Determination of avidity of IgG to Toxoplasma gondii (toxoplasmosis) in blood serum by the method of immunochemiluminescence
Toxoplasma gondii (T. gondii) is one of the representatives of intracellular parasites. Toxoplasmosis is a disease characterized by chronic or acute course with damage to the nervous system, liver, spleen, lymph nodes, skeletal muscles, myocardium. Toxoplasmosis is known to belong to the group of TORCH-infections, which are potentially dangerous for fetal development and cause severe organ damage in newborn children.
Avidity is the degree of binding strength of an antibody molecule to an antigen molecule. The primary immune response to antigens not previously encountered by the organism begins with the production of IgM.
IgG are late antibodies; in the primary immune response they replace the early IgM antibodies and accumulate in the body in much larger quantities. IgG have the highest avidity.
The blood test involves determining the antibodies formed in the body in response to toxoplasma invasion.
The acute phase of the disease is indicated by the presence of IgM antibodies. But the period of their presence in the body lasts from several weeks to months. The build-up of IgG levels occurs within a few weeks. At the same time, low-affinity antibodies are formed at the beginning of the immune response when the virus multiplies in the body and persist for 1-1.5 months from the onset of the disease.
Later, during the maturation of the immune response, high-affinity IgG antibodies appear in the body. High affinity antibodies remain in the body for a long time. Due to these antibodies, a rapid secondary immune response develops in case the pathogen is re-introduced into the body.
In cases of latent infection, IgM is detected in the blood during an exacerbation or reactivation of the infectious process. In cases where IgM is detected in the blood, it is necessary to distinguish between primary infection and exacerbation, as primary infection in pregnant women leads to severe consequences for the fetus. In addition, treatment regimens for primary and latent infections differ.
For this purpose, it is recommended to determine the avidity of IgG antibodies. If IgG with low avidity is detected in the blood in the presence of IgM, this indicates a primary (recent) infection. The presence of highly avid IgG antibodies (in the presence of IgM) indicates a secondary immune response in case of pathogen ingress or exacerbation (reactivation).
Thus, the determination of the avidity of antibodies to a given pathogen allows you to identify the primary infection, differentiate it from the period of reactivation or secondary penetration of the antigen into the body.
The use of IgG-antibody avidity as an indicator of the duration of primary infection was first proposed by Finnish researchers (Hedman K. M. et al., 1989). Detection in the serum of the presence of both IgG- and IgM-antibodies to the infectious agent can be interpreted as evidence of recent primary infection, since, as is known, the disappearance period of Ig M-antibodies is usually about 3 months from the beginning of the infection process.
But the circulation period of Ig M antibodies can vary significantly depending on the infectious agent and individual features of the immune response of the organism. In case of infection with Toxoplasma gondii, trace amounts of IgM antibodies to these infectious agents in some cases are detected for 1 - 2 or more years.