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The Immune Response

Biology: IB Diploma Higher Level, topic 11.2.2

Phagocytosis

Phagocytosis is the first stage in the immune response that takes place in white blood cells called phagocytes. The steps involved in this process are:

1) Detection of antigens

1) Detection of antigens

  • Foreign antigens bind to specific receptors on the cell surface of phagocytes.
2) Engulfing the pathogen

2) Engulfing the pathogen

  • The phagocyte moves towards the pathogen.
  • The phagocyte cytoplasm surrounds the pathogen and the pathogen is engulfed.
  • When the pathogen is engulfed, it is sealed into a phagosome (a vacuole) inside the cytoplasm.
3) Digestion of the pathogen

3) Digestion of the pathogen

  • Phagocytes have many organelles called lysosomes that contain proteolytic enzymes.
  • A lysosome fuses with the phagosome and releases the proteolytic enzymes into the phagosome.
  • The enzymes break down the pathogen.
4) Presenting the antigens

4) Presenting the antigens

  • The pathogen antigens are transported to the phagocyte cell membrane and presented on the cell surface.
  • The antigens can activate the other cells in the immune response.
Phagocytosis diagram

Phagocytosis diagram

T Lymphocytes

T lymphocyte cells are white blood cells that are involved in the cellular immune response. The steps involved in this process are:

1) Detection of antigens

1) Detection of antigens

  • The foreign antigens presented by phagocytes bind to specific receptors on the cell surface of T lymphocyte cells.
  • Binding of the antigens activates the T helper cells.
2) T helper cells

2) T helper cells

  • T helper (TH) cells are a specific type of T cell that activates several cells including:
  • Phagocytes.
  • T cytotoxic (TC) cells.
  • B cells.
3) T cytotoxic cells

3) T cytotoxic cells

  • TC cells are activated by TH cells.
  • TC cells release toxins that bind to and kill foreign cells and abnormal cells in the body.

B Lymphocytes

B lymphocyte cells are white blood cells that are involved in the humoral immune response. The steps involved in this process are:

1) Activation of B cells

1) Activation of B cells

  • TH cells activates B cells.
  • Activation of the B cells causes the B cells to divide into identical cells called plasma cells.
  • The selection of the correct B cell with a complementary receptor to a pathogen's antigen is called clonal selection.
  • Once the correct B cell has been selected, it divides multiple times to produce many identical plasma cells. Plasma cells make antibodies against specific antigens. This process is called clonal expansion.
2) Detection of antigens

2) Detection of antigens

  • B cells have specific proteins called antibodies on their cell surface membranes.
  • Each antibody is complementary (specific) to a specific antigen.
  • When an antigen binds to an antibody, an antigen-antibody complex is formed.
  • Binding of antigens to B cells also causes clonal selection.
3) Plasma cells

3) Plasma cells

  • Clonal selection leads to clonal expansion, which is the production of many plasma cells that have the specific antibodies for the antigens present in the body.
  • The antibodies are called monoclonal antibodies.
  • Monoclonal antibodies bind to the antigens of the pathogens.
4) Agglutination

4) Agglutination

  • Agglutination 'clumps' the pathogens together.
  • The pathogens are engulfed by phagocytes via phagocytosis.
  • The pathogens are destroyed.

Test yourself on The Immune Response

  • What are the steps involved in phagocytosis?

  • Which of the following statements about phagocytosis are true?

  • What structure is the pathogen sealed into when it is engulfed?

  • Which of these is NOT activated by T helper cells?

  • Which of the following statements about T lymphocytes are true?

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