In humoral immunity special lymphocytes (white blood cells), called B lymphocytes, produce circulating antibodies to act against a foreign substance. This type of immunity is based on the antigen-antibody response. An antigen is a substance, usually a protein, that stimulates the body to produce antibodies. An antibody is a globulin (protein) produced by the B lymphocytes as a defense against an antigen. Humoral immunity protects the body against bacterial and viral infections.
Specific antibodies are formed for a specific antigen, that is, chickenpox antibodies are formed when the person is exposed to the chickenpox virus (the antigen). This is called an antigen-antibody response. Once manufactured, antibodies circulate in the bloodstream, sometimes for only a short time and, at other times, for the life of the person. When an antigen enters the body, specific antibodies neutralize the specific invading antigen; this is called immunity. Thus, the individual with specific circulating antibodies is immune (or has immunity) to a specific antigen. Immunity is the resistance that an individual has against disease.
Cell-mediated and humoral immunity are interdependent, that is, CMI influences the function of the B lymphocytes, and humoral immunity influences the function of the T lymphocytes.
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