778
UNIT 4
Maintenance of the Body
21
Check Your Understanding
9.
What event (or observation) signals that a B or T cell has
achieved immunocompetence?
10.
Which of the following T cells would survive education in
the thymus? (a) one that recognizes neither MHC nor self-
antigen, (b) one that recognizes both MHC and self-antigen,
(c) one that recognizes MHC but not self-antigen, (d) one
that recognizes self-antigen but not MHC.
11.
Name three different APCs. Which is most important for T
lymphocyte activation?
12.
In clonal selection, “who” does the selecting? What is being
selected?
For answers, see Appendix H.
In summary, the two-fisted adaptive immune system uses
lymphocytes, APCs, and specific molecules to identify and de-
stroy all substances—both living and nonliving—that are in
the body but not recognized as self. Te system’s response to
such threats depends on the ability of its cells to (1) recognize
antigens by binding to them and (2) communicate with one
another so that the whole system mounts a response specific
to those antigens. As you will see, interactions between various
lymphocytes, and between lymphocytes and APCs, underlie
virtually all phases of the adaptive immune response.
Table 21.3
outlines major similarities and differences be-
tween B and ± lymphocytes. Tis will help orient you as you
delve into the details of humoral and cellular immunity.
Humoral Immune Response
Define humoral immunity.
Describe the process of clonal selection of a B cell.
Recount the roles of plasma cells and memory cells in
humoral immunity.
Primary response
(initial encounter
with antigen)
Antigen
Antigen binding
to a receptor on a
specific B lymphocyte
(B lymphocytes with
noncomplementary
receptors remain
inactive)
Proliferation to
form a clone
Activated B cells
Plasma cells
(effector B cells)
Secreted
antibody
molecules
Memory B cell—
primed to respond
to same antigen
Clone of cells
identical to
ancestral cells
Subsequent challenge
by same antigen results
in more rapid response
Secondary response
(can be years later)
Memory
B cells
Plasma
cells
Secreted
antibody
molecules
Adaptive defenses
Humoral immunity
Figure 21.11
Clonal selection of a B cell.
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