Five primary classes of antibodies exist with each class
playing a distinct role in the human immune response.
(Ig)(ɪmjʊnəʊˈɡlɒbjʊlɪn) - n: any of the five classes of proteins, (also known as antibodies).
An antibody (Ab), also known as an immunoglobulin (Ig), is a large Y-shaped protein produced by B-cells that is used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, called an antigen. Each tip of the "Y" of an antibody contains a paratope (a structure analogous to a lock) that is specific for one particular epitope (similarly analogous to a key) on an antigen, allowing these two structures to bind together with precision. Using this binding mechanism, an antibody can tag a microbe or an infected cell for attack by other parts of the immune system, or can neutralize its target directly (for example, by blocking a part of a microbe that is essential for its invasion and survival). The production of antibodies is the main function of the humoral immune system.
- IgG: These molecules are the most plentiful in circulation. They can cross blood vessels and even the placenta to provide protection to a fetus. Also secreted by plasma cells in the blood. The only antibody able to cross into the placenta and give passive immunity to the fetus. In it's four forms, IgG provides the majority of antibody based immunity against invading pathogens. The heavy chain type in IgG is a Gamma Chain.
- IgM: Of all of the immunoglobulins, these are the most massive. They contain five Y-shaped sections each with two light chains and two heavy chains. Each Y-shaped section is attached to a joining unit called a J chain. IgM molecules play a major role in the primary immune response as the initial respondents to new antigens in the body. Can be attached to the surface of a B cell or secreted into the blood. Also responsible for early stages of immunity. The heavy chain type in IgM is a Mu Chain.
- IgA: Located mainly in body fluids such as sweat, tears, breast milk, saliva, and mucus, these antibodies prevent antigens from infecting cells and entering the circulatory system. Also protects against pathogens. The heavy chain type in IgA is an Alpha Chain.
- IgD: The role of these antibodies in the immune response is currently unknown. IgD molecules are located on the surface membranes of mature B cells. Activate basophils and mast calls. Also functions mainly as an antigen receptor on B cells that have not been exposed to antigens. It's function is less defined than other Isotypes. The heavy chain type in IgD is a Delta Chain.
- IgE: Found mostly in saliva and mucus, these antibodies are involved in allergic responses to antigens. Protect against parasitic worms. Also binds to allergens and triggers histamine release from mast cells and basophils, and is involved in allergies. The heavy chain type in IgE is an Epsilon Chain.
Antibodies are secreted by a type of white blood cell called a plasma cell. Antibodies can occur in two physical forms, a soluble form that is secreted from the cell, and a membrane-bound form that is attached to the surface of a B cell and is referred to as the B cell receptor (BCR). The BCR is only found on the surface of B cells and facilitates the activation of these cells and their subsequent differentiation into either antibody factories called plasma cells, or memory B cells that will survive in the body and remember that same antigen so the B cells can respond faster upon future exposure. In most cases, interaction of the B cell with a T helper cell is necessary to produce full activation of the B cell and, therefore, antibody generation following antigen binding. Soluble antibodies are released into the blood and tissue fluids, as well as many secretions to continue to survey for invading microorganisms.
Antibodies are glycoproteins belonging to the immunoglobulin superfamily; the terms antibody and immunoglobulin are often used interchangeably. Antibodies are typically made of basic structural units, each with two large heavy chains and two small light chains. There are several different types of antibody heavy chains, and several different kinds of antibodies, which are grouped into different isotypes based on which heavy chain they possess. Five different antibody isotypes are known in mammals, which perform different roles, and help direct the appropriate immune response for each different type of foreign object they encounter.
Though the general structure of all antibodies is very similar, a small region at the tip of the protein is extremely variable, allowing millions of antibodies with slightly different tip structures, or antigen binding sites, to exist. This region is known as the hypervariable region. Each of these variants can bind to a different antigen. This enormous diversity of antibodies allows the immune system to recognize an equally wide variety of antigens. The large and diverse population of antibodies is generated by random combinations of a set of gene segments that encode different antigen binding sites (or paratopes), followed by random mutations in this area of the antibody gene, which create further diversity. Antibody genes also re-organize in a process called class switching that changes the base of the heavy chain to another, creating a different isotype of the antibody that retains the antigen specific variable region. This allows a single antibody to be used by several different parts of the immune system...
There are also a few sub-classes of immunoglobulins in humans. The differences in sub-classes are based on small variations in the heavy chain units
of antibodies in the same class. The light chains found in immunoglobulins exist in two major forms. These types are identified as kappa, and lambda chains.
Terms Used To
Immunoglobulin - antibody formed as a result of immune stimulus (exposure to foreign antigen).
Naturally Occurring - antibody formed without prior exposure to foreign antigen.
Auto-antibody - antibody formed to one's own antigens (abnormal condition).
Alloantibody - (unexpected, irregular, atypical): antibody formed to foreign antigens, but within the same species.
Agglutinin - antibody capable of causing agglutination when reacting with corresponding antigen.
Isoagglutinin - name commonly given to blood group antibodies anti-A and anti-B.
Saline Agglutinin - antibody capable of causing direct agglutination of antigens suspended in a saline medium without requiring any enhancement techniques.
Hemolysin - antibody capable of causing hemolysis when reacting with corresponding antigen.
Cold Antibody (cold agglutinin) - antibody whose optimal temperature of reactivity is less than 30*C.
Warm Antibody - antibody whose optimal temperature of reactivity is greater than 35*C.
The modification date for all health, and medical content on this page was last updated, and checked on June 6th, 2018 PST U.S.A.