124 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”6.10 Graft-Versus-Host: How the Guest takes
over the House
Under certain circumstances, a most dramatic scene plays out when the
transplanted tissue (non-self) attacks the host (self) and wreaks severe
havoc on the body. This can happen in bone marrow transplantation.
Certain medical conditions require replacement of bone marrow as a
treatment. If the donor’s bone marrow, which contains mature T lym-
phocytes, is immunologically incompatible with the host, the guest lym-
phocytes can mount an attack on virtually every part of the host, leading
to dire consequences. In this instance, the donor is the offensive and
the recipient becomes the defensive, a reverse of what usually occurs in
other types of tissue transplantation.
6.11 When Selves Play Hide-and-Seek: Parasite-Host
Interaction
In many instances the host and the invading organism engage in a war
of immunological hide-and-seek. One example is African trypanosome,
the protozoan parasite that causes a fatal disease called “sleeping sick-
ness.” The parasites evade their host’s immune recognition by constantly
changing their surface proteins, a process called antigenic variation.
A given trypanosome population can sequentially express hundreds of
these variant surface proteins in order to confuse and escape the host’s
immune recognition. This is an example of changing the molecular iden-
tity of self by an organism.^11
6.12 Immune Surveillance: Elimination of the Renegade
The strongest reason to suspect the presence of immune surveillance
against malignant tumors is that people who are immunologically com-
promised (such as those taking immune suppressive drugs) are more
likely to develop cancer. When normal cells turn cancerous, chemical
alterations occur, among which is a decrease in MHC protein (a marker