An antigen is a substance that is capable of inducing a specific immune response in the host into which it is introduced. The immune response is mediated via an immunoglobulin (protein) molecule, called an antibody, which is formed by B-lymphocytes and T-helper cells that are the basic ingredients of the host's immune system. "Antibody" is the generic name for any immunoglobulin thus produced, no matter how this occurs. Humans can produce many specific antibodies. This may be an active process by a healthy host in response to the challenge of exposure to a foreign antigen transmitted via the placenta or in maternal milk from mother to offspring, or it may be artificially induced by immunization with live attenuated organisms, killed organisms, or a protein derivative.
An antigen is an organic compound—a protein, polysaccharide or glycolipid. Sometimes it is an entire organ or tissue that has been transplanted into the host, which rejects it and attempts to destroy it. An antibody has the capacity to bind specifically to the (foreign) antigen and thereby neutralize it so it can be destroyed by the host's phagocytes.
The antibody is the basic ingredient of the host's defenses against infection. By measuring the concentration of specific antibodies in individuals and populations it is possible to determine levels of susceptibility and resistance to infection by specific pathogens. At the population level, this is called "sero-epidemiology."
An antigen is produced by living organisms, which evolve over generations and can therefore undergo subtle changes in protein composition, a process known as "antigen drift." A more sudden evolutionary change can lead to an abrupt change in protein structure, known as "antigen shift." Antigen drift renders an antibody less effective, and antigen shift makes an antibody ineffective in combating an antigen to which the host was immunized by exposure to the previous form of the antigen. Antigen drift and antigen shift account for recurrences of infection with viruses, such as those of the common cold and influenza.
JOHN M. LAST