Dictionary:

agglutinin

  (ə-glūt'n-ĭn)

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A substance, such as an antibody, that is capable of causing agglutination of a particular antigen, especially red blood cells or bacteria.

[AGGLUTIN(ATION) + –IN.]

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 also 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 also Blood groups.

1. a specific kind of antibody whose interaction with antigens is manifested as agglutination. 2. an antibody that agglutinates red blood cells or renders them agglutinable.

Any substance causing agglutination (clumping together) of cells, particularly a specific antibody formed in the blood in response to an invading agent. Such agglutinating antibodies (see immunoglobulin) function as part of the immune mechanism of the body. When the invading agents that bring about the production of agglutinins are bacteria, the agglutinins produced bring about agglutination of the bacterial cells both in vivo and in vitro.
Erythrocytes also may be agglutinated by agglutinins that are naturally present in the blood, such as the presence of anti A antibody in humans with the blood group B erythrocytes, or such agglutinins may also be formed in response to the entrance of noncompatible blood cells into the bloodstream. A transfusion reaction is an example of the result of agglutination of blood cells brought about by agglutinins present in the recipient's blood.

• cold a. — antibody that acts only at low temperature.
• cold a. disease — an autoimmune disease in which erythrocyte autoantibodies, usually IgM, are most active at temperatures below 98.6°F (37°C). Agglutination occurs in capillaries of the extremities (tail, ears, nose and feet), particularly on exposure to cold, resulting in tissue necrosis in those areas. Hemolytic anemia is a variable feature.
• group a. — antibody made against a particular organism. One that has a specific action on certain organisms, but will agglutinate other, usually related species as well.
• H a. — one that is specific for flagellar antigens of bacteria.
• immune a. — a specific antibody found in the blood after recovery from the disease or injection of the microorganism.
• incomplete a. — antibody that at appropriate concentrations fails to agglutinate the homologous antigen for steric reasons.
• normal a. — a specific antibody found in the blood of an animal or of humans that has had no known exposure to the antigen with which it combines; these may be natural antibodies such as those directed against A and B blood group antigens in humans or cross-reacting antibodies produced after infection with a related microorganism.
• O a. — antibody specific for somatic or cell wall antigens of a bacterium.
• partial a. — antibody which agglutinates organisms closely related to the specific antigen, but at a lower dilution.
• warm a. — an incomplete antibody that sensitizes and reacts optimally with erythrocytes at 98.6°F (37°C).