In some blood group systems, the plasma contains an antigen that is not synthesized by the red cell precursors but that contributes to the red cell phenotype. This antigen may be glycolipid (eg, ABH and Lewis) or glycoprotein (eg, Chido and Rodgers). The amount of ABH glycolipid in plasma is affected by the secretor and Lewis genes, and is greatest in those secretors who lack the Le gene. ABH taken up by the red cell from plasma is a type 1 glycolipid, whereas ABH glycolipid intrinsic to the red cell is type 2. Thus only those antibodies directed against type 1 glycolipid will detect ABH antigen acquired from the plasma. For example, type 1 A is detected by the antibody in group O donors but rarely by the antibody in group B donors. Lymphocytotoxic reactions which, at first, appear to be due to new specificities are sometimes due to the presence of type 1 glycolipids with ABH and Lewis specificity, acquired by lymphocytes from the plasma. Chido (Ch) and Rodgers (Rg) are antigens of C4, a glycoprotein. These antigens are acquired by the red cells when complement is activated.