Red‐Cell Abnormalities in HEMPAS (Hereditary Erythroblastic Multinuclearity with a Positive Acidified‐Serum Test)

S ummary . Abnormalities of the membrane of HEMPAS red cells have been demonstrated by certain serological tests and in freeze‐etch electron micrographs. There is also a marked increase in the activity of certain red‐cell enzymes. The positive acidified‐serum lysis test has proved useful in distinguishing HEMPAS from other kinds of hereditary dyserythropoietic anaemia. The greatly increased reactivity with anti‐i (although characteristic of HEMPAS) is a less specific finding, since enhancement of the i antigen is found in other dyserythropoietic anaemias, hereditary and acquired. Although the lysis of HEMPAS cells by acidified serum at first suggested that their membrane defect was similar to that present in PNH, further investigation showed that the mechanism of activation of complement is different in the two conditions. Lysis of HEMPAS cells depends on the binding of complement by a naturally‐occurring, cold antibody (“anti‐HEMPAS”) specific for an antigen present on HEMPAS cells, but not detectable on PNH or normal cells. Unlike PNH cells, HEMPAS cells are not lysed by complement in the absence of antibody. As judged by tests with anti‐I and human complement, PNH cells have a true increase in sensitivity to complement in that the binding of only a small number of complement sequences is sufficient to effect lysis. The increased lysis of HEMPAS cells by anti‐I appears to be due to the binding by antibody of many more complement sequences than are bound to PNH or normal cells. With some examples of anti‐I, an increased uptake of antibody by HEMPAS cells also contributes to their increased lysis.