Atypical pneumonia and persistent cough
Chlamydia pneumoniae is an intracellular pathogen that causes bronchitis, pharyngitis and so-called atypical pneumonia. It is characterized by a gradual onset, low temperature, a dry, debilitating cough that lasts for weeks, and scant data when listening to the lungs, which do not correspond to the severity of the complaints.
The practical importance of the diagnosis is that this pathogen is insensitive to penicillins and cephalosporins, the most prescribed antibiotics. The patient receives standard treatment, it does not work, the course is changed to another drug of the same class, and time is lost. Only macrolides, tetracyclines and respiratory fluoroquinolones that act inside the cell are effective.
IgM antibodies appear approximately two to three weeks after the onset of the disease - this is noticeably later than with most infections and explains why early testing is negative in cases where the disease has already developed. Therefore, in case of clinical suspicion, a negative result in the first days does not exclude infection, and the study is repeated or supplemented with the determination of IgG over time.
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.