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antigen-antibody reaction

The combination of an antigen with its specific antibody. It may result in agglutination, precipitation, neutralization, complement fixation, or increased susceptibility to phagocytosis. The antigen-antibody reaction forms the basis for B-cell–mediated immunity.

antigen binding site

Antigenic determinant.


(ant″i-jĕ-nē′mē-ă) [antigen + -emia] The presence of an antigen in the bloodstream.

antigenic determinant

The specific area of an antigen that binds with an antibody combining site and determines the specificity of the antigen-antibody reaction. SEE: antigen.

epitope a.d. The simplest form of an antigenic determinant within a complex antigenic marker. The epitope links with a paratope, one area of an antibody combining site.

antigenic sin

SEE: original antigenic sin.

antigen unit

The smallest quantity of antigen required to fix one unit of complement.


(ant″i-glob′yŭ-lĭn) [anti- + globulin] An antibody that binds with globulin and makes it precipitate out of solution. Antiglobulins are used in Coombs test to detect the presence of a particular antibody or to type blood groups.

antiglobulin test

A test for the presence in human blood of antibodies. The antibodies present in the blood do not, themselves, cause agglutination. It is the addition of an antibody made in animals (antiglobulin) that stimulates red blood cell clumping. The direct antiglobulin test (DAT) is used to diagnose autoimmune hemolytic anemia and hemolytic disease of the newborn. The indirect antiglobulin test (IAT), or Coombs test, is used to identify blood types. SEE: Coombs test.

direct a. t. ABBR: DAT. A laboratory test for the presence of complement or an antibody that is bound to a patient’s red blood cells (RBCs). The test is used in patients with autoimmune hemolytic anemia, hemolytic disease of the newborn, and transfusion reactions. After the patient’s RBCs are washed to remove unbound antibodies, they are mixed with antihuman globulin serum containing polyvalent antibodies that bind with the antibody or complement on the RBCs and cause them to agglutinate (clump). Monoclonal antibodies can be used to identify the specific class of antibody or complement component causing RBC destruction. SEE: Coombs’ test.


Serum antibody to hepatitis A virus. The presence of anti-HAV IgG in the blood is an indicator either of a successful immune response to vaccination or a prior hepatitis A infection. The presence of anti-HAV IgM antibodies suggests current infection.


Antibody to hepatitis B core antigen. The presence of anti-HBc in a sample of serum is an indication of infection (past or present) with hepatitis ...

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