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A substance that will cause a clumping of particles such as bacteria or erythrocytes. Of major importance are the specific or immune agglutinins, which are antibodies that will agglutinate bacteria containing the corresponding antigens on their surfaces. Agglutinins are readily determined, and their presence is of diagnostic value to indicate present or past host contact with the microbial agent sufficient to result in antibody formation. See Agglutination reaction, Antibody
Analogous reactions involve erythrocytes and their corresponding antibodies, the hemagglutinins. Hemagglutinins to a variety of erthyrocytes occur in many normal sera, and their amounts may be increased by immunization. The blood group isoagglutinins of humans and animals are important special cases which must be considered in all proposed blood transfusions lest transfusion reactions result. See Blood groups