ABO Blood Types
There are four blood types: A, B, AB, and O. Each
designates the presence or absence of specific
blood antigens and antibodies. Each type causes
an immune reaction to blood from its opposite
type. The exception is type O blood, which has no
antigens on its cell surfaces. Type AB blood has
antigens A and B on its cell surfaces. Doctors
sometimes refer to people who have type O blood
as universal donors because people of any blood
type can receive emergency transfusions of type O
blood (preferably as packed red blood cells con-
taining little of the antibody-carrying PLASMA), and
people who have type AB blood as universal
recipients because they can receive blood of any
type in emergency transfusions.
ABO BLOOD TYPESBlood Type Antigens on Erythrocytes Antibodies in Plasma
A A anti-B
B B anti-A
AB A and B none
O none anti-A and anti-B
Rh Blood TypesBlood typing further incorporates tests for the
presence or absence of a group of antigens known
collectively as the rhesus (Rh) factor. The name
derives from the rhesus monkey, the animal in
which researchers first isolated the antigens.
When the antigens are present on the erythro-
cytes the designation is Rh positive (Rh+); when
the antigens are not present the designation is Rh
negative (Rh–). Less than 15 percent of the Amer-
ican population has Rh– blood. Though collec-
tively there are more than 40 identified antigens
in the Rh blood type system, nearly all antibody
response arises from Rh-D antigens. Most blood
type classifications represent the ABO type and Rh
status together, as in AB+ or O–.
An immune response occurs when Rh+ and
Rh– blood mingle, irrespective of the ABO blood
type. The health risk is to the individual whose
blood is Rh–. Rh incompatibility is a serious threat
to the life of an unborn child and can manifest
when a mother who is Rh– conceives a child who
is Rh+. The first commingling of Rh-incompatible
blood typically does not result in adverse effects
because the mother’s Rh– blood does not yet con-
tain antibodies against the fetus’s Rh+ blood, but
the first exposure to Rh+ blood activates ANTIBODY
production in the mother. The first child in an Rh-
incompatible pregnancy typically is born without
complications; however, subsequent pregnancies
result in the mother’s body producing massive
Rh+ antibodies that cross the PLACENTAand destroy
the fetus’s Rh+ erythrocytes. This condition, called
hemolytic disease of the newborn (HDN) or ery-
throblastosis fetalis, often kills the unborn child.
The standard of PRENATAL CAREin the United
States includes blood tests to determine maternal
Rh status, with an injection of Rh IMMUNOGLOBULIN
to prevent antibody formation in women who
have Rh– blood. Rh incompatibility also can cause
transfusion reaction when a person who has Rh–
blood receives Rh+ blood. Such transfusion reac-
tions can cause serious ANEMIAand cell agglutina-
tion (clumping) that can result in death.Distribution of Blood Types
The most common blood type among Americans is
O+ (38 percent); the least common is AB– (1 per-
cent). Genes determine blood type (ABO as well
as Rh). Before the advent of DNAsequencing and
HUMAN LEUKOCYTE ANTIGENS (HLAS) typing, blood
type was the basis of paternity testing. There are
enough variations in blood type INHERITANCE PAT-
TERNS, however, to make blood type less than 100
percent reliable for determining parentage, and
blood type is no longer legal proof of parentage in
the United States.PERCENTAGES OF BLOOD TYPES AMONG AMERICANS
type O+ 38 percent
type A+ 34 percent
type B+ 9 percent
type O– 7 percent
type A– 6 percent
type AB+ 3 percent
type B– 2 percent
type AB– 1 percent
Source: American Association of Blood Banks, 2005See also BLOOD AUTODONATION; BLOOD DONATION.bone marrow The semigelatinous tissue within
the center of the BONE. Though its presence is
imperceptible in health, the bone marrow is thebone marrow 131