Antibodies bind to antigens, which can be disease-causing organisms, toxic molecules, or allergens. Antibodies consist of four polypeptide chains and are specific to a particular antigen. There are five classes of antibodies with different roles in the immune system, and agglutination can occur when multiple antibodies and antigens bind together.
Antibody binding occurs during an immune response between specific antibodies and antigens. Antigens are cells or molecules recognized by the body as foreign or not belonging to the self. Antigens can take many different forms from disease-causing organisms, such as bacteria and viruses, to toxic molecules or molecules that produce allergies, allergens. The presence of antigens within the body stimulates cells of the immune system, including those that produce and secrete antibodies.
Antibodies are proteins that have a distinctive basic structure. They consist of four polypeptide chains held together by disulfide bonds, i.e. 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, which allows for flexibility so that antibody binding can occur.
Each antibody is specific for a particular antigen. The specificity of the antibody is determined by the antigen binding site found at the end of each light chain. The amino acid sequence found at the end of each light chain forms a three-dimensional shape 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 to 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, with each class 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 the differences in heavy chains. There can be many different antibodies within each class with specific antibody binding occurring between the specific molecule and its complementary antigen.
Each of the different classes of antibodies binds to different types of antigens and has different roles in the immune system. For example, immunoglobulin G, IgG, can bind to two antigens, stimulate other cells of the immune system, and can 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, which allows it to bind to more than one antibody. As more and more antibodies and antigens bind together, they form a lump, or agglutinate, which aids in the destruction of cells with the antigens.
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