The immune response is our first line of defense against infection and probably against cancer cells as well. Substances that elicit an immune response are called antigens, and a specific immunoglobulin that binds to this substance is called an antibody. If the invading particle is large, like a cell, a virus, or a protein, many different antibodies may be elicited, each type binding specifically to a given antigenic determinant (or epitope) on the surface of the particle (Figure 7.29a). The major types of immunoglobulin molecules are listed below:
IgA - An immunoglobulin found in bodily secretions, including saliva, sweat, tears, and along the walls of the intestines. It is the major antibody of colostrum, the initial secretion from a mother's breasts after birth.
IgD - An immunoglobulin found on the surface of B cells.
IgE - An immunoglobulin associated with some of the body's allergic responses.
IgG - An immunoglobulin also known as gamma-globulin. It is the most abundant of circulating antibodies (see Figure 7.33).
IgM - An immunoglobulin produced during the early response to an invading microorganism that is the largest immunoglobulin. It contains 5 Y-shaped units of two light and two heavy chains each.
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