune privilege mechanisms. However, although microenvironment and the stromal cells that carry out the particular function may vary between tissues, and consequently the mechanisms that promote regulation may be unique to that cell or tissue, the goal is the same: limit collateral damage to preserve tissue integrity and maintain homeostasis to the extent possible, without compromising host defense.
Tolerance is the prevention of an immune response against a particular antigen. For instance, the immune system is generally tolerant of self-antigens, so it does not usually attack the body's own cells, tissues, and organs. However, when tolerance is lost, disorders like autoimmune disease or food allergy may occur. Tolerance is maintained in a number of ways:
Inhibitory NK cell receptor (purple and light blue) binds to MHC-I (blue and red), an interaction that prevents immune responses against self.
- When adaptive immune cells mature, there are several checkpoints in place to eliminate autoreactive cells. If a B cell produces antibodies that strongly recognize host cells, or if a T cell strongly recognizes self-antigen, they are deleted.
- Nevertheless, there are autoreactive immune cells present in healthy individuals. Autoreactive immune cells are kept in a non-reactive, or anergic, state. Even though they recognize the body's own cells, they do not have the ability to react and cannot cause host damage.
- Regulatory immune cells circulate throughout the body to maintain tolerance. Besides limiting autoreactive cells, regulatory cells are important for turning an immune response off after the problem is resolved. They can act as drains, depleting areas of essential nutrients that surrounding immune cells need for activation or survival.
- Some locations in the body are called immunologically privileged sites. These areas, like the eye and brain, do not typically elicit strong immune responses. Part of this is because of physical barriers, like the blood-brain barrier, that limit the degree to which immune cells may enter. These areas also may express higher levels of suppressive cytokines to prevent a robust immune response.
Fetomaternal tolerance is the prevention of a maternal immune response against a developing fetus. Major histocompatibility complex (MHC) proteins help the immune system distinguish between host and foreign cells. MHC also is called human leukocyte antigen (HLA). By expressing paternal MHC or HLA proteins and paternal antigens, a fetus can potentially trigger the mother's immune system. However, there are several barriers that may prevent this from occurring: The placenta reduces the exposure of the fetus to maternal immune cells, the proteins expressed on the outer layer of the placenta may limit immune recognition, and regulatory cells and suppressive signals may play a role.
Read more about MHC proteins in Communication.
Transplantation of a donor tissue or organ requires appropriate MHC or HLA matching to limit the risk of rejection. Because MHC or HLA matching is rarely complete, transplant recipients must continuously take immunosuppressive drugs, which can cause complications like higher susceptibility to infection and some cancers. Researchers are developing more targeted ways to induce tolerance to transplanted tissues and organs while leaving protective immune responses intact.
A pathogen or infectious agent is a biological agent that causes disease or illness to its host.
The term is most often used for agents that disrupt the normal physiology of a multicellular animal or plant.
However, pathogens can infect unicellular organisms from all of the biological kingdoms.
There are several substrates and pathways whereby pathogens can invade a host.
The human body contains many natural defenses against some of common pathogens in the form of the human immune system and by some "helpful" bacteria present in the human body's normal flora.
Some pathogens have been found to be responsible for massive amounts of casualties and have had numerous effects on afflicted groups.
Today, while many medical advances have been made to safeguard against infection by pathogens, through the use of vaccination, antibiotics and fungicide, pathogens continue to threaten human life.
Social advances such as food safety, hygiene, and water treatment have reduced the threat from some pathogens.
In the big picture of things, macrophages can ingest and destroy bacteria, clean up cellular debris and other harmful particles, as well as dead cells that contain microbes, such as bacteria or viruses. After macrophages ingest these dead cells, they will take some of the material from the microbe inside the cell—a snapshot of the intruder if you will—and present it to other cells in the immune system. In this way, macrophages can "sound the alarm" that a foreign invader is in the body and help other immune cells recognize that invader.
4 The Bugs in Your Guts Influence Your Daily Life
Medical science has always known that we're packed full of tiny critters, known as "intestinal microflora," which help or hinder us in various ways. But, it's only recently that we've been able to glimpse the full extent of their influence. So much so that many now consider our internal ecosystem to be an actual organ in and of itself, as important as the liver or the pancreas or the dong bladder.
What? Did we spell dong bladder wrong? We didn't pay much attention in biology class .
