Antibodies are immuoglobulin molecules that have a specific amino acid (protein) sequence that gives each one the ability to adhere to or interact with a specific antigen. It is this antigen-specific property of the antibody that is the essential basis in an immune response.
Antibodies are secreted by the B cells. Most are found in the gamma globulin part of the plasma proteins.
The initial response of an antibody to an antigen is called the primary response. The primary response is associated with a slow development and a low level of antibodies. This is because it takes time to build a resistance. For example, when such a disease as measles strikes, antibodies will develop against that viral molecule. The second time that the virus molecule appears in the system and an antibody comes across this same antigen, the response time will be much quicker in mounting a defense to get rid of the invader. This second challenge is referred to as the secondary response, and also called immunity. The level of antibodies in the blood is called an antibody titer, and reveals any previous exposure to a disease, or antigen.
Antibodies are classified according to their mode of action and response to an antigen. The ones that cause bacterial cells to clump together (agglutination) are called agglutinins. Those that cause bacterial cells to dissolve or liquify are called bacteriolysins. This happens when the antibody/antigen bond together with complement to destroy a bacterial cell by punching holes in the membrane. Still others coat the outside of a bacterial cell to make them more appealing to phagocytes. These are called opsonins. Antibodies that neutralize toxins of antigens are antitoxins and those that cause precipitation of antigens in a fluid medium are precipitins.
There are five major types of immuglobulins, or antibodies: IgA, IgD, IgE, IgG, and IgM, but the three most abundant are IgA, IgG, and IgM.
Class A (IgA) antibodies are exported into mucosal surfaces of exocrine glands found in the alimentary, respiratory, urinary, and genital tracts, and help protect against infections. IgA antibodies are also found in colostrum helping to protect an infant against many intestinal infections. The IgA molecule is made up of two Y-shaped molecules. Altogether there are about 10,000 million B cells that form IgA antibody. Some of them engage in immune responses, while others stand by until the need arises.
Class D (IgD) antibody is found in trace amounts in the serum and serves as a surface receptor for B cells.
Class E (IgE) antibody is a minor type, responsible for allergic reactions. It is formed by B cells that lie just below the respiratory and intestinal surfaces. IgE attach to mast cells in these areas. Stimulated by the presence of an antigen, IgE release a pulse of histamine in response. However, the same IgE response, useful in ridding the body of parasites for example, is also the same one responsible for hayfever, asthma, and eczema; and, on occasion, IgE is also responsible for a more severe reaction called anaphylaxis.
Class G (IgG) antibodies are the main ones found in plasma and body fluids and the main one that activates complement proteins. They consist of the basic Y-shaped structure that easily leaks out into tissues. For instance, small amounts are normally found in the cerebrospinal fluid surrounding the brain. IgG is particularly effective against certain bacteria, viruses, and various toxins.
Class M (IgM) is a very large type of antibody found in blood plasma. Each molecule is made up of five of the basic Y-shaped molecules. This means that it has five times as many sites that can combine with antigen making the IgM molecule much more powerful than IgG or IgA. IgM is the first to appear in an immune response, as much as a day or two before IgG, but is eventually replaced with IgG antibodies. It is also the first to appear in the development of a fetus, and the type of antibody seen in primitive fishes.