629
CHAPTER SIXTEEN
Lymphatic System and Immunity
with T cells and are abundant in lymph nodes, the spleen,
bone marrow, and the intestinal lining
(f g. 16.16)
.
Each person has millions of varieties of T and B cells.
The members of each variety originate from a single early
cell, so they are all alike, forming a
clone
(klo
¯n) of cells
(genetically identical cells originating from division of a sin-
gle cell). The members of each variety have a particular type
of antigen receptor on their cell membranes that can respond
only to a speciF
c antigen.
Table 16.4
compares the charac-
teristics of T cells and B cells.
PRACTICE
17
What is immunity?
18
What is the dif
erence between an antigen and a hapten?
19
How do T cells and B cells originate?
T Cells and the Cellular Immune Response
A lymphocyte must be activated before it can respond to an
antigen. T cell activation requires processed fragments of
antigen attached to the surface of another type of cell, called
Stem cells
in red bone
marrow give
rise to
lymphocyte
precursors.
Red
bone
marrow
Lymphocyte
precursors
Some lymphocyte
precursors are
processed within
the bone marrow
to become B cells.
B cell
Blood
transport
Blood
transport
Blood
transport
Some lymphocyte
precursors are
processed in the
thymus to become
T cells.
Thymus
T cell
T cell
Lymph
node
Both T cells and B cells
are transported through
the blood to lymphatic
organs, such as the
lymph nodes, lymphatic
ducts, and spleen.
1
2
3
4
B cell
FIGURE 16.15
Falsely colored scanning electron micrograph o± a
circulating lymphocyte (8,600×).
FIGURE 16.16
Bone marrow releases relatively unspecialized lymphocyte precursors, which a±ter
processing specialize as T cells (T lymphocytes) or B cells (B lymphocytes). In the ±etus, the medullary cavity
contains red marrow.
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