p r o c e ss
diagram
A n tib o d y -m ed iated im m unity • Figure 12.11
APC's active helper T cells activate B cells. B cells then proliferate and differentiate
into memory B cells and plasma cells [7 . Plasma cells secrete antibodies, which bind
to antigens and render them inactive in several ways
[2
.
B cell activation
B cell displaying processed
antigen is recognized by
helper T cell, which
releases costimulators.
B-cell antigen
receptor
B cell
recognizing
unprocessed
antigen
Clonal selection
(proliferation and
differentiation)
Costimulation
by interleukin-2 and
other proteins
Antibody structure consists of four polypep-
tide chains. The arms are flexible and carry a
variable region.
Antigen on surface
of microbe
Antibodies
Plasma cells
(secrete antibodies)
Antibody binding to antigens can do the
following:
• Immobilize bacteria (shown at right)
• Neutralize antigen (toxins) or prevent
attachment to cells (viruses)
• Clump antigens together (agglutination)
for subsequent phagocytosis
• Activate complement (see Figure 12.7)
• Attract phagocytes and enhance phagocytosis
Y-shaped antibody
In antibody-mediated immunity, the B cells process the
antigen and present it to helper T cells that then stimulate
activated B cells to undergo clonal activation to produce
memory B cells and plasma cells. The plasma cells subse-
quently produce antibodies (Figure 12.11). Because the
antibody “arms” can move somewhat, an antibody can as-
sume either a T shape or a Y shape. This flexibility allows an
antibody to bind to two identical antigens at the same time.
358 CHAPTER 12
The Lymphatic System and Immunity
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