To cause agglutination of red blood cells.
Substances capable of causing agglutination of red blood cells are known as hemagglutinins. Examples of which are the antibodies, viral capsid proteins, or some phytolectins (e.g. PHA).
Word origin: hem- » Greek haimo-, from haima + agglutinate.
Related forms: hemagglutination (noun), hemagglutinin (noun)
Variant: haemagglutinate (British)