CIE A Level Biology (9700) 2019-2021

Revision Notes

11.1.3 White Blood Cells

WBC Count & Infectious Diseases

  • Blood tests are regularly carried out by doctors to help them diagnose diseases and assess the success of treatments
  • The numbers of different types of white blood cells in the blood can provide information about what type of infection someone might have, the severity of the infection, or whether the treatment that has been prescribed is working
  • For example:
    • Neutrophils in the blood increase in number during bacterial infections and when tissues become inflamed and die
    • Lymphocytes in the blood increase in number during viral infections and tuberculosis
    • The human immunodeficiency virus (HIV) invades and destroys helper T cells, so blood tests of HIV+ patients are designed to record the numbers of specific types of T cells present. Monitoring declines in these T cells enables doctors to assess the harm the infection is causing on the immune system of HIV+ patients

WBC Count & Leukaemia

  • Stem cells in the bone marrow differentiate to form all white blood cell types
  • The two groups of stem cells in the bone marrow are:
    • Myeloid stem cells – differentiate to form red blood cells, neutrophils, monocytes and platelets
    • Lymphoid stem cells – differentiate to form lymphocytes (both B-lymphocytes and T-lymphocytes)
  • These stem cells divide in a rapid but controlled way, producing very large numbers of differentiated white blood cells that carry out specific functions within an immune response


  • Leukaemias are cancers of these bone marrow stem cells
  • When someone has leukaemia, these stem cells divide rapidly but uncontrollably, producing large numbers of cells that do not differentiate properly (known as malignant cells) and therefore do not function appropriately
  • The malignant cells also fill the bone marrow and disrupt the production of normal blood cells (including red blood cells and platelets). They then enter the blood and the lymphatic system
  • There are two types of leukaemia, each having a slightly different effect:
    • Myeloid leukaemias – neutrophil-producing stem cells divide rapidly and uncontrollably (number of immature neutrophil cells increases)
    • Lymphoblastic leukaemias – lymphocyte-producing stem cells divide rapidly and uncontrollably (number of immature lymphocyte cells increases)
  • These immature white blood cells disrupt the balance of components in the blood:
    • The number of red blood cells or platelets decreases, resulting in anaemia and an increased risk of excessive bleeding
    • The number of mature neutrophils and lymphocytes decreases, resulting in immunosuppression (increased susceptibility to infections)
  • Myeloid and lymphoblastic leukaemias can take two forms:
    • Acute form – acute leukaemias develop rapidly, have severe effects and must be treated as soon as a diagnosis is made
    • Chronic form – chronic leukaemias develop slowly (some take many years) and changes in blood cell counts are monitored over time so that treatment can be given when it has the highest chance of curing the disease


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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