There are five different classes of antibodies.
Antibody binding occurs during an immune response between antibodies and specific antigens. Antigens are any cell or molecule recognized by the body as foreign or non-self. Antigens can take many different forms, from disease-causing organisms, such as bacteria and viruses, to toxic or allergy-producing molecules, allergens. The presence of antigens in the body stimulates the cells of the immune system, including those that produce and secrete antibodies.
Antigen binding is an immunological process in which an antibody binds to an antigen.
Antibodies are proteins that have a different basic structure. They are made up of four polypeptide chains joined by disulfide bonds, which are bonds that form between sulfur atoms. Each antibody has two long or heavy chains and two short or light chains. Each light chain is at the end of one of the heavy chains. The four chains are held together in a Y-shape, allowing flexibility for antibody binding to occur.
Each antibody is specific for a particular antigen. The specificity of the antibody is determined by the antigen-binding site at the end of each light chain. The amino acid sequence found at the end of each light chain forms a three-dimensional shape that is complementary to the shape of the antigen. Since there are two light chains for each antibody, there are two antigen-binding sites, so each antibody can bind two antigens.
There are five different classes of antibodies. Because they are all globular proteins produced as part of the immune response, they are called immunoglobulins, and each class is designated by a letter, G, M, A, D, and E. Although all immunoglobulins share the same basic structure, the different classes they are based on differences in the heavy chains. There can be many different antibodies within each class, and specific antibody binding occurs between the specific molecule and its complementary antigen.
Each of the different classes of antibodies binds to different types of antigens and has different functions in the immune system. For example, immunoglobulin G, IgG, can bind to two antigens, stimulate other immune cells, and cause agglutination. Agglutination occurs when antibody binding occurs between multiple antibodies and antigens. This can happen when an antigen also has more than one binding site, allowing it to bind to more than one antibody. As more and more antibodies and antigens bind, they form a clump, or agglutination, that helps destroy cells with the antigens.