Chapter 21
The Immune System: Innate and Adaptive Body Defenses
785
21
CD4 cells (that usually become helper T cells) are restricted
to binding antigens only on class II MHC proteins, which
are typically displayed on antigen-presenting cell (APC)
surfaces.
CD8 cells (that become cytotoxic T cells) are activated by an-
tigen fragments on class I MHC proteins, also found on the
surface of APCs. Once activated, cytotoxic T cells look for
this same antigen presented on class I MHC proteins located
on any cell in the body.
±e MHC restriction for CD8 cells presents a problem for
APCs. As a rule, class I MHC proteins display endogenous
Class II MHC proteins loaded with foreign antigens gener-
ally signal CD4 cells that help is required. It’s as if they are show-
ing the CD4 cells what the foreign invader “looks like.” Most
oFen, the result is that the CD4 cell will help mount a defense
against the antigen.
MHC Restriction
CD4 and CD8 cells have different requirements for the class of
MHC protein that presents antigens to them. ±is constraint,
acquired during the education process in the thymus, is called
MHC restriction
.
Immature
lymphocyte
Maturation
CD4
cell
T cell
receptor
T cell
receptor
CD4
CD4 cells become
either
helper T cells
or
regulatory T cells
CD8 cells become
cytotoxic T
cells
APC
(dendritic cell)
APC
(dendritic cell)
Activation
Activation
Memory
cells
CD8
cell
CD8
Red bone marrow
Lymphoid
tissues and
organs
Blood plasma
Thymus
Class I MHC
protein displaying
antigen
Class II MHC
protein displaying
antigen
Adaptive defenses
Cellular immunity
Effector
cells
Figure 21.16
Major types of T cells.
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