Lymph nodes, white blood cells and antibodies

Antibodies are Y-shaped proteins called immunoglobulins (Ig), found in the blood and lymph. They are made by plasma cells, which come from activated B cells. There are five types: IgG, IgM, IgD, IgE and IgA. An antibody has four polypeptide chains: two long ones, the heavy chains, and two short ones, the light chains, held together by disulfide bonds. At the two tips of the Y are two identical antigen-binding sites. Their shape differs from one antibody to another, so the tips are called the variable regions, and the shape is set by the order of amino acids there. The rest of the antibody is the constant region, which has the same shape in all antibodies. An antibody fits its antigen like a key in a lock, and together they make an antigen-antibody complex. An antibody has only two binding sites, while an antigen has many. When a B cell meets its antigen for the first time it divides many times to make plasma cells, and each kind of plasma cell makes one kind of antibody against one kind of antigen. Antibodies stop antigens in five ways. In neutralization they cover the outer coat of a virus, so it cannot attach to host cells or enter them, or they keep its coat sealed so its nucleic acid cannot come out. In agglutination, antibodies such as IgM with many binding sites join several microbes into a clump, which weakens them. In precipitation, soluble antigens join antibodies to form insoluble complexes that settle out. In lysis, binding activates complements, which burst the coat of the antigen and dissolve its contents. In the antitoxin way, antibodies bind toxins and the complexes are made harmless with the help of complements. In every case phagocytes finish the job by swallowing the clumped, settled or burst material.