Acronym for a rare inherited anemia known as “Hereditary Erythroblastic Multinuclearity with a Positive Acidified Serum lysis test,” or congenital dyserythropoietic anemia, type II (CDA II). HEMPAS is important in blood banking due to the finding of polyagglutination when red cells from these patients are exposed to human serum in a minority of cases. Like other forms of polyagglutination, HEMPAS is associated with incomplete glycosylation of surface red cell antigens, leading to exposure of an underlying antigen that is not normally visible (cryptantigen). Antibodies against the cryptantigen are not as common as those present in T activation, so polyagglutination is only seen with exposure to about one third of non-self human sera. When present, however, the antibodies will lyse HEMPAS red cells when incubated at 37C.