Chapter 21
The Immune System: Innate and Adaptive Body Defenses
787
21
Activated T cells (like B cells) can become either effector cells or
memory cells. Next, we will focus on the three major groups of
effector T cells: helper, cytotoxic, and regulatory T cells. (±ere
are other minor “oddball” populations, but we will not consider
them here).
Helper T Cells
Helper T (T
H
) cells
play a central role in adaptive immunity,
mobilizing both its humoral and cellular arms
(Figure 21.18)
.
Once activated by APC presentation of antigen, T
H
cells help
activate B and T cells, and induce B and T cells to proliferate. In
fact, without the help of “director” T
H
cells, there is
no
adaptive
immune response. ±eir cytokines furnish the chemical help
needed to recruit other immune cells.
Activation of B Cells
Helper T cells interact directly with B
cells displaying antigen fragments bound to class II MHC re-
ceptors (Figure 21.18a). Whenever a T
H
cell binds to a B cell,
the T
H
cell releases cytokines that prod the B cells into dividing
more rapidly. ±en, like the boss of an assembly line, the T
H
Proliferation and Differentiation
Once activated by antigen binding and co-stimulation, a T cell
enlarges and proliferates. Cytokines (which we will discuss next)
released by APCs or T cells themselves promote this process.
Cells of the resulting clone differentiate to perform functions
according to their T cell class. ±is primary response peaks
within a week of exposure to the triggering antigen. A period
of apoptosis then occurs between days 7 and 30, during which
time the activated T cells die off and effector activity wanes as
the amount of antigen declines.
±is wholesale disposal of T cells has a critical protective
role because activated T cells are potential hazards. ±ey pro-
duce huge amounts of inflammatory cytokines, which contrib-
ute to infection-driven hyperplasia, and may promote cancer
in chronically inflamed tissue. Additionally, once they’ve done
their job, the effector T cells are unnecessary and thus dis-
posable. ±ousands of clone members become memory T
cells, persisting perhaps for life, and providing a reservoir of
T cells that can later mediate secondary responses to the same
antigen.
Cytokines
±e chemical messengers involved in cellular immunity belong to
a group of molecules called
cytokines
, a general term for media-
tors that influence cell development, differentiation, and responses
in the immune system. Cytokines include interferons and inter-
leukins. In
Table 21.6
on p. 790, you can see some of the large
variety of these molecules and their myriad effects on target cells.
Cytokines include hormone-like or paracrine-like glycopro-
teins released by a variety of cells. As we mentioned previously,
some cytokines promote T cell proliferation. For example,
inter-
leukin 1 (IL-1)
, released by macrophages, stimulates T cells to
liberate
interleukin 2 (IL-2)
and to synthesize more IL-2 recep-
tors. IL-2 is a key growth factor. Acting on the cells that release it
(as well as other T cells), it sets up a positive feedback cycle that
encourages activated T cells to divide even more rapidly.
Additionally, all activated T cells secrete one or more other
cytokines that help amplify and regulate a variety of adaptive
and innate immune responses. Some (such as
tumor necrosis
factor
) are cell toxins. Others (e.g.,
gamma interferon
) enhance
the killing power of macrophages; and still others are inflamma-
tory chemicals.
Check Your Understanding
17.
Class II MHC proteins display what kind of antigens? What
class of T cell recognizes antigens bound to class II MHC?
What types of cells display these proteins?
18.
What happens when antigens are bound in the absence of
co-stimulators?
For answers, see Appendix H.
Roles of Specific Effector T Cells
Describe T cell functions in the body.
1
Antigen
presentation
Dendritic cell engulfs
an exogenous
antigen, processes it,
and displays its
fragments on class II
MHC protein.
2
CD4 T cell
recognizes antigen-
MHC complex. Both
TCR and CD4 proteins
bind to antigen-MHC
complex.
Co-stimulatory
molecules bind
together.
3
2b
2a
Clone formation
Activated CD4 T cells
proliferate (clone),
and become memory
and effector cells.
Bacterial antigen
Dendritic
cell
Class lI MHC
protein
displaying
processed
bacterial antigen
Co-stimulatory
molecule
CD4 protein
CD4 T cell
Helper
T cells
Memory
CD4 T cell
Co-stimulatory
molecules
T cell
receptor
(TCR)
Clone
formation
Adaptive defenses
Cellular immunity
Double recognition
Figure 21.17
Clonal selection of T cells involves simultane-
ous recognition of self and nonself.
Activation of CD4 cells is
shown here, but activation of CD8 cells is similar.
previous page 821 Human Anatomy and Physiology (9th ed ) 2012 read online next page 823 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off