Detection of antibodies in human sera to streptococcal groups A and C carbohydrates by a radioimmunoassay.

Human antibodies to streptococcal groups A and C carbohydrates were measured quantitatively with a radioimmunoassay using tyrosylated 125I carbohydrate. Analysis of acute and convalescent sera from people with groups A or C streptococcal pharyngeal infection or persistent carriage revealed a significant rise in antibodies. Inhibition reactions with the cold carbohydrate indicated the specificity of the elicited antibodies. In some instances, group C as well as group A antibodies occurred after group A pharyngeal infections. Further clinical and epidemiological studies are required to determine the significance of group-specific antibodies. Antistreptolysin-O (ASO) rises were observed in individuals with group C antibody responses after persistent group C pharyngeal carriage. For this reason, epidemiological surveys that rely primarily on ASO surveys to determine the incidence of group A streptococcal infections must be interpreted with caution, at least in the developing countries, where group C pharyngeal carriage is common.