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The key to a healthy immune system is its remarkable ability to distinguish between the body’s own cells (self) and foreign cells (nonself). The body’s immune defenses normally coexist peacefully with cells that carry distinctive "self" marker molecules. But when immune defenders encounter cells or organisms carrying markers that say "foreign," they quickly launch an attack.
Anything that can trigger this immune response is called an antigen. An antigen can be a microbe such as a virus, or even a part of a microbe. Tissues or cells from another person (except an identical twin) also carry nonself markers and act as antigens. This explains why tissue transplants may be rejected. Each antibody is made up of two identical heavy chains and two identical light chains, shaped to form a Y. The sections that make up the tips of the Y's arms vary greatly from one antibody to another; this is called the variable region. It is these unique contours in the antigen-binding site that allow the antibody to recognize a matching antigen. The stem of the Y links the antibody to other participants in the immune defenses. This area is identical in all antibodies of the same class--for instance, all IgEs--and is called the constant region.
Immunoglobulins G, D, and E are similar in appearance. IgG, the major immunoglobulin in the blood, is also able to enter tissue spaces; it works efficiently to coat microorganisms, speeding their destruction by other cells in the immune system. IgD is almost exclusively found inserted into the membrane of B cells, where it somehow regulates the cell's activation. IgE is normally present in only trace amounts, but it is responsible for the symptoms of allergy. IgA--a doublet--guards the entrance to the body. It concentrates in body fluids such as tears, saliva, and secretions of the respiratory and gastrointestinal tracts. IgM usually combines in star-shaped clusters. It tends to remain in the bloodstream, where it is very effective in killing bacteria.
Source: National Cancer Institute, USA.
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Antibodies belong to a family of large
protein molecules known as immunoglobulins.
Different classes play different roles in the immune defense strategy.
Scientists have identified nine chemically distinct classes of human
immunoglobulins: four kinds of IgG and two kinds of IgA, plus IgM, IgE, and
IgD.