An antigen on the surface of a red cell that is not normally visible (it’s “in a crypt,” get it?). These antigens are “exposed” either through the actions of enzymes secreted by bacteria in acquired polyagglutination or as a result of an inherited defect leading to altered structure of the normal red cell antigens. Normally benign antibodies against cryptantigens are present in most human serum, and exposure of the usually invisible cryptantigens may result in polyagglutination. T, Tn, and Cad are well-known examples of cryptantigens.