Cambridge International A Level Biology Revision Guide

Chapter 11: Immunity
Phagocytes
Phagocytes are produced throughout life in the
bone marrow. They are stored there before being
distributed around the body in the blood. They are
scavengers, removing any dead cells as well as invasive
microorganisms.
Neutrophils are a kind of phagocyte and form about
60% of the white cells in the blood (Figure 11.3). They travel
throughout the body, often leaving the blood by squeezing
through the walls of capillaries to ‘patrol’ the tissues.
During an infection, neutrophils are released in large
numbers from their stores, but they are short-lived cells.
Macrophages are also phagocytes but are larger than
neutrophils and tend to be found in organs such as the
lungs, liver, spleen, kidney and lymph nodes, rather than
remaining in the blood. After they are made in the bone
marrow, macrophages travel in the blood as monocytes,
which develop into macrophages once they leave the blood
and settle in the organs, removing any foreign matter
found there.
Macrophages are long-lived cells and play a crucial role
in initiating immune responses, since they do not destroy
pathogens completely, but cut them up to display antigens
that can be recognised by lymphocytes.
Phagocytosis
If pathogens invade the body and cause an infection, some
of the cells under attack respond by releasing chemicals
such as histamine. These, with any chemicals released by
the pathogens themselves, attract passing neutrophils to
the site. (This movement towards a chemical stimulus is
called chemotaxis.) The neutrophils destroy the pathogens
by phagocytosis (Figure 4.19, page 87, and Figure 11.4).
The neutrophils move towards the pathogens, which may
be clustered together and covered in antibodies. The antibodies
further stimulate the neutrophils to attack the pathogens. This
is because neutrophils have receptor proteins on their surfaces
that recognise antibody molecules and attach to them. When
the neutrophil attaches to the pathogen, the neutrophil’s
cell surface membrane engulfs the pathogen, and traps it
within a phagocytic vacuole in a process called endocytosis.
Digestive enzymes are secreted into the phagocytic vacuole, so
destroying the pathogen.
Neutrophils have a short life: after killing and digesting
some pathogens, they die. Dead neutrophils often collect at
a site of infection to form pus.
QUESTION
11.2 a State the site of origin of phagocytes.
b Looking at Figure 11.2:
i describe the differences between the
neutrophil and the lymphocyte
ii calculate the actual size of the neutrophil.
1 Attraction (chemotaxis)
2 Recognition
and attachment
3 Endocytosis
neutrophil
cell surface
membrane
of neutrophil
nucleus
bacteria
bacterium attached directly
to neutrophil membrane
receptor for
antibody ‘marker’
bacterium ‘marked’
by an antibody
225
4 Bacteria within
a phagocytic vacuole
lysosome
phagocytic vacuole
5 Fusion of lysosomes
and phagocytic
vacuole
Figure 11.3 A transmission electron micrograph of two
neutrophils that have ingested several Staphylococcus
bacteria (× 4000). Notice at the extreme right, one bacterium
being engulfed. Compare this photograph with Figure 11.4.
6 Killing and digestion
Figure 11.4 The stages of phagocytosis.
bacteria digested
by enzymes such
as proteases