An antigen is part of a molecule that is recognized by an antibody. Erythrocytes have antigens on their surface that determine blood type. The ABO system classifies blood types according to the presence or absence of A and B antigens. Blood type A has only the A antigen; B has B antigen; AB has both antigens; O has neither antigen. Individuals have antibodies to AB antigens that are not on their own cells. Blood type A has B antibodies; B has A antibodies; AB has no antibodies; O has A and B antibodies.
Blood contains antibodies for AB antigens that are not on their own erythrocytes. Agglutination is the clumping of cells due to reaction of an antibody with a matching cell surface antigen. Transfusion with unmatched blood may cause agglutination. Blood typing tests reactions of blood with antibodies to surface antigens. Agglutination with the added antibody means the antigen is present on the erythrocyte surface.
Rh (rhesus factor) is an antigen that may (Rh+) or may not (Rh-) be present on the red cell surface. Rh antigen is typed by reaction with anti-D antibody. Rh antibodies are only found in blood previously exposed to Rh (such as Rh negative mother who carried Rh positive baby).
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