The aims of this study were: 1) to characterize the IgG and IgM antibody responses to MsgC fragments in HIV-infected and HIV-uninfected children hospitalized with PcP (PcP+) and other causes of pneumonia (PcP-); 2) to identify specific host factors that are independent predictors of these antibody levels; 3) to determine if any of the antibody responses are independent predictors of mortality from PcP. The gene discussed is CD40LG; the disease is pneumocystosis.