The body's major line of defense is the immune system. It protects the body against infection, toxins, and cancer. These defenses can be either nonspecific or specific. Lymphocytes are the major types of cells in the immune system and are classified as either B cells or T cells.

When the body is first exposed to an antigen, a primary immune response occurs. This response is less specific and slower than a secondary immune response, which occurs the next time the body is exposed to the same antigen. An intact immune system is important because the body is attacked by numerous invaders every day. In order to effectively perform aseptic technique and infection control, the medical assistant must have a working knowledge of the immune system.