Immune function risk factors for acute lower respiratory tract infections.

The immune response of pneumonia patients and controls to Streptococcus pneumoniae was investigated by enzyme-linked immunosorbent assay (ELISA). Patients less than 6 months of age had significantly lower levels of anti-pneumococcal polysaccharide antibodies than their age-matched controls. Patients and controls 6-14 months of age had lower antibody levels than the children 0-5 months of age. Adult patients and controls did not differ in their antibody status. However, patients and Papua New Guinean controls had depressed cell-mediated immunity and low T-cell numbers. Low levels of antibodies to pneumococcal polysaccharides in children less than 6 months of age appear to be an important risk factor for acute lower respiratory tract infections but other factors may be important in older infants and adults.