When a horse comes into contact with certain substances (especially infectious agents), the immune system (certain white blood cells) produces antibodies (glycoproteins) that bind to particular parts of the offending agent (called an antigen).

Antibodies bind to invading pathogens and help neutralize and remove the offenders.

An antibody titer is a count of the number of antibodies to a particular agent (bacteria, virus or allergen) found in the horse's blood.

A positive titer indicates that the horse has been exposed to the agent in question. A negative titer generally means that the horse has not been exposed. A higher titer generally indicates greater exposure and a higher likelihood that the agent is indeed causing disease. Vaccines also produce high titers.

A blood sample is submitted to a laboratory. There are a variety of methods of determining titer, but the basic principle is to use the antigen (infectious agent itself or part of it), bound to some sort of visible marker. This antigen will bind to the antibodies in the blood, and the marker will be visible in the sample.

In recent years, it has become popular to test antibody levels in horses to determine their need for vaccination. The idea is that if the antibody level is high, the horse has no need to be vaccinated. While this makes intuitive sense, it may not always be reliable.