122 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”6.8 When Self Attacks Self: Autoimmunity
How does the immune system avoid attacking its own cells? While
working hard to eliminate non-self intruders, the immune system of an
animal must walk the fine line of not attacking itself. This important
task is accomplished by a process called “negative selection” during
the developmental stage of the immune system. By a mechanism not
totally understood, the bone marrow (where B cells mature) and thymus
(where T cells mature) manage to express an entire spectrum of self-
proteins belonging to the animal. Young lymphocytes with “receptors”
(surface antibodies) that bind strongly to these self-proteins are elim-
inated (negative selection). Only lymphocytes that survive this initial
pruning are released to the lymph nodes, where they will be activated by
non-self antigens through positive selection. In effect, the bone marrow
and thymus serve as “training camps” where lymphocytes are “educated”
to distinguish self from non-self.
Like other processes in nature, this complicated system of cell
selection is not perfect, and sometimes aberrant outcomes do occur,
leading to either overreaction (autoimmune diseases) or under-reaction
(immunodeficiency). Examples of autoimmune diseases include myas-
thenia gravis, in which auto-antibodies attack acetylcholine receptors
in neuromuscular junctions, resulting in muscle weakness; rheumatoid
arthritis, where T lymphocytes attack joint tissue lining, leading to inflam-
matory joint disease; and multiple sclerosis, where T cells destroy myelin
proteins, causing multiple neurologic symptoms. One way to treat these
diseases is to give immunosuppressive drugs, such as corticosteroids,
cytokines, and agents toxic to immune cells. Some of the autoimmune
diseases are rare, but as medical progress prolongs life span, many rare
autoimmune disorders surface and become more prevalent.
6.9 When Self Tolerates another Self: Pregnancy
One instance when self naturally tolerates non-self is the mother-
fetus relationship. Fetuses inherit some of the paternal proteins, which