Cell Wall Components and Antigenic Structure
Characteristically, Coxiella exists antigenically in two forms: (a) phase 1 antigen and (b) phase 2 antigen.
Phase 1 antigen: It is a cell wall LPS antigen with a complexcarbohydrate structure. This antigen usually prevents inter-action of antibodies with surface protein. Coxiella in phase 1 antigen is highly infectious. Phase 1 is highly immunogenic and elicits a strong antibody response to both 1 and 2 antigens.
Phase 2 antigen: This occurs by following repeated passage inyolk sac in which the LPS is modified by exposing the surface proteins to antibodies. In this phase, the surface proteins act with host antibodies. Coxiella occurring in phase 2 is a less infective form of the bacterium.
C. burnetii either in phase 1 or phase 2 do not show anyantigenic sharing with rickettsia or Proteus bacillus antigen. Therefore, Weil–Felix test is not useful for detection of C.burnetii infection. The antibodies in serum of Q fever do notreact with Proteus antigens used in the Weil–Felix test.
Acute stage of Q fever is characterized by the presence of antibody titers against phase 2 antigen, whereas chronic dis-ease is characterized by presence of antibodies against both phase 1 and phase 2 antigens. These high titers of antibodies appear to be responsible for pathological changes in various organs during the disease. The cell-mediated immunity is asso-ciated with recovery from the clinical illness.