Q.An unconscious patient is rushed into the emergency room and needs a fast blood transfusion. Because there is no time to check her medical history or determine her blood type, which type of blood should you as her doctor, give her?
a. A2
b. AB
c. O1
d. O2

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 …