ADAPTIVE IMMUNITY
To adapt means to become suitable, and adaptive
immunity can become “suitable” for and respond to
almost any foreign antigen. Adaptive immunity is spe-
cific and is carried out by lymphocytes and macro-
phages.
The majority of lymphocytes are the T lympho-
cytes and B lymphocytes, or, more simply, T cellsand
B cells. In the embryo, T cells are produced in the
bone marrow and thymus. They must pass through
the thymus, where the thymic hormones bring about
their maturation. The T cells then migrate to the
spleen, lymph nodes, and lymph nodules, where they
are found after birth.
Produced in the embryonic bone marrow, B cells
migrate directly to the spleen and lymph nodes and
nodules. When activated during an immune response,
some B cells will divide many times and become
plasma cells that produce antibodies to a specific for-
eign antigen.
The mechanisms of immunity that involve T cells
and B cells are specific, meaning that one foreign anti-
gen is the target each time a mechanism is activated. A
macrophage has receptor sites for foreign chemicals
such as those of bacterial cell walls or flagella, and may
phagocytize just about any foreign material it comes
across (as will the Langerhans or dendritic cells). T
cells and B cells, however, become very specific, as you
will see.
The first step in the destruction of a pathogen or
foreign cell is the recognition of its antigens as for-
eign. Both T cells and B cells are capable of this, but
the immune mechanisms are activated especially well
when this recognition is accomplished by macro-
phages and a specialized group of T lymphocytes
called helper T cells(also called CD4 T cells). The
foreign antigen is first phagocytized by a macrophage,
and parts of it are “presented” on the macrophage’s
cell membrane. Also on the macrophage membrane
are “self ” antigens that are representative of the
antigens found on all of the cells of the individual.
Therefore, the helper T cell that encounters this
macrophage is presented not only with the foreign
antigen but also with “self ” antigens for comparison.
The helper T cell becomes sensitized to and specific
for the foreign antigen, the one that does not belong
in the body (see Box 14–2: AIDS).
The recognition of an antigen as foreign initiates
one or both of the mechanisms of adaptive immunity.
These are cell-mediated immunity (sometimes
called simply cellular immunity), in which T cells and
macrophages participate, and antibody-mediated
immunity(or humoral immunity), which involves T
cells, B cells, and macrophages.Cell-Mediated Immunity
This mechanism of immunity does not result in the
production of antibodies, but it is effective against
intracellular pathogens (such as viruses), fungi,
malignant cells, and grafts of foreign tissue. As men-
tioned earlier, the first step is the recognition of the
foreign antigen by macrophages and helper T cells,
which become activated and are specific. (You may
find it helpful to refer to Fig. 14–7 as you read the fol-
lowing.)
These activated T cells, which are antigen specific,
divide many times to form memory T cellsand cyto-
toxic(killer) T cells(also called CD8 T cells). The
memory T cells will remember the specific foreign
antigen and become active if it enters the body again.
Cytotoxic T cells are able to chemically destroy for-
eign antigens by disrupting cell membranes. This
is how cytotoxic T cells destroy cells infected with
viruses and prevent the viruses from reproducing.
These T cells also produce cytokines, which are chem-
icals that attract macrophages to the area and activate
them to phagocytize the foreign antigen and cellular
debris.
It was once believed that another subset of T cells
served to stop the immune response, but this may not
be so. It seems probable that the CD4 and CD8 T
cells also produce feedback chemicals to limit the
immune response once the foreign antigen has been
destroyed. The memory T cells, however, will quickly
initiate the cell-mediated immune response should
there be a future exposure to the antigen.Antibody-Mediated Immunity
This mechanism of immunity does involve the pro-
duction of antibodies and is also diagrammed in Fig.
14–7. Again, the first step is the recognition of the for-
eign antigen, this time by B cells as well as by
macrophages and helper T cells. The sensitized helper
T cell presents the foreign antigen to B cells, which
provides a strong stimulus for the activation of B cells
specific for this antigen. The activated B cells begin to
divide many times, and two types of cells are formed.
Some of the new B cells produced are memory B
cells, which will remember the specific antigen and330 The Lymphatic System and Immunity