Hemagglutinin

Hemagglutinin or haemagglutinin (British English) refers to a substance that causes red blood cells to agglutinate. This process is called hemagglutination or haemagglutination.

Antibodies and lectins are commonly known hemagglutinins.

Types
Examples include:
 * Influenza hemagglutinin
 * Measles hemagglutinin
 * Parainfluenza hemagglutinin-neuraminidase
 * Mumps hemagglutinin-neuraminidase
 * The PH-E form of phytohaemagglutinin

Uses in serology
Hemagglutination can be used to identify RBC surface antigens (with known antibodies) or to screen for antibodies (with RBCs with known surface antigens).

Using anti-A and anti-B antibodies that bind specifically to either the A or to the B blood group surface antigens on RBCs it is possible to test a small sample of blood and determine the ABO blood group (or blood type) of an individual.

The bedside card method of blood grouping relies on visual agglutination to determine an individual's blood group. The card has dried blood group antibody reagents fixed onto its surface and a drop of the individual's blood is placed on each area on the card. The presence or absence of visual agglutination enables a quick convenient method of determining the ABO and Rhesus status of the individual.

Agglutination of red blood cells is used in the Coombs test.