The serum concentration of antibody to a specific antigen is known as what?

The term used to describe the serum concentration of antibody to a specific antigen is known as "titer." This is a quantitative measure that indicates the amount of a particular antibody present in the serum, often determined through serial dilution methods where the highest dilution that still demonstrates a positive reaction is recorded. The titer provides valuable information regarding the immune response of an individual, typically in contexts such as vaccination effectiveness or the presence of an infection.

In research and clinical settings, understanding the titer can aid in evaluating an animal's immune status and can guide further steps in treatment or preventive measures. The other options do not specifically denote this quantitative measure of antibodies in the same way as the term titer does. Concentration generally refers to the amount of a substance in a given volume but lacks the specificity related to antibodies; antibody level and immunoglobulin index, while related to the presence of antibodies, are not standardized terms used to describe the degree of antibody response as effectively as titer.