Agglutinin

An agglutinin is a substance that causes particles to coagulate to form a thickened mass [1]. Agglutinins can be antibodies that cause antigens to aggregate by binding to the antigen-binding sites of antibodies. Agglutinins can also be any substance other than antibodies such as sugar-binding protein lectins. Agglutinins work by clumping on particles causing the particles to change from fluid-like state to thickened-mass state. When an agglutinin is added to a uniform suspension of particles such as bacteria or blood, agglutinin binds to the agglutinin-specific structure on the particle causing the particles to aggregate and fall to the bottom leaving a clear suspension. This phenomenon known as agglutination is of great importance to the medical world as it serves as a diagnostic tool.

Reaction of particles with agglutinin is used to indicate present or past host contact with a pathogen [2]. A host infected with a pathogen produces antibodies to neutralize the pathogen. As a result, the blood of a host applied to a diagnostic kit causes the aggregation of the pathogenic particles due to the antigen-agglutinin interaction. Conversely, agglutination can also be used to identify new bacteria or cells with a specific antigen by exposing them to serum containing known agglutinins.

Agglutination, using blood agglutinins known as hemagglutinins, is used diagnostically to identify blood types of human beings based on the reaction between the erythrocyte (Red blood cell) antigens and agglutinins [3]. Human erythrocytes have only two types of antigens (Antigen A and B) expressed in different combinations to give either erythrocytes that express only antigen A, antigen B, antigen A and B together or no antigen at all. When erythrocytes are exposed to hemagglutinins (anti-A and Anti-B antibodies), those expressing antigen A or B coagulate upon contacting anti-A and anti-B hemagglutinins respectively. Erythrocytes expressing both antigens coagulate upon contacting either anti-A or anti-B hemagglutinins while those not expressing any antigen do not coagulate upon contact with any hemagglutinin. This technique serves as a quick and effective method for identifying the blood type of a person and it is essential when blood transfusion has to be performed quickly to replenish lost blood.

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Wikipedia