The bugs in your guts help to regulate the metabolism, assist the immune system, and fight disease. And they're no small deal -- there are about 100 trillion of them living inside you right now, accounting for about 1 to 2 kilograms of your total body weight. That's right: You're not fat, you're just big bacteria-ed.
But, not all gut microbes are benevolent. After the germs learned how to hijack our body like the Statue of Liberty in Ghostbusters II, some of them decided to use that power for evil. Recent studies have suggested that certain kinds of gut microbes have evolved to influence the vagus nerve (the nerve that links the brain and the gut), as they can force us to crave certain foods.
These microbes happen to prefer sugary or high fat foods to healthier alternatives, and they may be part of the reason some of us have the uncontrollable urge to binge on cheese until we sweat skim milk. By releasing chemicals that stimulate our nervous system to affect our cravings, or to light up our satisfaction nodes when their demands are met, they can effectively dictate our dietary choices according to their own whims. As such, it's thought that these greedy internal puppet masters might be partly responsible for today's epidemics of obesity and diabetes.
With aging, the immune system becomes less effective in the following ways:
The immune system becomes less able to distinguish self from nonself, making autoimmune disorders more common.
Macrophages destroy bacteria, cancer cells, and other antigens more slowly, possibly contributing to the increased incidence of cancer among older adults.
T cells respond less quickly to antigens.
There are fewer lymphocytes that can respond to new antigens.
The aging body produces less complement in response to bacterial infections.
Although overall antibody concentration does not decline significantly, the binding affinity of antibody to antigen is decreased, possibly contributing to the increased incidence of pneumonia, influenza, infective endocarditis, and tetanus and the increased risk of death due to these disorders among older adults. These changes may also partly explain why vaccines are less effective in older adults.
Lymphocytes --- Heart of the Immune System
How lymphocytes recognize antigens
When an antigen invades the body, normally only those lymphocytes with receptors that fit the contours of that particular antigen take part in the immune response. When they do, so-called daughter cells are generated that have receptors identical to those found on the original lymphocytes. The result is a family of lymphocytes, called a lymphocyte clone. with identical antigen-specific receptors.
A clone continues to grow after lymphocytes first encounter an antigen so that, if the same type of antigen invades the body a second time, there will be many more lymphocytes specific for that antigen ready to meet the invader This is a crucial component of immunologic memory.
How lymphocytes are made
Some lymphocytes are processed in the bone marrow and then migrate to other areas of the body --- specifically the lymphoid organs (see Lymphatic System). These lymphocytes are called B lymphocytes, or B cells (for bone-marrow-derived cells). Other lymphocytes move from the bone marrow and are processed in the thymus, a pyramid-shaped lymphoid organ located immediately beneath the breastbone at the level of the heart. These lymphocytes are called T lymphocytes, or T cells (for thymus-derived cells).
These two types of lymphocytes --- cells and T cells --- play different roles in the immune response, though they may act together and influence one another's functions. The part of the immune response that involves B cells is often called humoral immunity because it takes place in the body fluids. The part involving T cells is called cellular immunity because it takes place directly between the T cells and the antigens. This distinction is misleading, however, because, strictly speaking, all adaptive immune responses are cellular --- that is, they are all initiated by cells (the lymphocytes) reacting to antigens. B cells may initiate an immune response, but the triggering antigens are actually eliminated by soluble products that the B cells release into the blood and other body fluids. These products are called antibodies and belong to a special group of blood proteins called immunoglobulins When a B cell is stimulated by an antigen that it encounters in the body fluids, it transforms, with the aid of a type of T cell called a helper T cell (see "T cells"), into a larger cell called a blast cell. The blast cell begins to divide rapidly, forming a clone of identical cells.
Some of these transform further into plasma cells --- in essence, antibody-producing factones. These plasma cells produce a single type of antigen-specific antibody at a rate of about 2,000 antibodies per second. The antibodies then circulate through the body fluids, attacking the triggering antigen.
Antibodies attack antigens by binding to them. Some antibodies attach themselves to invading microorganisms and render them immobile or prevent them from penetrating body cells. In other cases, the antibodies act together with a group of blood proteins, collectively called the complement system, that consists of at least 30 different components. In such cases, antibodies coat the antigen and make it subject to a chemical chain reaction with the complement proteins. The complement reaction either can cause the invader to burst or can attract scavenger cells that "eat" the invader.
