Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
T-lymphocytes
Mature T cells have specific cell surface receptors called
T cell receptors (Figure 11.12). T cell receptors have a
structure similar to that of antibodies, and they are
each specific to one antigen. T cells are activated when
they encounter this antigen on another cell of the host
(that is, on the person’s own cells). Sometimes this cell
is a macrophage that has engulfed a pathogen and cut
it up to expose the pathogen’s surface molecules, or it
may be a body cell that has been invaded by a pathogen
and is similarly displaying the antigen on its cell surface
membrane as a kind of ‘help’ signal. The display of
antigens on the surface of cells in this way is known as
antigen presentation. Those T cells that have receptors
complementary to the antigen respond by dividing by
mitosis to increase the number of cells. T cells go through
the same stages of clonal selection and clonal expansion as
clones of B cells.
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There are two main types of T cell:
helper T cells
killer T cells (or cytotoxic T cells).
When helper T cells are activated, they release hormonelike
cytokines that stimulate appropriate B cells to
divide, develop into plasma cells and secrete antibodies.
Some T helper cells secrete cytokines that stimulate
macrophages to carry out phagocytosis more vigorously.
Killer T cells search the body for cells that have become
invaded by pathogens and are displaying foreign antigens
from the pathogens on their cell surface membranes.
Killer T cells recognise the antigens, attach themselves to
the surface of infected cells, and secrete toxic substances
such as hydrogen peroxide, killing the body cells and
the pathogens inside (Figure 11.13). Some helper T cells
infected body cell with
antigens displayed
230
In bone marrow,
In immature bone marrow, T cells
In immature divide bone by marrow, mitosis. T cells
immature divide by mitosis. T cells
divide by mitosis.
helper T cell
divides
by mitosis
killer T cell
divides
by mitosis
In the thymus, each
In T cell the matures. thymus, each
In T cell the matures. thymus, each
T cell matures.
production of T cell receptors
production of T cell receptors
production of T cell receptors
T cell receptors in cell surface membrane
T cell receptors in cell surface membrane
T cell receptors in cell surface membrane
Mature T cells
Mature circulate, T cells some as
Mature circulate, helpers, T some cells
as as
circulate, helpers, killers. some some as as
helpers, killers. some as
killers.
mature T cells, each with a
mature different T cells, T cell each receptor with a
mature different T cells, T cell each receptor with a
Figure 11.12 Origin and maturation
different
of
T
T-lymphocytes.
cell receptor
As T
cells mature in the thymus gland they produce T cell receptor
proteins. Each cell has a specific receptor. Some cells become
helper T cells, others become killer T cells.
secretes cytokines that
stimulate B cells to
divide and form
plasma cells and
memory cells
killer T cells
‘punch’ holes in
cell surface membrane
of infected body cell
divides to
form memory
cells
vacuoles in killer
T cell containing
toxins
hole in cell surface
membrane of
infected body cell
divides to
form memory
cells
infected
body cell
with antigens
displayed
Figure 11.13 The functions of T-lymphocytes during an
immune response. Helper T cells and killer T cells with T cell
receptor proteins specific to the antigen respond and divide by
mitosis. Activated helper T cells stimulate B cells to divide and
develop into plasma cells (Figure 11.6, page 227). Killer T cells
attach themselves to infected cells and kill them.