plement
also
provides
a means
of attracting
macrophages, which scavenge
the target cells
or their
products.
Alternatively, the antigen-
antibody
complex may
be taken up directly by
macrophages
(scavenger
cells)
and destroyed.
The cell-mediated response is executed
by a class of T lymphocytes called
cytotoxic
T
cells
(also
called
killer T
cells). The basic func-
tion of the T cell
in
recognizing a target antigen
is indicated in r;r..,iri'rir
l:;r.li.
A
cell-mediated
response typically is elicited by an intracellular
parasite,
such as a virus that infects the body's
own cells.
As
a
result
of the viral infection, frag-
ments of foreign
(viral)
antigens are displayed
on the surface
of the
cell.
These
fragments are
recognized by the T cell receptor
(TCR),
which
is the T cells' equivalent of the antibody
pro-
ducedbyaBcell.
A
crucial
feature
of this
recognition reac-
tion
is
that
the antigen must
bepresented by a cel-
lular
protein
that is a member of the IVIHC
(maj
or
histocompatibility complex). The MHC
pro-
tein has a
groove
on
its
surface that binds a
pep-
tide
fragment derived from the foreign antigen.
The combination of
peptide
fragment
and
MHC
protein
is recognized by the T cell receptor. Every
individual has a characteristic
set of
MHC
pro-
teins. They are
important
in
graft
reactions; a
graft
of tissue
from
one
individual
to another
is
rejected because of the difference in MHC
pro-
teins between
the donor and recipient, an issue
of
major medical importance. The
demand
that
the T lymphocytes
recognize
both foreign anti-
gen
and
MHC
protein
ensures
that
the cell-
mediated response acts only on host cells that
have
been
infected with a foreign antigen.
(We
discuss the division of
MHC
proteins
into the
general
types of class
I
and class
II later in Sec-
tion
2).20, The Maj or Histocompatibility Locus
Codes for
Many
Genes of the
Immune
System.)
The
purpose
of each type of immune
response is to attack a foreign target.
Target
recognition is the
prerogative
of B-cell
im-
munoglobulins
and T
cell
receptors. A crucial
aspect of their
function lies in
the ability
to dis-
tinguish
"self"
from
"nonself."
Proteins and cells
of the body
itself must never be attacked. For-
eign targets
must
be destroyed entirely.TLre
prop-
erty
of failing to attack
"self"
is
called tolerance.
Loss
of
this ability results in an autoimmune
disease,
in
which
the immune system attacks
its
own
body, often with disastrous conse-
quences.
What
prevents
the
lymphocyte
pool
from
responding to
"self"
proteins?
Tolerance
prob-
ably arises early
in
lymphocyte
cell develop-
ment when
B cells and
T cells that
recognize
"self"
antigens
are
destroyed.
This
is called
clonal deletion.
In addition
to this
negative
selection, there
is also
positive
selection
for
T
cells carrying
certain
sets
of T cell
receptors.
A
corollary
of tolerance
is that
it
can
be dif-
ficult to obtain
antibodies
against
proteins that
are closely related
to those
of the organism
itself.
As
a
practical
matter,
therefore,
it
may be diffi-
cult to use
(for
example)
mice
or rabbits
to
obtain antibodies
against
human
proteins
that
have been
highly conserved
in mammalian
evo-
lution. The tolerance
of
the mouse
or
rabbit for
its
own
protein may extend
to the
human
pro-
tein in such cases.
Each of
the three
groups
of
proteins
required for
the
immune
response-im-
munoglobulins,
T cell
receptors,
and
MHC
proteins-is
diverse.
Examining
a Iarge
num-
ber of
individuals,
we
find many
variants
of
each
protein.
Each
protein is coded
by a large
family
of
genes; in the case
of antibodies
and
the T cell
receptors,
the
diversity
of the
i:il.i"l[{f
i]jl.i.r
In cet[-mediated
immunity,
k'itter
T
cel.l.s
use the
T ce[[ receptor
to
recognize
a
fragment of
the
for-
eign antigen
that
is
presented
on
the surface
of the
tar-
get
cetl
by the
MHC
protein.
23.1
Introduction
573