Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

OVERVIEW OF THE ADAPTIVE IMMUNE SYSTEM
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(A) resting T or B cell
1 µm
(C) effector T cell
1 µm
(B) effector B cell (plasma cell)
hematopoietic system, including erythrocytes, leukocytes, and platelets (see Figure
22–32).
Because they are sites where lymphocytes develop from lymphoid progenitor
cells, the thymus and bone marrow are referred to as central (primary) lymphoid
organs (see Figure 24–12). As we discuss later, most B and T cells die in
the central lymphoid organs soon after they develop, without ever functioning.
Others, however, mature and migrate via the blood to the peripheral (secondary)
lymphoid organs—mainly the lymph nodes, spleen, and epithelium-associated
lymphoid tissues in the gastrointestinal tract, respiratory MBoC6 tract, m25.07/24.15 and skin. It is in
these peripheral lymphoid organs that foreign antigens activate B and T cells (see
Figure 24–13).
B and T cells become morphologically distinguishable from each other only
after antigen has activated them: resting B and T cells look very similar, even in
an electron microscope (Figure 24–14A). After activation by an antigen, both proliferate
and mature into effector cells. Effector B cells secrete antibodies; in their
most mature form, called plasma cells, they are filled with an extensive rough
endoplasmic reticulum that is busily making antibodies (Figure 24–14B). In contrast,
effector T cells (Figure 24–14C) contain very little endoplasmic reticulum
and secrete a variety of cytokines rather than antibodies. Whereas B‐cell-derived
antibodies are widely distributed by the bloodstream, T‐cell-derived cytokines
mainly act locally on neighboring cells, although some are carried via the blood
and act on distant host cells.
1 µm
Figure 24–14 Electron micrographs of
resting and effector lymphocytes.
(A) This resting lymphocyte could be
either a B cell or a T cell, as these cells
are difficult to distinguish morphologically
until antigen activates them to become
effector cells. (B) An effector B cell (a
plasma cell). It is filled with an extensive
rough endoplasmic reticulum (ER), which is
distended with antibody molecules that are
secreted in large amounts. (C) An effector
T cell, which has relatively little rough ER
but is filled with free ribosomes; it secretes
cytokines, but in relatively small amounts.
The three cells are shown at the same
magnification. (A, courtesy of Dorothy
Zucker-Franklin; B, courtesy of Carlo
Grossi; A and B, from D. Zucker-Franklin
et al., Atlas of Blood Cells: Function and
Pathology, 2nd ed. Milan, Italy: Edi. Ermes,
1988; C, courtesy of Stefanello de Petris.)
Immunological Memory Depends On Both Clonal Expansion and
Lymphocyte Differentiation
The most remarkable feature of the adaptive immune system is that it can respond
to millions of different foreign antigens in a highly specific way. Human B cells, for
example, collectively, can make more than 10 12 different antibody molecules that
react specifically with the antigen that induced their production. How can B cells
and T cells respond specifically to such an enormous diversity of foreign antigens?
The answer for both B and T cells is the same. As each lymphocyte develops in
a central lymphoid organ, it becomes committed to react with a particular antigen
before ever being exposed to the antigen. It expresses this commitment in
the form of cell-surface receptors that specifically bind the antigen. When a lymphocyte
encounters its antigen in a peripheral lymphoid organ, the binding of the
antigen to the receptors (with help from co-stimulatory signals, discussed later)