Agglutinin is an antibody found in blood plasma which causes the agglutination (sticking together) of antigens such as foreign red blood cells and bacteria. Each agglutinin acts on a specific antigen, removing it from the blood. An agglutinin is produced in large quantities after immunization with its particular antigen.

Agglutinins which agglutinate erythrocytes (red blood cells) are called isohemagglutinins, and the blood group of an individual is determined by which of these are present in his or her blood. Group O blood contains isohemagglutinins anti-A and anti-B; group A contains anti-B; group B contains anti-A, and group AB contains neither.