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blood groups

The surfaces of red blood cells carry molecules known as antigens. Certain other cells release substances called antibodies into the blood. An antibody in the blood which matches an antigen, the way a key matches a lock, can bind to the antigen. In doing so it starts an immune reaction which leads to the death of that red blood cell.

Blood can be classified by the antibodies with which it will react. Such classification is essential for transfusing blood safely. More than 20 different blood group systems are recognized in medicine. Of these, the best known are the ABO system and the Rh system.

Blood Groups and Paternity

Using tests for the ABO, MN, and Rh blood systems, a randomly chosen person has about a 50% chance of demonstrating he or she is not the child's biological parent. Most courts in the United States admit the results of tests for blood group systems ABO, MN, and Rh as evidence in paternity cases. They haven't, however, accepted such results as enough to prove paternity. DNA testing has made using blood groups for this purpose obsolete.

Mother	Baby	Father can't be
A	B	O or A
	AB	O or A
	O	AB
B	A	O or B
	AB	O or B
	O	AB
AB	AB	O
O	A	O or B
	B	O or A
	O	AB
M	MN	M
N	MN	N
Any group	M	N
Any group	N	M
Rh+	Rh-	Rh+
Rh-	Rh+	Rh-

The Major ABO Blood Groups

One parent's blood type	Other parent's blood type	Person's blood type	Prevalence in
One parent's blood type	Other parent's blood type	Person's blood type	Americans of Western European descent	Americans of African descent (NY, 1955)	West Africans (Ewe, 1951)	Native Americans (pure Cherokee, 1958)
A1	A1	A1	35%	21%	19%	4%
A1	A2
A1	O
A2	A2	A2	10%	6%	2%
A2	O	A2	10%	6%	2%
B	B	B	8%	17%	29%	1%
B	O	B	8%	17%	29%	1%
A1	B	A1B	3%	1%	4%	0%
A2	B	A2B	1%	3%	4%	0%
O	O	O	43%	51%	43%	96%
h	h	Oh	very rare	very rare	very rare	very rare

Marion E. Reid and Christine Lomas-Francis.
The Blood Group Antigens Fact Book.
New York: Academic Press, 1997.