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A brief overview of the immune system, how it functions and how it malfunctions, which might be useful in understanding diseases relating to it.

Introduction

The immune system serves two functions, the protection of the body from invading microbes, and the maintenance of the body’s normal internal environment (doing such things as clearing away dead or damaged cells). These functions are performed by a system involving three components: barriers, immune system cells and circulating immune proteins. These are all interrelated, but are thought about separately to better understand the immune system’s function and how it breaks down.

Barriers such as the skin and tissues provide physical walls that keep invading microbes out of the body. The importance of skin becomes obvious in burns, cuts and some skin disorders, which make people much more susceptible to infections. The second component, immune system cells, form the cellular immune system, and have the jobs of breaking down tissues, coordinating the immune response, swallowing up invading microbes to destroy them, and killing cells that are infected by microbes.

The circulating proteins, called the humoral immune system, includes many molecules that the body makes to promote the immune response, poison invading microbes, and identify invading microbes as foreign (not belonging in the body). Some of the important proteins in this system are called antibodies, which some immune system cells can make in response to a specific infection. The antibodies help to clear the infection, and then persist afterwards in case the body encounters that specific infection again.

The Immune Response

One of the major problems the immune system faces is how to distinguish foreign, damaged or invading things from the many normal tissues in the body. There are two ways the immune system handles this problem, and this divides the immune response into two different categories, the innate and adaptive immune response. The body naturally makes certain molecules and receptors which can identify proteins as either friendly or foreign to the body. There are certain proteins and sugars that are associated with specific bacteria or viruses, and the innate immune response recognizes these as foreign. The strength of this arm of the immune response is its ability to instantly attack foreign substances (for example, bacteria or viruses). It does not need to have ever been exposed to or "seen" these substances before. Its weakness is that there are a huge number of possible foreign substances, so there is no way it can be prepared ahead of time for every possible invader. Some key parts of the innate immune response are:

1. Neutrophils: cells that eat bacteria and kill them.
2. Eosinophils, mast cells and basophils: cells that defend against parasites, and have important roles in allergy as well.
3. Macrophages: clean-up cells that can break down a wide variety of tissues. These cells are also key links between the innate immune response and the other arm of the immune response, the adaptive immune response.

The adaptive immune response can identify and respond to almost any foreign substance in existence. It does this by being “educated” during its development about what the body’s normal tissues are, and identifying anything else as foreign. Its weakness is that it needs to have “seen” the foreign substance at least once before in order to recognize it and develop its best response to it. It is this part of the immune system that makes vaccines work, (People can get chickenpox twice). This arm of the immune system also plays an important role in coordinating the immune response overall. The innate and adaptive immune systems work together and communicate through another cell called a macrophage. . The macrophages from the innate immune response show those foreign proteins to the adaptive immune system cells, called lymphocytes, and the lymphocytes then respond to them.

Antibodies

There are two main types of lymphocytes, B cells and T cells. B cells are the lymphocytes that make antibodies. Antibodies are proteins that can bind to almost any foreign protein the body has seen once before. They then identify what they bind to as foreign, necessary to be destroyed. There are five kinds of antibodies the body makes, IgA, IgM, IgD, IgE and IgG. IgG is the most important and prevalent antibody. IgE is important in allergy (and parasitic infection?!), IgA protects the body surfaces, and IgM is the first kind of antibody to bind to new foreign proteins. The role of IgD is not well understood. When antibodies are missing, the body is particularly at risk for certain types of infections.

T cells are the other type of lymphocytes, and there are two subtypes of these. CD4 positive T cells, called helper T cells, coordinate the immune response and are the type of cell destroyed by the HIV virus, which is why the immune system works poorly in AIDS. CD8 positive T cells, called killer T cells, are responsible for destroying cells that are infected by viruses, and may have a role in destroying cancer cells as well. They take care of infected cells and foreign proteins that the antibodies can't reach.

Immune system malfunction

The two main ways the immune system can malfunction are by either weakness infighting infection, called immunodeficiency, or by inappropriately attacking the body’s own normal tissues, called autoimmunity. There is a great variety of different ways for immunodeficiency and autoimmunity to show up in a person , and the different diseases in this database describe these. Immunodeficiencies and autoimmunity have historically been difficult to treat, largely due to a lack of understanding about the manner in which the immune system operates. Fortunately, the amount of information known about the immune system increases almost daily, and many new medications and techniques are being developed to deal with immunodeficiency and autoimmunity. As a result, many of these diseases have become treatable.