Q.Surface antigens on RBCs are called ________.

Blood typing exists because every person's red blood cells carry a personal pattern of surface antigens, and mismatching that pattern between donor and recipient triggers a dangerous immune reaction. The ABO system recognises four groups - A, B, AB and O - based on which of two antigens (A and/or B, called agglutinogens) sit on the RBC surface; the plasma simultaneously carries natural antibodies (agglutinins) against whichever antigen a person's own cells lack, so group A blood carries anti-B antibodies, group B carries anti-A, group AB carries neither antibody (since both antigens are already present), and group O carries both anti-A and anti-B antibodies (since neither antigen is present). Transfusing incompatible blood causes the recipient's antibodies to attack the donor's antigen-bearing cells, so compatibility must always be checked. The second system, Rh typing, is based on a single surface protein (the D antigen), present in roughly 80% of people (Rh-positive) and absent in the rest (Rh-negative). Rh mismatch matters especially in pregnancy: if an Rh-negative mother carries an Rh-positive foetus, foetal blood cells that cross into her circulation (mainly at a first delivery) can sensitise her immune system to produce anti-D antibo …