What is hemaglutination and what are its uses in medicine?

Briefly,: It is the measure of agglutination of red blood cells and can be used as a measurement of the relative concentration of antibody in serum. The red blood cell acts as the antigen and antibody in the serum 'agglutinates' the antigen. Can also be utilized to quantitate virus and bacteria. Although not a medical site this reference is good: http://en.Wikipedia.Org/wiki/hemagglutination.