Adaptive Immunity Allows You to
Respond to a Variety of Invaders
All innate defenses are
nonspecific;
they try to block every-
thing that could possibly cause harm. However, your body
also has the ability to adapt to specific types of infections,
such as specific strains of viruses and bacteria, or specific
antigens, including pollen, animal dander, and antibiot-
ics. This ability is called adaptive immunity. Adaptive
immunity can respond quickly—it can “remember” the
particular antigens that have invaded your body—and
can recognize the difference between foreign cells or sub-
stances and your own cells or substances.
There are two types of adaptive immunity: cell-mediated
immunity
and
antibody-mediated
immunity
(Figure
12.8). Cell-mediated immunity is effective against in-
tracellular pathogens
(in tra -
= within), such as viruses,
bacteria, and fungi located inside cells; it can also be used
against some cancer cells and foreign tissue transplants.
Cell-mediated immunity involves T lymphocytes, also
called T cells, which are made in the red bone marrow
and mature in the thymus gland. (The
T
stands for
th y -
m u s.)
Thus, cell-mediated immunity always involves cells
attacking cells. Antibody-mediated immunity is effec-
tive against extracellular pathogens
(extra-
= outside),
such as viruses, bacteria, and fungi in blood and body
fluids. Antibody-mediated immunity involves B lympho-
cytes, also called B cells, which are made and mature in
red bone marrow. (The
B
stands for
bone m a rro w )
When an antigen invades the body, many copies of
that antigen spread throughout the body’s tissues and flu-
ids. Some copies of the antigen may be present inside body
cells, while other copies may be present in extracellular
fluid. Therefore, cell-mediated immunity and antibody-
mediated immunity often work together to rid the body of
the large number of copies of a particular antigen.
Regardless of the type, adaptive immunity basically
involves four phases:
Phase 1: Production and maturation of T and B cells in
the primary lymphatic organs (the thymus and
red bone marrow), with subsequent migration
to secondary lymphatic organs. These T and B
cells are inactive.
Phase 2: Activation of helper T cells by antigen-presenting
cells. This leads to clonal selection, a vast
proliferation and differentiation of cells into active
cells and memory cells.
Phase 3: Activation of B cells and cytotoxic T cells with
the aid of active helper T cells. B cells and
cytotoxic T cells also undergo clonal selection.
O verview o f a d a p tiv e im m une resp o n se s
• Figure 1 2 .8 __________________________________
The phases of adaptive immune responses for cell-mediated im-
munity and antibody-mediated immunity are shown. Phase 1
oc-
curs throughout life, while phases 2-4 occur only in response to
presentations of various pathogens (such as infections, illnesses,
and allergies).
Phase
1
T cells and B cells develop and mature in primary lymphatic
organs. Antigen receptors are placed in their cell membranes.
T cells and B cells migrate to secondary lymphatic organs,
where they are inactive.
Phase 4: Actions of active cytotoxic T cells and active B
cells. Cytotoxic T cells are T cells that kill
infected body cells. Active B cells differentiate
into
plasma
cells, which
secrete
antibodies.
Antibodies act on antigens in various ways, as you
will learn shortly.
354 CHAPTER 12
The Lymphatic System and Immunity
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