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.

edit ABO system:

Genotype	Blood type	Agglutinogen (antigen)	Agglutinin (antibody)
OO	O	-	Anti A & Anti B
OA/AA	A	A	Anti B
OB/BB	B	B	Anti A
AB	AB	A & B	-

• 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.

RBC group	Agglutination with Anti-A	Agglutination with Anti-B
O	-	-
A	+	-
B	-	+
AB	+	+

edit 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.

edit 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).