Not all of the cells from the clone formed from the original B cell transform into antibody-producing plasma cells some serve as so-called memory cells. These closely resemble the original B cell, but they can respond more quickly to a second invasion by the same antigen than can the original cell. T cells. There are two major classes of T cells produced in the thymus: helper T cells and cytotoxic, or killer, T cells. Helper T cells secrete molecules called interleukins (abbreviated IL) that promote the growth of both B and T cells. The interleukins that are secreted by lymphocytes are also called lymphokines. The interleukins that are secreted by other kinds of blood cells called monocytes and macrophages are called monokines. Some ten different interleukins are known: IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-7, interferon, lymphotoxin, and tumor necrosis factor. Each interleukin has complex biological effects.
Cytotoxic T cells destroy cells infected with viruses and other pathogens and may also destroy cancerous cells. Cytotoxic T cells are also called suppressor lymphocytes because they regulate immune responses by suppressing the function of helper cells so that the immune svstem is active onlv when necessary.
The receptors of T cells are different from those of B cells because they are "trained" to recognize fragments of antigens that have been combined with a set of molecules found on the surfaces of all the body's cells. These molecules are called MHC molecules (for major histocompatibility complex). As T cells circulate through the body, they scan the surfaces of body cells for the presence of foreign antigens that have been picked up by the MHC molecules. This function is sometimes called immune surveillance.
Acquired immune system (B cells and T cells)
The human acquired immune system is responsible for the destruction of foreign particles once they have entered the body. Before it has seen a foreign particle, it is actually quite ignorant about how to destroy it. During the first exposure to an invader (which could be a virus, a bacteria or any unwanted particle), the acquired immune system must ‘learn’ how to attack and destroy the foreign particle. This means that it is not as good as the innate immune system for keeping out things that it has never encountered before. Once the acquired immune system has created a response, however, a protective response can be made more quickly and with greater force, allowing it to protect the body from harm.
The cells of the acquired immune system are mainly the B cells and the T cells, but there are also other important parts of the acquired immune system, such as the ‘complement cascade‘ and the production of antibodies. The acquired immune system also plays the key role in the rejection of implanted tissue.
Activation of the acquired immune system
Unlike the innate immune system, the acquired immune system needs to have seen a substance before in order to attack it effectively. This is because the way that the acquired immune system attacks a target is very specific and takes time to prepare.
All agents foreign to your body have unique patterns on their surfaces that allow the cells of the acquired immune system to detect them. When the cells of the aquired immune system detect these patterns, the agents are recognised as foreign, and the immune system can therefore mount an attack. Anything that the immune system can detect and attack is called an antigen.
The activation of the acquired immune system initially requires the help of other cells. The cells of the acquired immune system are coated in receptors. These are highly specific molecules designed to recognise certain substances. The receptors are so specific that each receptor can only recognise one substance and nothing else. There are many immune cells in the blood, each with its own different receptor. This means that the body can be protected against many different things.
Cells called macrophages (which means ‘big eater’) can speed up the process of activation. Macrophages are found in lots of places throughout the body, and eat anything that they do not recognise. After they have eaten something, the macrophages break it down into its basic proteins and present these to the immune cells. This causes a better, more accurate and more damaging response than the macrophages alone are capable of producing.
Lymphocytes are a type of white blood cell. There are two types of lymphocytes of the acquired immune system: T cells and B cells. There is a third type of lymphocyte known as natural killer (NK) cells, but these are a part of the innate immune system.
T cells: Mediated immunity
T cells account for about 80% of all lymphocytes. They are named T cells because they mature in the thymus, a gland found in the chest. There are three types of T cell lymphocytes:
These cells all play a role in the direct destruction of problem cells in the body, such as cells infected with a virus, or cells with DNA damage (e.g. some cancer cells).
T cell production
T cells start out as stem cells (early types of cells that have not yet fully grown) and are produced by bone marrow. To mature, these stem cells move to the thymus, where they can stay for up to three weeks. About 99% of T cells do not make it to maturity. This is because the body is very selective about what T cells are produced so that they do not cause damage to the body’s own cells. In the thymus, the T cells are given T cell receptors, of which there are several types. The type of receptor received determines what type of T cell it will be, what its role is, and which cells it can interact with.
