Why IgA for intestinal infections?
Giardia parasitizes on the surface of the mucous membrane of the small intestine, without penetrating into the blood. In this case, the immune response is formed primarily local, and the leading role in it is played by secretory immunoglobulins of class A, produced by the mucous membrane. Their appearance in serum reflects the activity of the process in the intestine better than IgG.
Clinically, giardiasis is manifested by bloating, rumbling, unstable stools with alternating diarrhea and constipation, nausea, pain in the umbilical region, and in children - decreased appetite, weight loss and irritability. Skin manifestations are common: atopic dermatitis and urticaria, which are difficult to respond to conventional treatment.
It is important to understand the place of serology in diagnosis. Direct detection of Giardia - cysts in stool or antigen using ICA - remains the main method of confirmation, since it identifies the pathogen itself. However, cysts are released periodically, and a single negative stool test does not exclude invasion. Determination of antibodies completes the picture, especially with repeated negative coproscopies and persistent complaints.
What do IgM and IgG mean?
The immune system produces different classes of antibodies at different times. Immunoglobulins M appear first, usually in the first or second week after infection, last for several weeks or months and then disappear. Their detection indicates an acute or recent process.
Immunoglobulins G appear later, after a few weeks, and persist for years, and often for life. They mean that a meeting with the pathogen took place in the past, and in many cases indicate formed immunity. Therefore, positive IgG in itself is not a reason for treatment.
Why dynamics are important, not just a number
A single result is often ambiguous. Antibody levels vary from person to person, with borderline levels falling into what is known as a gray area. A reliable sign of active infection is a fourfold increase in IgG titer in paired sera taken at intervals of 10–14 days.
This is why serology is rarely prescribed as a single test. The doctor either combines IgM and IgG, or repeats the test, or confirms the result with a direct method - PCR, which detects the DNA or RNA of the pathogen itself.
Limitations of the method
Antibodies do not appear immediately: in the first days of the disease, serology can be negative if the infection has already begun - this is the period of the serological window. In case of immunodeficiency and against the background of immunosuppressive therapy, antibodies may not be produced at all.
Cross-reactions with related pathogens are also possible, giving a false-positive result. Therefore, treatment is never prescribed “based on analysis” - only on the basis of a combination of clinical data and research results.