Phosphorylcholine is the hydrophilic polar head group of some phospholipids, which is composed of a negatively charged phosphate bonded to a small, positively charged choline group. Phosphorylcholine is part of the platelet-activating factor; the phospholipid phosphatidylcholine and sphingomyelin, the only phospholipid of the membrane that is not built with a glycerol backbone. Treatment of cell membranes, like those of RBCs, by certain enzymes, like some phospholipase A2, renders the phosphorylcholine moiety exposed to the external aqueous phase, and thus accessible for recognition by the immune system. Antibodies against phosphorylcholine are naturally occurring autoantibodies that are created by CD5+/B-1 B cells and are referred to as non-pathogenic autoantibodies.