What best defines an antibody titer?

• A. A specific type of antibody
• B. A measurement of the amount of antibody present
• C. The response to a second exposure to the same antigen
• D. The lack of response to an antigen

Answer: (B)

Antibody titer describes how much antibody is present in a sample. It is determined by serially diluting the serum and testing for detectable antibody activity; the titer is the reciprocal of the highest dilution that still shows a reaction (for example, detectable at 1:64 but not at 1:128 gives a titer of 64). This measures quantity, not the specific type of antibody, nor the immune response after a second exposure, and it doesn’t by itself indicate lack of response. Titers are used to gauge immune status or vaccine response and can be reported for binding antibodies or, in some tests, for neutralizing (functional) antibodies.