hemagglutinate

The agglutination of red blood cells caused by an antibody either for red blood cell antigens or for antigens that coat red blood cells or by the presence of viruses or other microbes.

Agglutination of erythrocytes usually by either antibodies, viruses or certain plant lectins; abbreviated HA.

• indirect h. test — see hemagglutination inhibition test (below).
• h. inhibition (HI) test — an assay for the presence of specific antiviral antibodies in a test serum. The serum, usually a twofold dilution series, is mixed with a standard number, usually 4 to 8 HA units, of virus and incubated prior to the addition of a standard suspension of erythrocytes. The highest dilution of serum that inhibits hemagglutination is the HI titer of the serum.
• passive h. test — see passive agglutination test.
• h. (HA) test — hemagglutinating viruses directly agglutinate erythrocytes by binding to specific receptor sites on the surface of the erythrocyte and this characteristic can be used in detection, identification and quantitation of the virus.
• h. viruses — viruses capable of agglutinating red blood cells of a variety of animals, e.g. adenoviruses, parvoviruses, togaviruses, some coronaviruses, picornaviruses, orthomyxoviruses and paramyxoviruses. Useful in classifying viruses and assaying amounts of virus and antibody. See also hemadsorption.

The verb has one meaning:

Meaning #1: cause the clumping together of red blood cells
  Synonym: haemagglutinate