Mycoplasma pneumoniae is one of the smallest free-living organisms. It is an atypical bacterium that forms no cell wall and has limited biosynthetic capabilities.

In addition toChlamydia and Legionella, M. pneumoniae is one of the primary causes of community-acquired pneumonia (CAP). Differentiation of the pathogens is crucial for choosing the suitable antibiotic therapy. 

Mycoplasma are transmitted by droplet infection and affect both the upper and lower respiratory tract. They often cause mild to severe respiratory disease. M. pneumoniae affects all age groups, though it is most frequently found in children and adolescents. Because infection does not lead to life-long immunity, reinfection is possible.

In addition to direct detection of the pathogen with cultures and nucleic acid amplification tests, serological detection of pathogen-specific antibodies with ELISA is the most common method used to diagnose infections caused by M. pneumoniae. Antibodies against M. pneumoniae’s adhesins are particularly useful for this. These antibodies are characterized by their high sensitivity and specificity. Adhesins are special surface molecules that help bacteria to stick to the mucous membranes of the respiratory tract.

The individual detection of IgG, IgM, and IgA antibodies is sufficient to differentiate between primary infections and reinfections. IgM antibodies appear seven to ten days after onset of the infection, IgM antibodies are often absent in reinfections. The IgG titres slowly increase as the disease progresses, usually reaching a maximum about five weeks after the onset of clinical symptoms and then slowly decrease again. 

A significant increase in the IgG titre within two weeks is indicative of an acute infection. High IgA titres are the best indicator of acute infection in adults; however low titres of IgA antibodies can persist over longer periods of time.