The Immune System: Innate and Adaptive Body Defenses
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
Helper T Cells
Helper T (T
play a central role in adaptive immunity,
mobilizing both its humoral and cellular arms
Once activated by APC presentation of antigen, T
activate B and T cells, and induce B and T cells to proliferate. In
fact, without the help of “director” T
cells, there is
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
cell binds to a B cell,
cell releases cytokines that prod the B cells into dividing
more rapidly. ±en, like the boss of an assembly line, the T
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
±e chemical messengers involved in cellular immunity belong to
a group of molecules called
, a general term for media-
tors that inﬂuence cell development, diﬀerentiation, and responses
in the immune system. Cytokines include interferons and inter-
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,
leukin 1 (IL-1)
, released by macrophages, stimulates T cells to
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
) are cell toxins. Others (e.g.,
the killing power of macrophages; and still others are inﬂamma-
Check Your Understanding
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?
What happens when antigens are bound in the absence of
For answers, see Appendix H.
Roles of Speciﬁc Effector T Cells
Describe T cell functions in the body.
Dendritic cell engulfs
antigen, processes it,
and displays its
fragments on class II
CD4 T cell
MHC complex. Both
TCR and CD4 proteins
bind to antigen-MHC
Activated CD4 T cells
and become memory
and effector cells.
Class lI MHC
CD4 T cell
CD4 T cell
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.