T cell function
T cells function both through the release of substances into the blood, and by signalling B cells through contact. They have several different roles:
- Signalling for growth and activation of B cells
- Activation of cells that can ‘eat’ foreign substances
- Stimulation of cytotoxic T cells during a viral infection
- Signalling growth in cells, including other T cells, macrophages and eosinophils
T cell activation
T cells cannot detect foreign substances without assistance, and require a complex system to help them work. They need the help of cells called antigen presenting cells (APCs). These cells will eat the foreign substance, be it a bacteria, virus-infected cell or toxin, break it down, and present part of it to the T cell so that it can mount a response. APCs have a special type of molecule on their surface that allows them to communicate with helper T cells. Once a response is activated, lots and lots of T cells of different types are released into the blood stream. The released cells are responsible for the destruction of the foreign substance.
Helper T cells
Helper T cells are by far the most common T cell. They make up more than three quarters of the T cell population.Helper T cells help the immune system in many different ways, and serve as a major regulator of virtually all immune functions in the body. They mainly act through the release of substances that help control the other parts of the immune system. These substances (called lymphokines) stimulate the other types of T cells to grow and attack. They also help B cells grow and mature into their active form.In acquired immunodeficiency syndrome (AIDS), there is a loss of helper T cells, leaving the body open to infection. Also, due to the influence of helper T cells on B cells, B cells may be inactive in cases where the T cells are damaged.
Cytotoxic T cells
Cytotoxic T cells are important in defending against virally infected cells, in the rejection of tissue grafts, and in the immune response to certain tumour types. Cytotoxic T cells require activated APCs, and rely on the presence of helper T cells.
Following activation by helper T cells, cytotoxic T cells prepare for the destruction of their target. Inside the cells, substances are formed which are incredibly dangerous to cells. They create a protein called perforin, named because it has the ability to ‘perforate’ infected cells by punching holes in them. Cytotoxic T cells can also release enzymes that destroy the cell structure.
To destroy a cell, a cytotoxic T cell first latches on to it, then releases the aforementioned substances directly onto the cell. Consequently, there is no damage to any other cell that happens to be nearby. The released substances cause the cell to self-destruct rather than explode. Thus, if there are any viral particles in the cell, they will be destroyed with the cell rather than be released to spread. After the cell is destroyed, the cytotoxic T cell can detach itself and leave to destroy other infected or otherwise damaged cells.
Suppressor T cells
Suppressor T cells are, as the name suggests, capable of suppressing the functions of both helper and cytotoxic T cells. It is believed that suppressor cells function as regulators of the other cells of the immune system, stopping them from causing excessive damage to the body’s own tissues. It is probable that suppressor T cells play an important role in protecting against autoimmune attack.
B cells: Humoral immunity
B cells account for 10–15% of circulating lymphocytes. They are called B cells because they were first discovered to mature in the ‘bursa of Fabricius’ organ in birds. Humans no longer have this organ, and so B cell maturation now takes place in human bone marrow. B cells circulate around the body in the blood stream. When activated, they release huge amount of antibodies.
B cell production
B cells start out as the same type of stem cell as the T cells. Instead of moving to the thymus, however, B cells move to bone marrow to mature. There they are given cell receptors, and are then released into the blood. Once released, they move to the lymphoid tissue of the body, where they are located nearby, but distinctly separate from, the T cells.
The production of B cells involves some incredibly complex genetics that is not worth speaking about here. Generally, imagine that the body has an enormous pile of building blocks with which to make something that will recognise a foreign substance. These building blocks can be placed together in millions of different combinations, and each will be able to recognise a certain substance. The body makes almost all possible combinations in the form of immunoglobulins and places them onto the surface of B cells, allowing them to recognise foreign substances.
B cell function
B cells play two major roles in the protection of the body:
- Ensuring antibody production against the appropriate target antigen and
- Presenting antigens to T cells and providing signals for T cell activation.
B cell activation
The majority of B cell activation takes place in the lymph nodes. Certain types of cells in the lymph nodes eat anything foreign and present them to B and T cells. Any B cell that shares a receptor for this substance will be activated and start to multiply. B cells can also be activated by helper T cells. After activation, active B cells migrate around the body and change into plasma cells.
