A titer is most accurately described as?

• A. carry antibodies against a particular antigen
• B. measure of antibody affinity
• C. amount of antigen present in a sample
• D. rate of antibody production

Answer: (A)

A titer measures the concentration of antibodies against a specific antigen in a sample, usually determined by serially diluting the serum until the reaction is no longer detectable. The titer is reported as the reciprocal of the highest dilution that still gives a positive result, so a higher titer means more antibody is present. This reflects antibody quantity, not how tightly they bind (affinity), nor the amount of antigen in the sample, nor the speed at which antibodies are produced. For example, a positive reaction at 1:160 but not at 1:320 indicates a titer of 160, indicating the level of antibodies against that antigen.