The serologic reactions of the tissue and blood-cell antigens of most animals are normally characteristic of the species. Significant serologic cross reactions usually occur only with antisera to the corresponding antigens of closely related species. The numerous groups of heterophile antigens—of which the Forssman antigens are the best studied—constitute significant exceptions. Heterophile antigens link the species hog-ox-human (blood group A), cat-horse, and dog-hog-cat-human, while several heterophile groups link otherwise diverse microorganisms. Links between pneumococcus type XIV and the human blood groups are also known, as well as similarities between antigens in mammalian hearts and the cell walls of the group A hemolytic streptococcus—a bacterium that is a common cause of rheumatic fever in humans. The cross reactions between the Proteus bacillus and the Rickettsiae are important in the diagnosis of typhus fever.