HUMAN BIOLOGY

162 Chapter 9

overview of adaptive Defenses

will see, however, activation requires that the B or T cell meet

and recognize the one antigen for which it has receptors.

When a B or T cell does interact with an antigen, the

cell divides, the resulting new cells divide again, and so on

until a huge number of identical copies exist. Because each

copy is identical to the “parent” cell, it is called a clone and

the copying process is called clonal expansion. All the cells

in the resulting lymphocyte army have exactly the same

receptors, so all recognize the same antigen, but not all will

continue down the same developmental path. Many dif-

ferentiate into effector cells that can begin destroying the

enemy right away. Others become memory cells. Instead

of joining the first battle, memory cells are set aside. If the

threat returns, they will be available to mount a larger,

faster response to it (Figure 9.8). Memory cells are what

make you “immune” to a given cold or flu virus once you

have recovered from the first infection.

Effector B cells are called plasma cells. Plasma cells

make proteins called antibodies, so the B cell response is

called antibody-mediated immune response. Antibod-

ies target antigens of pathogens and toxins that are outside

cells, in blood or tissue fluid.

n in adaptive immunity, b cells, T cells, and phagocytes are

mobilized to fight specific threats.

n Links to White blood cells 8.1, self markers 8.4

adaptive immunity has four key features

In Section 9.1 you read that the responses of adaptive

immunity are not preset. They change throughout life as

new threats enter the body. Now we are ready to consider

four features that are central to adaptive immunity.

1. Recognition of self versus nonself: Like the ABO self
   markers on red blood cells, all body cells have self
   markers called MHC markers. (They are named after
   the genes that code for them.) MHC markers are some
   of the proteins that stick out above a cell’s plasma
   membrane. T cells have receptors that recognize MHC
   markers and other self tags on body cells. These and
   other receptors also can recognize antigens as nonself.


2. Specificity: Each B or T cell makes receptors for only one
   kind of antigen. A receptor and its antigen fit together,
   something like a lock and key. B cells make B cell
   receptors, and T cells make T cell receptors.
   3. Diversity: B and T cells collec-
   tively may have receptors for
   more than 2 billion different
   antigens.
   4. Memory: Some of the B and
   T cells formed during a first
   response to an invader are held in
   reserve for future battles with it.
   Let’s now learn a little more about
   these features, which are the defining
   characteristics of adaptive defenses.

Lymphocytes become

specialized for different

roles in adaptive immunity

As you know, lymphocytes arise from

stem cells in red bone marrow. Form-

ing B cells continue developing in

bone marrow, but cells that will spe-

cialize as T cells travel via the blood

to the thymus gland, where they com-

plete their development. When B and

T cells are mature, most move into

lymph nodes, the spleen, and other

lymphatic system tissues. Each cell is

studded with its unique receptors and

is capable of becoming active in an

adaptive immune response. As you

9.5

antibodies Proteins made
by activated B cells that are
the cells’ antigen receptors.

antibody-mediated
immune response An
adaptive immune response
in which antibodies are pro-
duced against an antigen.

B cell receptor Antigen
receptor on a B cell.

effector cell B or T cell
that has been sensitized
to an antigen as an adap-
tive immune response gets
under way. Effectors begin
to act as soon as they are
sensitized.

memory cell B or T cell
that has been sensitized to
an antigen but remains in
reserve and acts in a sec-
ondary response.

MHC marker Self-
recognition protein on a
body cell.

plasma cell Cell derived
from an activated B cell;
it produces antibodies that
operate in antibody-
mediated immunity.

T cell receptor Antigen
receptor on a T cell. Figure 9.8^ Animated!^ Activated lymphocytes produce
effector cells and memory cells. (© Cengage Learning)

effector cells

effector cells

memory cells memory cells

B or

T cell

primary

immune response

secondary

immune response

Magnitudeof responsefirst

exposure

second

exposure

Weeks

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