Two previously undescribed extracellular streptococcal antigens detectable with naturally occurring human antibodies

Two streptococcal extracellular antigens (X and Y), which are detectable with naturally occurring human antibodies, have been isolated and shown to represent entities distinct from previously described streptococcal enzymes, toxins, and antigens. They are each synthesized by both group C and group A streptococci and appear to be proteins. Both antigens were found to be nonhemolytic and nontoxic for human leucocytes as well as for isolated beating newborn rat heart cells in tissue culture. In addition to being distinct from known streptococcal enzymes, they were also shown to be devoid of several other enzymatic activities.