b. Development of Antibodies. The sugar-sugar linkages that confer A, B, and
H activity on molecules in the red blood cell membrane also occur in other biologic
materials. Many bacteria have structural products with the same sugar groups; the
antigenic properties of these bacterial materials are similar to those of human red blood
cells. Bacteria are everywhere in the environment, and it appears that dust, foodstuffs,
and other widely distributed agents constitute a powerful, continuing antigenic stimulus.
Persons with normal immune systems react to these stimuli by producing antibodies
against those antigens foreign to their own system. Thus, anti-A occurs in serum from
persons of group O or B, and anti-B in group O or A serum. AB persons, having both
antigens, lack both antibodies. Except for the rare hh persons who lack H, everyone
has some element of H in his cellular makeup. Anti-H appears regularly in the serum of
hh individuals, but rarely in other serums. A few persons with strongly active A1, or A1
and B, transferases convert virtually all their H substance to A, or to A and B. These
people may have a weak anti-H in their serum, but it never achieves the strength of the
anti-H in hh persons, or of anti-A or anti-B.
(1) Time of appearance. Antibody production does not normally begin until
after birth. Newborns have their mother's IgG antibodies, but these are passively
received through placental transfer, not actively produced. Anti-A and anti-B production
begins in the first few months of life, with titers rising for the first 5 or 6 years, and then
remaining functionally the same until late in adult life. In very old people, levels of anti-A
and anti-B are significantly lower than in young adults.
(2) Antibody behavior. Agglutination is the most conspicuous "in vitro"
serologic effect of anti-A and anti-B. Other effects can and do occur under appropriate
circumstances. Hemolysis, an important "in vitro" effect, sometimes occurs under "in
vitro" conditions, and should be sought in observing every serum test. ABH antibodies
sometimes coat cells without causing agglutination. When coating and agglutinating
antibodies of the same specificity are present in a serum, only agglutination is apparent,
unless the agglutinating antibodies are neutralized or inactivated. Coating antibodies
are usually IgG. They are clinically important in hemolytic disease of the newborn
since, like all IgG antibodies, they readily cross the placenta. Small amounts of IgG anti-
A or anti-B can be found in group B or group A serum, but they are seldom of clinical
significance. Hemolytic disease of the newborn occurs primarily in the A or B offspring
of group O mothers or, rarely, the B offspring of A2 mothers.
(3) Group O serum. In addition to separable anti-A and anti-B, serum from
group O subjects may contain an antibody that reacts with either A cells or B cells. The
two antibody specificities cannot be separated by differential absorption. Either A or B
cells can be used for absorption, and the eluate from either will react with both A and B
cells. This antibody has been called inseparable anti-A and B or anti-A, B. Anti-A, B
from group O serum reacts more strongly with some variant examples of A and B
antigens than does either individual anti-A or anti-B. Blood banks use an anti-A, B
reagent, prepared from group O serum, to detect weakly reacting cells that are not
agglutinated by anti-A or anti-B, and might otherwise be classified as group O.