Toxoplasma gondii, avidity of IgG antibodies

Why this test?

To determine the fact and history of infection with Toxoplasma gondii, in order to exclude or verify a recent primary infection.

In what cases is it prescribed?

The determination of T. gondii antibodies is not usually a routine test for pregnant women, but may be necessary if the patient has been in contact with cat feces, raw (or undercooked) meat, or contaminated food (water), or if the doctor wants to determine whether the women previously had toxoplasmosis.

If the antibody test is negative, but toxoplasmosis is still suspected, the test can be repeated one or more times during pregnancy.

This test is sometimes prescribed to people with weakened immune systems who have flu-like symptoms, as well as those who have symptoms or complications typical of toxoplasmosis.

Test information

Toxoplasmosis is an infectious disease caused by the intracellular parasite Toxoplasma gondii, which affects the central nervous system, eyes, liver and lungs.

Cats, which are infected by eating rodents, birds, and meat of domestic animals, are of primary importance in the spread of the disease. A person becomes infected by eating raw or insufficiently heat-treated meat containing parasite eggs, as well as by ingesting toxoplasma eggs from the environment with food and water in case of non-compliance with the rules of personal hygiene. From the place of introduction (most often it is the organs of the digestive system), toxoplasma with the lymph flow penetrates into the regional lymph nodes, where they multiply, and then enter the blood in large quantities (parasitemia occurs). As a result, pathogens spread throughout the body and lesions appear in various organs. With a normal immune response, granulomas form around the foci - a focus of inflammation, in which the pathogen is separated from healthy tissue by a capsule formed by protective cells, and the disease passes from an acute phase to a chronic one. In unfavorable situations for the body (stress, acute diseases), the protective membrane can be destroyed and the pathogen can enter the blood again and affect new cells. This is an exacerbation of chronic toxoplasmosis. Infection in the described way leads to a form of the disease called acquired toxoplasmosis.

However, if a pregnant woman becomes ill with acquired toxoplasmosis, during parasitemia, the causative agent enters the placenta and enters the fetus from it with the blood stream. This disease is called congenital toxoplasmosis. Toxoplasmosis is included in the group of diseases united by the Ministry of Health as TORCH - it is an infection, the primary disease of which is dangerous for the development of the fetus.

In response to toxoplasma antigens, specific IgG and IgM antibodies are produced in the human body. The first to appear in response to infection are IgMs, which circulate in the blood for several weeks. Then their number gradually decreases, and the concentration of IgG, on the contrary, increases and stabilizes at a certain level, providing long-term protection. Antibodies of the IgM class are traditionally considered indicators of early infection (recent infection). However, with toxoplasmosis, they can circulate in the blood for quite a long time, which calls into question the determination of the age of infection by the number of antibodies of this class.

For a more accurate diagnosis of the time of infection, determination of IgG avidity is used. Avidity is the binding force of an antibody to a specific antigen. IgG produced by lymphocytes initially has a low avidity, which increases over time in newly synthesized immunoglobulins. An increase in avidity leads to better binding, which means antigen destruction, and is a natural step in improving the quality of the immune response. When studying avidity, the antigen-antibody complex is treated with a solution that causes protein denaturation. At the same time, the connection of low-avidity antibodies with the antigen is destroyed, and high-avidity antibodies are preserved. The avidity of IgG is estimated using a calculated indicator - the avidity index.