A titer is a measurement of the concentration of a substance in a solution, but in infectious disease, the term nearly always refers to the amount of antibodies a person has against a particular pathogen. It’s expressed as a ratio, such as 1:40. A blood test can reveal the antibody titers, or blood titers, for measles, for example. For most diseases, there is a threshold that is thought to represent the minimum amount of titers needed for a person to have immunity against that disease, but it’s rarely simple to use titers to determine level of immunity. For one, the amount of titers needed isn’t always clear with some pathogens.
Further, titers only reveal the concentration of antibodies the person has and cannot reveal any information about the quantity or quality of memory B cells or memory T cells specific to that pathogen. Memory B cells, for example, remember how to make antibodies against a particular pathogen. A person might have very low titers against a disease, but if they have a lot of strong memory B cells, then getting infected with the disease may lead those B cells to ramp up production of enough antibodies to fight the infection before a person ever develops symptoms, which means they do have some immunity to that disease.
Titers also do not provide any information about long-lived plasma cells, a type of plasma cells in the bone marrow that also contributes to immunity against a specific disease by producing antibodies over time.