Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
224
Internal defence system
If pathogens do successfully enter the body, white blood
cells (Chapter 8) can recognise them as something foreign
and destroy them.
White blood cells are part of the immune system
and they recognise pathogens by the distinctive, large
molecules that cover their surfaces, such as proteins,
glycoproteins, lipids and polysaccharides, and the waste
materials which some pathogens produce. Any molecule
which the body recognises as foreign is an antigen.
There are two types of white blood cell, namely
phagocytes and lymphocytes. Before looking at their
structure and function in detail, it will be useful to look
at an example of the immune response in humans. This
example introduces further important features of the
immune system, namely the ability to distinguish between
self and non-self, and the production of antibodies.
Antibodies are glycoprotein molecules that act against
specific antigens (page 228).
Each of us has molecules on the surfaces of our cells
that are not found in other organisms, or even in other
humans. These are often called cell surface antigens.
Although they do not stimulate production of antibodies
in us, they will do if they enter someone else. The cell
surface antigens of the human ABO blood group system
are a good example. If you are blood group A, then you
have certain glycolipids on your red blood cells that are not
on the red cells of people who are blood group B. If blood
of type A is given to someone who has blood of type B
during a transfusion, then the recipient will recognise these
blood cells as foreign and start to produce antibodies. The
recipient’s immune system has recognised the antigens on
blood cells of type A as non-self. If blood of type B is used
during the transfusion, as it should be, then the recipient’s
immune system recognises the antigens on the red blood
cells as self and no antibodies are produced.
The response of lymphocytes to the presence of a
foreign antigen is known as the immune response. In
some cases lymphocytes respond by producing antibodies;
in others they respond by killing cells that have become
infected by pathogens.
QUESTION
11.1 a Explain the terms antigen, antibody and
immune response in your own words.
b Explain why blood of type B is not given to
someone with blood type A during a blood
transfusion.
Cells of the immune system
The cells of the immune system originate from the bone
marrow. There are two groups of these cells involved
in defence:
■■
■■
phagocytes (neutrophils and macrophages)
lymphocytes.
All of these cells are visible among red blood cells when
a blood smear is stained to show nuclei as shown in
Figure 11.2.
Figure 11.2 A monocyte (left), which will develop into a
macrophage, a neutrophil (centre) and a lymphocyte (right),
together with red blood cells in a blood smear which has been
photographed through a light microscope. The cytoplasm of
the neutrophil contains vacuoles full of hydrolytic enzymes
(× 1000).
An antigen is a substance that is foreign to the body and
stimulates an immune response.
An antibody is a glycoprotein (immunoglobulin) made
by plasma cells derived from B-lymphocytes, secreted
in response to an antigen; the variable region of the
antibody molecule is complementary in shape to its
specific antigen.
The immune response is the complex series of
responses of the body to the entry of a foreign antigen; it
involves the activity of lymphocytes and phagocytes.
Non-self refers to any substance or cell that is
recognised by the immune system as being foreign and
will stimulate an immune response.
Self refers to substances produced by the body that the
immune system does not recognise as foreign, so they
do not stimulate an immune response.