Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

The Animal Self: Molecular Recognition 123

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

are foreign to the mother. Normally the conflict is avoided by the non-
mixing of maternal and fetal circulations. Nevertheless, some of these
foreign cells may escape from the fetal to the maternal circulation
(through small tears in the placenta during delivery), and this induces
antibodies from the mother against the fetus. During subsequent preg-
nancies, some of these antibodies may pass through the placental bar-
rier and attack the second fetus, if it also expresses the same offending
antigen. A common example of this situation is blood group incompat-
ibility, leading to hemolysis (breakage of red blood cells) and jaundice
in the newborn. In severe cases, blood transfusion may be needed to
save the baby.
The immune system can be “coaxed” to accept a foreign antigen by
introducing the antigen in small doses over a long period of time. This
phenomenon, called anergy, occurs naturally in the body against some
self proteins as part of immune system maturation. The same principle
can be used to desensitize allergy, a type of immune reaction.
In an experimental setting, an animal can be fooled to accept an
organ transplant by introducing the donor cell to the recipient when the
latter is at a very young age, during a critical period when the immune sys-
tem is being “trained” to discriminate between self and non-self (Fig. 6.5).

Fig. 6.5. How self can be “fooled” to accept non-self. A skin graft was successfully
transplanted from an adult brown mouse to an adult white mouse. The white mouse
tolerated the foreign skin because earlier (at the time of birth) it received an injection
of bone marrow cells (the precursors of all immune cells) from the brown mouse, thus
changing its immunological makeup. [See Note 14; permission Mosby, Inc.]