CIE A Level Biology (9700) 2019-2021

Revision Notes

11.1.2 Primary Immune Response

Primary Immune Response

  • Lymphocytes are another type of white blood cell
  • They play an important part in the specific immune response
  • They are smaller than phagocytes
  • They have a large nucleus that fills most of the cell
  • They are produced in the bone marrow before birth
  • There are two types of lymphocytes (with different modes of action). The two types of lymphocytes are:
    • B-lymphocytes (B cells)
    • T-lymphocytes (T cells)

B-lymphocytes

  • B-lymphocytes (B cells) remain in the bone marrow until they are mature and then spread through the body, concentrating in lymph nodes and the spleen
  • Millions of types of B-lymphocyte cells are produced within us because as they mature the genes coding for antibodies are changed to code for different antibodies
  • Once mature, each type of B-lymphocyte cell can make one type of antibody molecule
  • At this stage, the antibody molecules do not leave the B-lymphocyte cell but remain in the cell surface membrane
  • Part of each antibody molecule forms a glycoprotein receptor that can combine specifically with one type of antigen
  • If that antigen enters the body, B-lymphocyte cells with the correct cell surface receptors will be able to recognise it and then divide by mitosis (clonal selection)
  • During a primary immune response, B-lymphocytes divide repeatedly by mitosis (clonal expansion) and differentiate into two main types of cell:
    • Plasma cells
    • Memory cells
  • These two cell types each have a specific function

T-lymphocytes

  • Immature T-lymphocytes leave the bone marrow to mature in the thymus
  • Mature T-lymphocytes have specific cell surface receptors called T cell receptors
  • These receptors have a similar structure to antibodies and are each specific to one antigen
  • T-lymphocytes are activated when they encounter (and bind to) their specific antigen that is being presented by one of the host’s cells (host cells being the human’s own cells)
  • This antigen-presenting host cell might be a macrophage or a body cell that has been invaded by a pathogen and is displaying the antigen on its cell surface membrane
  • These activated T-lymphocytes (those that have receptors specific to the antigen) divide by mitosis to increase in number (similar to the clonal selection and clonal expansion of B-lymphocytes) and differentiate into two main types of T cell:
    • Helper T cells
    • Killer T cells
  • These two T cell types each have a specific function

Author:

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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