Blood Groups


 * Blood groups 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 1st pregnancy and before 2nd to kill 1st fetus’s cells that remained in mother’s circulation)--Artemis 20:30, 1 June 2011 (CEST)