632
UNIT FOUR
When an activated helper T cell encounters a B cell already
combined with an identical foreign antigen, the helper cell
releases certain cytokines. These cytokines stimulate the B cell
to proliferate, thus enlarging its clone of antibody-producing
cells
(f g. 16.18)
. The cytokines also attract macrophages and
leukocytes into infl amed tissues and help keep them there.
Some members of the activated B cell’s clone differenti-
ate further into
memory cells
(f
g. 16.19)
. Like memory T
cells, these memory B cells respond rapidly to subsequent
exposure to a speciF
c antigen.
In 2008, researchers discovered that memory B cells are incredibly
long-lived. B cells taken from living survivors of the 1918 flu pan-
demic produced antibodies in the laboratory that, when injected into
mice, fully protected them against any form of inF
uenza.
PRACTICE
20
How do T cells become activated?
21
What are some functions of cytokines?
22
Name three types of T cells.
23
How do cytotoxic T cells destroy antigen-bearing cells?
B Cells and the Humoral Immune
Response
Sometimes a B cell may become activated when it encoun-
ters an antigen whose molecular shape F
ts the shape of the B
cell’s antigen receptors. In response to the receptor-antigen
combination, the B cell divides repeatedly, expanding its
clone. However, most of the time B cell activation requires
T cell “help.”
Clone of
B cells
Activated B cell
Proliferation
B cells
Antigen
Antigen
receptor
Cytokines
Stimulation by
activated helper T cell
Activation
1
2
3
FIGURE 16.18
B cell clone. 1. When a B cell encounters an antigen that ±
ts its antigen receptor, it becomes activated. 2. Stimulated by helper T
cell cytokines, the B cell proliferates. 3. Proliferation enlarges the B cell clone. All cells in the clone have the same antigen receptor.
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