This is one of the most commonly confused facts about blood groups. The key is understanding the difference between DONATING and RECEIVING. When O− people DONATE: Their RBCs have no A antigen, no B antigen, and no Rh(D) antigen. When transfused into any recipient — whether A+, B−, AB+, O− — the recipient's immune system finds nothing to attack on the donated RBCs. Hence O− is the "universal donor" for RED BLOOD CELL transfusions. When O− people RECEIVE: Their plasma has Anti-A AND Anti-B AND Anti-Rh antibodies. If you give them A+ blood, for example, their Anti-A antibodies attack the A antigens on the donated RBCs → haemolytic reaction. They can only receive blood from O− (which has none of these antigens). This asymmetry is the core of transfusion medicine — the donor's RBCs must be compatible with the RECIPIENT's antibodies. Important nuance: "Universal donor" strictly applies to RBC transfusions. O− whole blood (which contains plasma with Anti-A and Anti-B antibodies) can be problematic in large transfusions because the Anti-A and Anti-B antibodies could attack the recipient's RBCs. This is why modern transfusion medicine mostly uses component therapy (packed RBCs, fresh frozen plasma, platelets separately). For UPSC: Memorise — O− donates to all (no antigens on RBCs) but receives from O− only (has antibodies against A, B, Rh). AB+ receives from all (no antibodies in plasma) but donates RBCs to AB+ only (has A, B, and Rh antigens).

In blood group science, these two terms have very specific and testable definitions: Antigens (also called agglutinogens in old terminology): Proteins or carbohydrate structures on the SURFACE of Red Blood Cells. They are markers of identity — they tell the immune system "I belong to this person." In ABO, the antigens are glycoproteins (A and B are sugar-linked proteins). The H antigen is the precursor. Rh antigen (D) is a protein. Antibodies (also called agglutinins): Proteins in the PLASMA (liquid part of blood) that recognise and attack foreign antigens. In blood groups, they are "natural antibodies" — you are born with antibodies against whatever antigens you do NOT have on your own RBCs. Type A person has A antigens on RBCs, so develops Anti-B antibodies (not Anti-A — that would destroy their own cells = autoimmune disaster). The rule: You always have antibodies against the antigens you DON'T have. The reaction: When incompatible blood is transfused, antibodies in the recipient's plasma bind to foreign antigens on donated RBCs → form antigen-antibody complexes → agglutination (clumping) → haemolysis (RBC rupture) → haemolytic transfusion reaction. For UPSC pattern: Antigens are on RBCs; antibodies are in plasma. Type AB — both antigens, no antibodies. Type O — no antigens, both antibodies. Always opposite.

Haemophilia is a genetic blood clotting disorder — the body cannot make adequate amounts of specific clotting factors. Haemophilia A (most common — 80% of cases): Deficiency of Clotting Factor VIII. Haemophilia B (Christmas disease): Deficiency of Clotting Factor IX. Both are X-linked recessive disorders — the faulty gene is on the X chromosome. Why mostly males? Males have one X and one Y chromosome (XY). If the single X chromosome carries the haemophilia gene, there's no "backup" — they have haemophilia. Females have two X chromosomes (XX). They need faulty genes on BOTH X chromosomes to have haemophilia. If only one X is affected, the other provides enough Factor VIII/IX — they are "carriers" (pass it to sons) but usually don't have the disease. Why "royal disease"? Queen Victoria of Britain (1819–1901) was a carrier of Haemophilia B. She passed it to several of her children and grandchildren who married into European royal families — spreading the condition across European royalty. Her grandson Alexei Romanov (heir to the Russian throne) had haemophilia — this influenced Russian politics and arguably contributed to the conditions leading to the Russian Revolution (1917). Treatment: Infusions of the missing clotting factor (recombinant Factor VIII or IX). Gene therapy for haemophilia is now available — HEMGENIX (approved 2022 by FDA) — first gene therapy for Haemophilia B. For UPSC: X-linked recessive; males affected; females are carriers; royal disease (Queen Victoria); Factor VIII deficiency (Haemophilia A); Factor IX deficiency (Haemophilia B).