Plasma cells are B cells that remain committed to the production and secretion of a single antibody type. This secretion gives rise to the antibodies found in the circulation. Immunity is kept for as long as the plasma cell continues to secrete antibodies.
Memory B cells
Memory B cells can also be formed after stimulation. These cells migrate to the lymph nodes, where they remain ready for further rounds of activation should the specific antigen ever be encountered again. If it is, then a very quick response can be made because memory B cells are ready and waiting to multiply.
Antibodies are proteins that have the ability to bind antigens. While the terms ‘antibodies’ and ‘immunoglobulins’ mean slightly different things, they are often referred to interchangeably. Antibodies have many uses within the body, including:
- Targeting an infective organism
- Recruitment of immune cells
- Neutralisation of toxins
- Removal of foreign antigens from circulation
- Complement activation
- Stimulation of cells that can ‘eat’ foreign substances
- Activation of mast cells
The typical immunoglobulin (Ig) is made up for four parts called ‘chains’. Two of the parts are slightly heavier, and so the Ig molecule is said to be made of two heavy chains and two light chains. These are arranged in a Y-shaped configuration. At the end of the two ‘arms’ is an area that has the ability to recognise foreign substances.
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There are several different types of immunoglobulin.
Immunoglobulin G (IgG)
IgG is the most abundant immunoglobulin, and is one of the major activators of the complement pathway. Part of the IgG molecule is able to interact with lots of cells of the immune system, and so it has the ability to stimulate a very direct attack on anything that it recognises. IgG is also the only type of immunoglobulin that is transferred across the placenta from mother to foetus. This gives unborn children some protection from diseases to which they have not yet been exposed.
Immunoglobulin A (IgA)
IgA is the second most abundant immunoglobulin. An immunoglobulin (or antibody) is a protein which is involved in immune responses. IgA plays an important role in the cellular defence of mucosal surfaces. Mucosal surfaces are the moist, soft surfaces of the body (e.g. the inside of the mouth, all along the digestive system). IgA is excreted into bodily secretions such as tears, saliva and breast milk. A lack of IgA often causes major infections of mucosal surfaces, such as the mouth, throat and lungs.
Immunoglobulin M (IgM)
IgM looks different than the other immunoglobulins, but is in fact just five normal immunoglobulins joined at their bases. IgM is the first immunoglobulin synthesised in an antibody response. It is a strong activator of the complement cascade.
Immunoglobulin E (IgE)
IgE is the largest immunoglobulin, but is present in extremely low levels in a healthy individual. IgE levels rise in response to parasitic infections and in individuals who are strongly allergic to something. The main action of IgE is binding to and activating mast cells. Mast cells cause local and occasionally generalised effects (e.g. swelling, redness, pain and itchiness). Hayfever is a condition caused by too much IgE activity.
The complement cascade
The complement system or cascade is a combination of 20 proteins that have the ability to directly destroy foreign cells. The complement pathway can be activated either as part of the innate immune system, or via the ‘classical pathway’ discussed here.
The classic pathway for complement activation involves the binding of an antibody to an antigen, whereby a foreign substance is coated with antibodies secreted from B cells. These antibodies bind directly to the ‘first’ complement enzyme. Multiple different substances are formed by this process. Each is designed to prevent damage to the body’s tissues while causing maximum damage to the invader.
Complement activation has the following effects on foreign substances (e.g. bacteria):
- Signalling and eating: Complement coats bacteria with signals that highlight it for the body’s ‘eaters’, leading to its destruction.
- Direct destruction: One substance formed can punch holes in bacteria walls, causing it to burst.
- Agglutination: Alteration of the bacterial surface by complement proteins cause them to stick to each other, making their destruction easier.
- Neutralisation of viruses: Complement proteins can directly attack viruses, rendering them harmless.
- Cell signalling: Complement causes neutrophils and macrophages to move next to antigens.
- Activation of mast cells and basophils: Activating mast cells and basophils causes the release of substances that lead to increased local blood flow, increased leakage of damaging proteins, and other factors that help inactivate or immobilise bacteria.
- Inflammation: Several complement proteins act to increase inflammation.
