Antibodies are gamma globulin molecules found in the blood and in other tissues. Each antibody molecule has four protein chains --two heavy and two light chains. There are five classes of antibody molecules. Theses are IgA, IgG, IgD, IgE, IgM. IgM antibodies are the important gamma globulins produced in the primary immune response.. They are found mainly in plasma. IgA molecules are found primarily in sweat, saliva, breast milk and in intestinal secretions. IgD antibodies are exist mainly B-cells, where they assist in B-cells binding to antigens. IgG molecules constitute over 80% of all antibody molecules produced. These molecules are found in plasma and lymph. IgE molecules form the smallest proportion of antibodies secreted. They may be found in skin, in GI mucosal lining, in the respiratory mucosa and in tonsils. They are important in the body's reactions to allergies, and parasitic infestations.
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Each immunoglobulin molecule has two heavy chains (H) and two light chains (L).. Each peptide chain has a variable (V) and a constant region(C), that have different functions. The name of the immunoglobulin class is based on the constant region,.The constant region also determines functions and capabilities of antibody molecules: they determine functions like complement fixation, precipitation , antigen neutralization, and agglutination. Some antibodies can cross the placenta ; some reside in skin and mucosae; others circulate in blood, and lymph. These functions and abilities of antibody classes are determined by constant regions Variable regions function in antigen recognition and binding.