Human Physiology, 14th edition (2016)

Blood, Heart, and Circulation 413

donor red blood cells. (Donor plasma could agglutinate recipi-

ent red blood cells if the transfusion volume were too large.)

Because of the dangers involved, use of the universal donor

and recipient concept is strongly discouraged in practice.

Rh Factor

Another group of antigens found on the red blood cells of

most people is the Rh factor (named for the rhesus monkey,

in which these antigens were first discovered). There are a

number of different antigens in this group, but one stands out

because of its medical significance. This Rh antigen is termed

D, and is often indicated as Rho(D). If this Rh antigen is pres-

ent on a person’s red blood cells, the person is Rh positive; if it

is absent, the person is Rh negative. The Rh-positive condition

is by far the more common (with a frequency of 85% in the

Caucasian population, for example).

The Rh factor is of particular significance when Rh-

negative mothers give birth to Rh-positive babies. The fetal

and maternal blood are normally kept separate across the pla-

centa (chapter 20, section 20.6), and so the Rh-negative mother

is not usually exposed to the Rh antigen of the fetus during

donated is small, since plasma from a type O person would
agglutinate type A, type B, and type AB red blood cells. Like-
wise, type AB people are universal recipients because they
lack anti-A and anti-B antibodies, and thus cannot agglutinate

Figure 13.5 Agglutination reaction. People with
type A blood have type A antigens on their red blood cells and
antibodies in their plasma against the type B antigen. People with
type B blood have type B antigens on their red blood cells and
antibodies in their plasma against the type A antigen. Therefore,
if red blood cells from one blood type are mixed with antibodies
from the plasma of the other blood type, an agglutination
reaction occurs. In this reaction, red blood cells stick together
because of antigen-antibody binding.

Type A Type B

Antigens on

red blood

cells

Antibodies

in plasma

Agglutination

reaction

Figure 13.6 Blood typing. Agglutination (clumping) of

red blood cells occurs when cells with A-type antigens are mixed

with anti-A antibodies and when cells with B-type antigens are

mixed with anti-B antibodies. No agglutination would occur with

type O blood (not shown).

Anti-B Anti-A

Type A

Type B

Type AB

Table 13.3 | The ABO System of Red Blood
Cell Antigens

Genotype

Antigen

on RBCs

Antibody

in Plasma

I AI A; I Ai A Anti-B

I BI B; I Bi B Anti-A

ii O Anti-A and anti-B

I AI B AB Neither anti-A nor anti-B