Biology of Disease

Approximately 75% of Caucasians attach A, B and H antigens (depending on

their blood group) to soluble proteins and secrete them into saliva and other

body fluids, and are known as secretors. Secretor status is dependent on the

expression of a fucosyl transferase, which is encoded by the SECRETORgene

on chromosome 19. In the past, the presence or absence of secreted ABH anti-

gens has been used in forensic science for the purpose of identification.

Antibodies of the ABO Blood Group System

Transfusion of ABO incompatible blood causes problems because individuals

possess plasma antibodies to the complementary antigens. Thus, people of

blood group A have antibodies to the B antigen, those with blood group B

have antibodies to the A antigen (Table 6.5). People of blood group AB have

neither antibody, while those of blood group O possess both. These antibodies

will agglutinate erythrocytes of the complementary group. The pattern of

agglutination when serum and erythrocytes of different ABO types are mixed

in vitro is shown in Table 6.6.

Blood group Antibodies in plasma

A anti-B

B anti-A

AB neither anti-A nor anti-B

O anti-A and anti-B

Table 6.5Antibodies of the ABO system

Serum

Cells A B AB O

A negative positive negative positive

B positive negative negative positive

AB positive positive negative positive

O negative negative negative negative

Table 6.6 Patterns of hemagglutination when sera and cells of different ABO types are mixed

Antibodies directed against the ABO antigens generally belong to the IgM

class though IgG antibodies (Chapter 4) may also be found. IgM antibodies are

pentameric molecules, which are efficient agglutinators of cells and activa-

tors of complement. These antibodies, which are sometimes called isohemag-

glutinins because they agglutinate erythrocytes, are sometimes also termed

‘natural antibodies’ because they are present even though people have never

been ‘immunized’ with blood of an inappropriate group. In fact, the blood

group antigens are also found on some common bacteria and the antibodies

that are produced following infection are capable of cross-reacting with blood

group antigens. Of course, people do not make antibodies to antigens that

they themselves express, because of immunological tolerance (Chapter 5).

Consequences of ABO Incompatible Transfusions

An example of an incompatible blood transfusion would be when a patient

of blood group A, the recipient, is given blood from a donor who is blood

group B. In this case the anti-B antibodies present in the plasma of the

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