Transplants, skin grafts and rejection
All cells of the immune system play a role in the rejection of transplants (e.g. skin grafts and organ transplants). There seems to be more than one mechanism for the rejection of transplanted organs. In organs and tissue taken from an individual of the same species, the parts attacked by the recipient’s immune system are referred to as alloantigens. When foreign tissue from another human is placed into the recipient, the body detects differences between the donor tissue and the body’s own tissue. An immune reaction may then cause the destruction of the donor tissue.
Types of rejection
Hyperacute rejection is caused by preformed antibodies in the recipient that are directed against the cells from the donor. It occurs minutes to hours after transplantation. An example of this is when transplants are made across the ABO blood group barrier. If tissue from a group A donor is transplanted into an O patient, the O patient already has lots of antibodies against the A blood type within their circulation, and the foreign tissue is rejected. This type of rejection can also be due to the recipient having previously seen similar tissue before. An example of this is when white blood cells from a blood transfusion share some patterns with the recipient.
This form of rejection causes the formation of clots, and eventually the death of the implanted organ.
Acute rejection is mediated by T cells and usually becomes apparent 7 days post-transplant. The recipient’s T cells recognise alloantigens and cause white blood cell infiltrations of the transplanted organ.
Chronic rejection appears months or years after successful transplantation, and is the major cause of long term graft loss. Blood vessel damage may predispose to chronic rejection, but the exact mechanism is poorly understood. It is mostly characterised by cell proliferation in the vascular walls and narrowing of the blood vessels, stopping adequate blood flow.
Graft versus host reactions
Graft versus host reactions arise when stem cells that can produce active immune cells are transplanted from a donor. It is most common following bone marrow transplantation, but has also been described following liver transplantation and blood transfusion. There are two forms of graft versus host reactions:
- Acute: Occurs within 1–2 months of a transplant
- Chronic: Develops 2–3 months post-transplant.
Graft versus host disease typically affects the liver, skin, intestinal tract and immune system within days or weeks of a bone marrow transplant. In its mild form, it can manifest as redness of the palms, soles and ears. There can also be signs of mild liver disease, and gastrointestinal signs such as mild diarrhoea. In more severe disease, there can be death of the skin, severe liver abnormalities and copious diarrhoea that can cause life-threatening dehydration.
Mild graft versus host disease may resolve spontaneously, or with immunosuppression. The severe form of the disease is usually resistant to treatment and has a fatal outcome.
Immunity in babies
Unborn babies actually get a lot of immunity from their mother, because lots of the proteins and antibodies of immunity can easily cross the placenta (an organ that connects the mother to the baby in the womb). However, the unborn baby does not create many antibodies for quite a while.
After birth, when the mother is no longer passing antibodies to the child in the womb, the baby’s immunity can fall for a month or so. The antibodies from the mother can provide protection for about six months, and the baby will start creating antibodies well before then.
As has been mentioned before, the acquired immune system needs to encounter a foreign substance before it can mount an effective attack. This is why children seem to get more colds and other infections than adults. As children age and their immune systems encounter more and more infectious agents, they can fight infections more effectively.
Infections in the elderly
The elderly also have a decreased immune response, but while children get infections because their immune systems are young and hve not seen much, the reason is different for the elderly. As people get older, lots of their cells start to ‘slow down’ or become less effective. This includes the immune cells, and they are unable to mount as quick or as strong a response as a younger person.
The immune system can be affected by lots of other things besides age. Smoking makes people far more likely to get infections because it ‘slows’ the immune system. Some illnesses and medications can also cause the immune system to be less effective.
Mucus in the Mouth and Stomach
Saliva in the mouth contains mucus with a thin consistency. This mucus is an excellent lubricant and makes swallowing food easier.
The stomach lining is covered by a protective layer of mucus. Glands in the stomach produce mucus, hydrochloric acid, and an inactive enzyme called pepsinogen. In the stomach cavity, the hydrochloric acid changes the pepsinogen into an active digestive enzyme called pepsin. This enzyme digests proteins. The mucus layer acts as a barrier that prevents the stomach lining from being attacked by pepsin and acid.
If the mucus layer in the stomach is thinned or removed, which may happen during an infection by a bacterium called Helicobacter pylori, pepsin and acid may be able to attack the stomach lining. The infection can cause inflammation (gastritis) and sores called ulcers.
This is part of the mucosa lining the small intestine. The folds are known as villi. They increase the surface area for the absorption of digested food.
