A meaurement of the amount of antibodies in the blood. The test to measure antibodies is usually performed by making a number of dilutions of the blood and then measuring at what dilution there is sufficient antibody to react in the test. For example, a titer of 1:8 (one to eight) means the blood can be diluted to one part blood and seven parts saline and still produce a positive reaction in the test. The higher the titer (1:16 is higher than 1:8), the more antibody is present. (NOTE: The word 'titer' may also be used when discussing the amount of antigen present, e.g., a high titer vaccine has a large number of virus particles.) The reciprocal of the highest dilution of serum that gives a reaction in an immunological test.