Antigens
Antigens,
or immunogens,
are substances or molecules that are foreign to the body but when introduced
trigger the production of antibodies by B lymphocytes leading to the ultimate
destruction of the invader. They are usually large macromolecules (>10,000
Da) such as proteins, polysaccharides, lipids, and free nucleic acids. Antigens
are recognized by specific receptors present on the surface of lymphocytes and
by the antibodies or immunoglobulins secreted in response to the
antigen. Antigens can take the form of any foreign substance including
bacteria, fungi, viruses, protozoa, parasites, and nonmicrobial agents such as
plant pollens, insect venom, and
transplanted
organs.
Antigens
possess immunologically active sites called antigenic determinants, or epitopes. These are smaller, discrete
components of the antigen that have a unique molecular shape, which can be
recognized by and bound to a specific Ig receptor found on the surface of the
lymphocyte or by an antigen-binding site of a secreted antibody (Fig. 13.4). It
is not unusual for a single antigen to possess several antigenic determinants
and, therefore, be capable of stimulating several different T and B
lymphocytes. For example, different proteins that comprise the influenza virus
may function as unique antigens (A, B, C, H, and N antigens), each of which
contains several antigenic determinants. Hundreds of antigenic determinants are
found on structures such as the bacterial cell wall.
Low
molecular weight molecules (<10,000 Da) may contain antigenic determinants
but alone are usually unable to stimulate an immune response. These molecules
are known as haptens. When they are complexed with an immunogenic carrier
(usually a protein), they function as antigens. Many haptens exist in nature
and frequently create problems for humans. Urushiol is a toxin found in the
oils on poison ivy that is responsible for initiating an allergic reaction.
An allergic response to the
antibiotic penicillin is an example of a medically important reaction due to
hapten–carrier complexes. The penicillin molecule is very small (350 Da) and
usually nonantigenic. However, in susceptible people it can complex with
carrier proteins in the body, which are then recognized as “foreign” and
capable of initiating an antigen–antibody reaction.
