Page 322 - Anatomy and Physiology of Farm Animals, 8th Edition

Body Defenses and the Immune System / 307

and intrinsic cell membrane proteins foreign material or microbes (Fig. 16‐1).
These antigens are derived from microbes
VetBooks.ir known as major histocompatibility com- or material that the APCs take up by
plex (MHC) proteins.
The genes that code for MHC proteins
phagocytosis or pinocytosis. APCs process
are found on a single chromosome, and materials in phagocytic or pinocytic vesi-
MHC proteins are continuously synthesized cles to produce antigens that are com-
in all cells of an animal’s body except for plexed to class II MHC proteins and
erythrocytes, which do not have nuclei and inserted into the APC cell membrane.
are not capable of protein synthesis. After As with B cells, clones of T cells exist
synthesis, MHC proteins enter the outer that are specific for a given antigen, and the
cell membrane so that a portion is exposed process of antigen recognition permits a
to the exterior. As MHC proteins prepare specific clone to be selected. However, the
for insertion in the cell membrane, they selection and activation of T cells also
form complexes with antigenic materials requires interaction between the MHC
found within the cell. When the complex proteins holding the antigenic material and
enters the membrane, the antigen is exposed other proteins in the cell membranes of the
to the exterior. The exposed antigen is then various T cells known as cluster of differ-
in a position to be recognized by T cells. entiation markers (CD markers). These
Cell membranes hold two major classes marker proteins determine the interac-
of MHC proteins, class I and class II tions that are possible for the different
(Fig. 16‐1). Class I MHC proteins are found types of T cells. Helper T cells have CD4
in all cells except for erythrocytes. Class I markers (Fig. 16‐1), which interact with
MHC proteins continuously present class II MHC proteins, and cytotoxic T
potential antigenic materials on the sur- cells have CD8 markers, which interact
face of all cells. These materials include with class I MHC proteins. Thus, helper T
peptides and other antigenic materials cells can recognize antigenic material pre-
normally produced in cells, as well as anti- sented by APCs, and cytotoxic T cells can
genic materials produced as a result of an detect foreign antigenic material presented
abnormality in cell function. If a foreign by almost any type of cell (Fig. 16‐1).
cell (i.e., not self) is introduced into an After recognizing antigens presented by
animal’s body, the antigenic material pre- APCs, the selected helper T cells are acti-
sented by its class I MHC proteins is unique vated by interactions between other of their
for that foreign cell and differs from endog- membrane proteins and membrane pro-
enous self cells. When viruses infect nor- teins found in the APCs. This interaction
mal cells and alter their synthetic pathways, (costimulation) leads to full helper T cells
they change the antigenic materials pre- activation. The activated helper T cells
sented by the class I proteins of the infected multiply and produce a subset to serve as
cell. By recognizing these new antigens, T memory cells (memory T cells). A major
H
cells recognize foreign or infected cells, function of the other activated helper T
allowing initiation of a cellular immune cells is to secrete cytokines to promote and
response directed at cells bearing the for- amplify all aspects of the innate and spe-
eign antigen. cific immune responses. This includes
Only selected cell types have class II attraction and stimulation of additional
MHC proteins. These include lympho- macrophages and NK cells, promoting the
cytes, free and fixed macrophages, micro- action of cytotoxic T cells, and promoting
glia in the central nervous system, and a the development of selected B cells. A key
population of cells in the spleen and lymph cytokine produced by helper T cells to
nodes known as dendritic cells. Cells that stimulate macrophages, NK cells, cytotoxic
contain class II MHC proteins are termed cells, and B cells is interleukin‐2.
antigen‐presenting cells (APCs), for they Costimulation by cells presenting the
process and present antigens derived from specific antigen to selected cytotoxic T cells