Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
226
Lymphocytes
Lymphocytes are a second type of white blood cell. They
play an important role in the immune response.
Lymphocytes are smaller than phagocytes. They have a
large nucleus that fills most of the cell (Figure 11.2). There
are two types of lymphocyte, both of which are produced
before birth in bone marrow.
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B-lymphocytes (B cells) remain in the bone marrow
until they are mature and then spread throughout
the body, concentrating in lymph nodes and the spleen.
T-lymphocytes (T cells) leave the bone marrow and
collect in the thymus where they mature. The thymus is
a gland that lies in the chest just beneath the sternum.
It doubles in size between birth and puberty, but after
puberty it shrinks.
Only mature lymphocytes can carry out immune
responses. During the maturation process, many different
types of B- and T-lymphocyte develop, perhaps many
millions. Each type is specialised to respond to one
antigen, giving the immune system as a whole the ability
to respond to almost any type of pathogen that enters
the body. When mature, all these B and T cells circulate
between the blood and the lymph (Chapter 8). This ensures
that they are distributed throughout the body so that they
come into contact with any pathogens and with each other.
Immune responses depend on B and T cells interacting
with each other to give an effective defence. We will look
in detail at the roles of B and T cells and how they interact
in the following section. Briefly, however, some T cells
coordinate the immune response, stimulating B cells to
divide and then secrete antibodies into the blood; these
antibodies recognise the antigens on the pathogens and
help to destroy the pathogens. Other T cells seek out and
kill any of the body’s own cells that are infected with
pathogens. To do this they must make direct contact with
infected cells.
B-lymphocytes
As each B cell matures, it gains the ability to make
just one type of antibody molecule. Many different
types of B cell develop in each of us, perhaps as many as
10 million. While B cells are maturing, the genes that code
for antibodies are changed in a variety of ways to code
for different antibodies. Each cell then divides to give a
small number of cells that are able to make the same type
of antibody. Each small group of identical cells is called a
clone. At this stage, the antibody molecules do not leave
the B cell but remain in the cell surface membrane. Here,
part of each antibody forms a glycoprotein receptor,
which can combine specifically with one type of antigen.
If that antigen enters the body, there will be some mature
B cells with cell surface receptors that will recognise it
(Figure 11.5).
Figure 11.6 shows what happens to B cells during the
immune response when an antigen enters the body on two
separate occasions. When the antigen enters the body for
the first time the small numbers of B cells with receptors
complementary to the antigen are stimulated to divide by
mitosis. This stage is known as clonal selection. The small
clone of cells divides repeatedly by mitosis in the clonal
expansion stage so that huge numbers of identical B cells
are produced over a few weeks.
In bone marrow,
immature
B cells divide
by mitosis.
Still in bone
marrow, each
B cell matures.
Mature B cells
circulate and
concentrate in
liver and spleen.
immature
B cells
production of
antibody receptors
antibody receptors in
cell surface membrane
mature B cells each with a different antibody receptor
Figure 11.5 Origin and maturation of B-lymphocytes. As they
mature in bone marrow, the cells become capable of secreting
one type of antibody molecule with a specific shape. Some of
these molecules become receptor proteins in the cell surface
membrane and act like markers. By the time of a child’s birth,
there are millions of different B cells, each with a specific
antibody receptor.
QUESTIONS
11.3 State the sites of origin and maturation of
B-lymphocytes (B cells) and T-lymphocytes (T cells).
11.4 Suggest why the thymus gland becomes smaller
after puberty.