Antibody formation occurs in response to the presence of a substance perceived by the immune system as foreign. The foreign entity is generically called an antigen. There are a myriad of different antigens that are presented to the immune system. Hence, there are a myriad of antibodies that are formed.

The formation of innumerable antibodies follows the same general pattern. First, the immune system discriminates between host and non-host antigens and reacts only against those not from the host. However, malfunctions occur. An example is rheumatoid arthritis, in which a host response against self-antigens causes the deterioration of bone. Another example is heart disease caused by a host reaction to a heart muscle protein. The immune response is intended for an antigen of a bacterium called Chlamydia, which possess an antigen very similar in structure to the host heart muscle protein.

Another...