Antibody Avidity

Antibody avidity (from Latin avidus — greedy) is an indicator reflecting the overall strength and stability of the bond between specific antibodies (mainly immunoglobulins of class G) and corresponding antigens.

This parameter directly indicates the degree of maturity of the body’s immune response.

Etiology and pathophysiology

Upon first contact with a new virus, the immune system produces antibodies that initially bind weakly to antigens (have low avidity, antigen-antibody complex binding is weak).

Over a period of weeks or months (depending on the type of infection), the affinity of antibody binding to antigens increases, and high-avidity antibodies appear in the blood.

Thus, by the type of antibodies, it is possible to determine the timing of infection:

• presence of low-avidity antibodies — indicates a recent infection;
• detection of high-avidity IgG — means that the infection occurred at least 3–4 months ago.

Clinical significance

Determining avidity is an important test for pregnant women during TORCH screening. For example, if antibodies to toxoplasma or cytomegalovirus are detected for the first time in a pregnant woman, the physician needs to know exactly whether the infection occurred during pregnancy or if the woman was previously infected.

High avidity in the first trimester excludes the risk of primary infection during pregnancy, sparing the patient from unnecessary invasive procedures and psychological stress.

In laboratory practice, avidity is measured in percentages: the higher the percentage, the “older” the immunity to the given disease.