One of the most often used and least understood immunological terms is antibody titer. But what does titer mean, and how can this information be used in assessing the immune status of a horse?

A titer is defined as the concentration of specific antibodies in the blood that recognize a particular agent, such as equine influenza virus. The titer is determined by serially diluting the serum fraction of blood and assaying (testing) each dilution for the antibody of interest (for example, equine influenza virus antibodies). The last dilution of a serum sample that responds in the assay determines the titer. The greater the concentration of the specific antibody in the serum sample, the higher the titer. For example, a titer for an influenza hemagglutination inhibition assay of 1:10 would be very low; a titer of 1:320 would be high. A low or undetectable titer indicates very little antibody present in the serum