Cambridge International A Level Biology Revision Guide

Chapter 11: Immunity
secrete cytokines that stimulate macrophages to carry out
phagocytosis more vigorously, or that stimulate killer
T cells to divide by mitosis and to differentiate by
producing vacuoles full of toxins.
Memory helper T cells and memory killer T cells
are produced, which remain in the body and become
active very quickly during the secondary response to
antigens.
QUESTION
11.11 Outline the functions of B-lymphocytes (B cells)
and T-lymphocytes (T cells) and describe how they
interact during an immune response.
Numbers of white blood cells
Blood tests are routinely carried out to help doctors
diagnose diseases and to assess the success of treatments.
Blood samples are taken from patients and sent for
analysis in laboratories that use automated cell counters.
The results usually include the numbers of red and
white blood cells and platelets. Platelets are small cell
fragments that do not have a nucleus; they are formed
from the break-up of cells in the bone marrow (Figure
8.13, page 163). They release substances that stimulate
blood clotting. The results of such blood tests are given as
absolute values as in the second column of Table 11.1. The
results for specific white blood cells, such as neutrophils
and lymphocytes, are given as absolute numbers or as
percentages of the white cell count. There is considerable
variation in these numbers between people. The third
column of Table 11.1 gives examples of these normal ranges.
The number of neutrophils in the blood increases
during bacterial infections and whenever tissues become
inflamed and die. The number of lymphocytes in the
blood increases in viral infections and in TB. Most of the
lymphocytes that circulate in the blood are T cells. In
some blood tests, the numbers of T cells are recorded.
The human immunodeficiency virus (HIV) invades
helper T cells and causes their destruction, so blood
tests for people who are HIV+ record the numbers of
specific T cells. The normal value is between 500 and 1500
cells mm −3 . The specific T cell numbers provide useful
information on the progress of the disease and the success
of treatments, by monitoring decline in T cell number
and therefore assessing the deleterious effect on the
immune system.
QUESTION
11.12 Use the typical values in the second column of
Table 11.1 to answer the following questions.
a Calculate the total numbers of red blood cells
and white blood cells in 1 mm 3
i in males
ii in females.
b Calculate the white blood cell counts as
percentages of the totals you calculated in a.
c Express the results for neutrophils and
lymphocytes as percentages of the white blood
cell counts for the males and females.
d Calculate the numbers of neutrophils and
lymphocytes in 1 dm 3 of blood. Express your
answers using standard form.
231
Cellular component of blood
Numbers mm −3 of blood
Percentage of cellular component
Typical values
Normal range of values within all cellular components
red blood cells
5500 000 (males)
4800 000 (females)
4600 000–6200 000 93–96
platelets 300 000 150 000–400 000 4–7
white blood cells 7500 4500–10 000 0.1–0.2
Percentages of all white blood cells
of which: neutrophils 4500 3000–6000 30–80
B lymphocytes 400 70–600
15–40
T lymphocytes 1500 500–2500
other white blood cells 1100 800–2000 5–14
5807 500 (males)
Totals
5107 500 (females)
Table 11.1 The results of blood tests are given as absolute numbers and compared with the normal ranges. They are often
given as the numbers in 1 mm 3 which is the same as 1 μl (microlitre) or 1 × 10 −9 dm 3 of blood. As numbers of blood cells vary
considerably, results are often expressed as percentages, as in the fourth column.