Antigens are surface proteins found on all cells including blood cells. In the case of blood groups, an individual's blood type reflects the presence or absence of specific antigens. An antigen-antibody response is initiated if the individual receives a transfusion of blood containing antigens that it identifies as being "foreign." Antibodies found in a person's blood bind to the foreign antigen, causing agglutination, or clumping. The antigen-antibody complexes clog the small blood vessels, and the foreign RBCs are lysed, releasing hemoglobin into the bloodstream. The most serious complication of a transfusion reaction is kidney failure due to blockage of the kidney tubules by the hemoglobin molecules.
