Blood group antibodies are made by CD5 and by CD5 B cells

The B1 subset of B lymphocytes is associated with the production of low‐affinity, polyspecific antibodies. B1 cells are generally recognized by their expression of CD5, and comprise the majority of neonatal B cells. The neonate responds to a restricted range of antigens, and generally makes low‐affinity IgM antibody. Published data suggest that antibodies against the blood group antigens A and B are found occasionally in cord blood, and develop rapidly in infants as a result of cross‐reactivity with bacterial carbohydrate antigens. This suggests that CD5+ B1 cells may be specialized to make antibodies against such carbohydrate antigens. In this study we evaluated the appearance of antibodies against the blood group (ABO) antigens in human infants, using reagents which specifically distinguish between IgM (made by the infant) and IgG (mainly of maternal origin) and immunofluorescence to detect low levels of antibody. Having established that antibody is always detectable by 8 months of age, and frequently much earlier, we developed a plaque assay to examine the phenotype of cells making antibody against blood group antigens. At 8 months of age, CD5+ and CD5+ cells were both capable of making anti‐blood group antibody.