7 'Useless' Body Parts Explained
June 4, 2014 -- intro: Every day, people have their tonsils, appendix, and wisdom teeth removed--and after the pain subsides, they proceed without a hitch. The truth is, it's not all that apparent why many parts of your body are there, or what they actually do.
“Evolution moves toward features that are advantageous over others, so at some point all anatomical features were important to our early ancestors,” says Anthony Weinhaus, Ph.D., director of the University of Minnesota’s Human Anatomy program. Some of these still serve a purpose--just not necessarily a function crucial to our survival anymore.
Here are real explanations for these seven seemingly pointless body parts.
quicklist: 1category: 7 'Useless' Body Parts Explainedtitle: Nipplesurl:text: Let’s get the biggest news out of the way: All men start off as women.
“All embryos begin female, and if it masculinizes, it becomes male but keeps much of the same anatomy,” says Weinhaus.
Nipples are the same in men and women, but without an influx of hormones like estrogen, they're simply chest ornaments on men.
quicklist: 2category: 7 'Useless' Body Parts Explainedtitle: Armpit Hairurl:text: There’s no definitive story for underarm hair, but its location offers a clue.
There are two types of sweat glands in your body: eccrine and apocrine, the latter of which are mostly in your armpits, explains Daniel Lieberman, Ph.D., professor of human evolutionary biology at Harvard University. You use apocrine for sexual signaling. Presumably, the hair holds on to the secreted odors so they'll stay around long enough for a potential mate to catch a whiff, he explains.
quicklist: 3category: 7 'Useless' Body Parts Explainedtitle: Eyebrowsurl:text: The evolutionary purpose of eyebrows is debatable: In one camp, scientists believe brows keep sweat and rain off your eyes, which would help in primitive hunting and navigation. Lieberman favors the hypothesis that eyebrows serve to communicate your emotions, but they may also communicate your identity.
Behavioral neuroscientists from Massachusetts Institute of Technology found that people were less likely to recognize pictures of celebrities without their eyebrows than without their eyes. The researchers speculate that eyebrows have remained because they’re crucial to identifying faces and navigating social circumstances.
quicklist: 4category: 7 'Useless' Body Parts Explainedtitle: Appendixurl:text: The appendix is a vestigial organ, which means it has lost most of its ancestral function.
“One idea is the human appendix is remnant of what used to be a larger fermenting chamber in our gut,” Lieberman says.
Since humans stopped eating uncooked or low-quality foods like grass, this chamber is no longer useful. Recent research, though, suggests the appendix might be an essential hangout spot for healthy bacteria.
“Your microbiome is very important to digestive tract function, so this reservoir would allow microbes to recolonize your gut after inflammation or digestive issues,” he explains.
quicklist: 5category: 7 'Useless' Body Parts Explainedtitle: Tonsilsurl:text: Tonsils are technically lymph nodes--part of the lymphatic system, which is vital to your immunity.
“Your oral cavity is an entryway to your body, so immune cells in your throat can help you fight respiratory infections,” Lieberman explains.
Sometimes your tonsils get inflamed and infected, which is when they're removed. Your lymph nodes are incredibly important, but there's some redundancy, so if a pair is taken away, you can survive without them, Lieberman adds.
quicklist: 6category: 7 'Useless' Body Parts Explainedtitle: Wisdom Teethurl:text: Like monkeys, men have three permanent sets of molars. Until recently, wisdom teeth were never an issue for humans.
“Teeth don’t change size. They’re grown before you use them, and then they erupt to the surface,” says Lieberman.
Jaws are bone and, like the rest of your body, need to be supported and used in order to grow properly. Since humans now eat soft, cooked foods as children, our jaws don’t grow to the full capacity.
quicklist: 7category: 7 'Useless' Body Parts Explainedtitle: Foreskinurl:text: Male foreskin takes years to separate from the glans (head), which is unusual enough of a process to suggest one if its main functions may help prevent infection, especially in infants.
It helps shield the opening of the urethra from any contaminates or bacteria, explains Weinhaus. It also protects your reproductive chances: Without a foreskin, the glans rubs against objects, like your clothes, and develops a thick layer of skin to desensitize itself, Weinhaus says. Foreskin keeps men more sexually sensitive, which would’ve encouraged our ancestors to reproduce more.