DP IB Biology: HL

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First teaching 2014

Last exams 2024

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11.1.2 Specific Immune Response

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Specific Immune Response

  • T-Helper cells (a type of lymphocyte that responds to specific antigens) and mature B cells (another type of lymphocyte) have specific receptors located on their cell surface membranes
    • These receptors have a similar structure to antibodies and are each specific to one antigen
    • Note that lymphocytes are a type of white blood cell involved in the specific immune response; there are several different types of lymphocyte
  • When phagocytes engulf pathogens, they present the pathogen antigens on their own cell surface membrane
    • A cell with non-self antigens on its surface membrane is known as an antigen presenting cell
  • The T-helper cell with the complementary receptor proteins to the antigen will bind to the antigen and become activated by the phagocyte
  • Activated T-helper cells then bind with complementary receptors on the surface membrane of specific B-lymphocytes
  • On binding, the T-helper cells releases signalling proteins and activate these B-cells

t-helper-cell-and-b-cell-activation

Antigens activate complementary T-helper cells which go on to activate complementary B-cells

Plasma Cells

  • During an immune response, B-lymphocytes mature to form two types of cell: plasma cells and memory cells
  • Plasma cells produce large volumes of antibodies specific to the single type of antigen that triggered the immune response
  • The cells are specialised with large amounts of rough endoplasmic reticulum which promotes protein synthesis to make the required antibodies
  • As B-cells only produce one type of antibody, only a small proportion of the genes are expressed in the nucleus

Clonal Selection & Expansion

  • Clonal selection involves identifying and activating a B-cell with the complementary receptor to the target antigen
  • Clonal expansion can then occur
    • The activated B-cell divides by mitosis to create many clones of itself
      • Each clone will produce the exact same antibody, complementary to the target antigen
  • Some of these mature B-lymphocytes differentiate into plasma cells 
  • The other B-lymphocytes become memory cells that  remain and circulate in the blood
    • Whilst the antibodies produced by the plasma cells are only present for a matter of weeks or months, memory cells form the basis of immunological memory – the cells can last for many years and often a lifetime

The primary and secondary immune response

    • A primary immune response occurs in response to a newly encountered antigen
      • This is a relatively slow response as the immune system takes time identifying the complementary antibody for each new antigen it encounters
      • The infection may result in symptoms being presented whilst the immune system identifies and manufactures the correct antibodies
    • Secondary immune response in response to a previously encountered antigen
      • The memory cells with the correct antibody, are already circulating in the blood so the response is more rapid, producing more antibodies than the primary response, in a much shorter time frame
      • Symptoms do not develop as the pathogen can be destroyed before significant cell damage occurs

The function of B-lymphocytes during a primary and secondary immune response (1)_1, downloadable AS & A Level Biology revision notesThe function of B-lymphocytes during a primary and secondary immune response (2)_1

During a secondary immune response, memory cells that remained in the blood divide very quickly into plasma cells (to produce antibodies) and more memory cells.  Whereas a primary response occurs much more slowly.

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