Antibodies are immunoglobulin proteins secreted by B-lymphocytes after stimulation by a specific antigen. The antibody formed binds to the specific antigen in order to mark the antigen for destruction.
The type of antigenic exposure occurring in the body determines if the antibody is a naturally occurring or immune antibody.
Naturally occurring antibodies can be formed after exposure to environmental agents that are similar to red cell antigens, such as bacteria, dust, or pollen. Sensitization through previous transfusions, pregnancy, or injections is not necessary.
These antibodies are usually IgM and react best at room temperature or lower.Most of these antibodies are not clinically significant with the exception of ABO antibodies.
Examples of naturally occurring antibodies include anti-A, anti-B, anti-Cw, anti-M, and antibodies in the Lewis and P systems.