Pathogenesis and Immunity - Mycoplasma pneumonia

Pathogenesis and Immunity

Mycoplasmas are primarily extracellular pathogens that adhere to surface of ciliated and nonciliated epithelial cells.

◗ Virulence factors

The adhesion protein called P1 is the key virulence factor of the Mycoplasma. The bacteria usually do not cause invasion of theblood to produce systemic manifestation of the disease.

P1 antigen: P1 antigen is a membrane-associated protein,which helps in adhesion of mycoplasmas to epithelial cells. This protein or adhesin combines specifically with sialated gly-coprotein receptors present at the base of the cilia on epithelial surface. This same receptor is also present on the surface of the erythrocytes. The antibodies against P1 antigen can also act as an autoantibody against RBCs causing their agglutination.

◗ Pathogenesis of Mycoplasma infections

Following attachment, Mycoplasma cause direct damage to the epithelial cells in which first cilia and then ciliated epithelial cells are destroyed. Loss of the cells interferes with normal functioning of the upper respiratory tract. This results in the lower respiratory tract to become infected with microbes and mechanically irritated. The mechanical irritation causes persis-tent cough typically seen in patients with respiratory infection caused by M. pneumoniae.

      M. pneumoniae behaves as a super antigen. This causes migra-tion of inflammatory cells to the site of infection and produces cytokines, such as tumor necrosis factor-alpha, interleukin-1, and interleukin-6. These cytokines help in clearing of the bacte-ria and of the disease. The bacteria usually do not cause invasion of the blood to produce systemic manifestation of the disease.

        Mycoplasma rarely penetrates the submucosa except in rarecases of immunosuppression or following instrumentation. In these conditions, they may invade the blood stream and cause infection in different organs of the body.

◗ Host immunity

M. pneumoniae infection does not induce any protective immu-nity. Individuals suffering from M. pneumoniae infection are susceptible to reinfection by the bacteria. The P1 antigen induces production of antibodies. These antibodies are found in nearly 50% of patients infected with M. pneumoniae. The anti-body against P1 antigen is an autoantibody that cross-reacts with antigen I of RBCs and is not protective.