Blood Groups


 * Blood groups (blood types) are caused by antigens on RBCs.
 * There are about 30 commonly occurring antigens, but most are weak so are only used in paternity tests.

ABO system:

 * ABO blood group antigens are derived from H antigen, which is a carbohydrate sequence with other carbohydrates (D-galactose, N-acetylglucosamine etc.) linked mainly to protein; the A allele encodes for a glycosyltransferase that bonds N-acetylglucosamine, producing A antigen.
 * The B allele encodes for a glycosyltransferase that bonds D-galactose, producing B antigen.
 * In the case of O allele, H antigen remains unchanged.
 * The associated anti-A antibodies and anti-B antibodies are usually IgM antibodies, which are usually produced in the first years of life by sensitization to environmental substances such as food, bacteria and viruses


 * In case of mismatch in transfusion, the following occur:
 * 1) RBCs clump together (agglutinate) as a result of agglutinins attaching to RBCs.
 * 2) The clumps block small vessels.
 * 3) Physical distortion or attack by phagocytic WBCs destroy membranes of agglutinated cells.
 * 4) Hemoglobin released into plasma.
 * 5) Hemolytic anemia.


 * BLOOD TYPING: this is the determination of the blood types of recipient and donor that is necessary before giving a transfusion, so that the bloods can be appropriately matched.
 * 1) RBCs are separated from plasma and diluted with saline.
 * 2) One portion is mixed with anti-A agglutinin and another portion with anti-B.
 * 3) Mixtures are observed under microscope after several minutes.
 * 4) If RBCs have become agglutinated, then an antibody-antigen reaction has occurred.
 * 5) Note that O RBCs don’t have antigens thus do not agglutinate.

RH system:

 * Difference with ABO: spontaneous agglutinins almost never occur therefore agglutination is much slower.
 * 6 antigens: C, D, E, c, d, e (each of which is called an Rh factor).
 * If D is present – Rh+ (because it is considerably more antigenic than the others).


 * Rh immune response – transfusion reactions: When Rh+ is injected in Rh− person, anti-Rh+ antibodies develop slowly (up to 2–4 months). When the antibodies do develop they agglutinate with the antigen on Rh+ RBC forming clumps which are hemolyzed by macrophages. Therefore a transfusion reaction does occur but is mild. On subsequent transfusions however, reactions are more severe because Rh+ already exists in blood.

Erythroblastosis fetalis (Hemolytic disease of newborn)

 * 1) Characterized by agglutination of RBCs and subsequent phagocytosis leading to hemolytic anemia.
 * 2) Occurs mostly in Rh− mothers bearing an Rh+ child; the mother develops anti-Rh+ agglutinins from exposure to fetus's Rh+ antigen; these agglutinins then diffuse through the placenta into the fetus and cause RBC agglutination.
 * 3) Incidence rises progressively with subsequent pregnancies.
 * 4) Prevention: administration of Rh immunoglobulin globin (an anti-D antibody) to the expectant mother at 28–30 weeks of gestation, to prevent sensitization of mother to D antigen of fetus (usually administered after first pregnancy and before second to kill first fetus’s cells that remained in mother’s circulation).