Edexcel International A Level Biology

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6.10 Lymphocytes: Types & Roles

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Lymphocytes: Types & Roles

  • There are two types of lymphocyte that play a particular role in the specific immune response
    • T cells
    • B cells
  • Note that lymphocytes are a type of white blood cell

T cells

  • T cells, sometimes known as T lymphocytes, are produced in the bone marrow and finish maturing in the thymus, which is where the T in their name comes from
  • Mature T cells have specific cell surface receptors called T cell receptors
  • These receptors have a similar structure to antibodies and are each specific to a particular type of antigen

_The maturation of T-lymphocytes

Mature T cells have many different types of receptor on the cell surface membrane; these receptors will bind to different antigens on antigen presenting cells

  • T cells are activated when they encounter and bind to their specific antigen on the surface of an antigen presenting cell 
    • This antigen-presenting cell might be a macrophage, an infected body cell, or the pathogen itself
  • These activated T cells divide by mitosis to increase in number
    • Dividing by mitosis produces genetically identical cells, or clones, so all of the daughter cells will have the same type of T cell receptor on their surface 
  • There are three main types of T cell
    • T helper cells
      • Release chemical signalling molecules that help to activate B cells
      • Release chemical signalling molecules that help to activate T killer cells
      • Release chemicals called cytokines, that label pathogens and infected cells for phagocytosis
    • T killer cells
      • Bind to and destroy infected cells displaying the relevant specific antigen
    • T memory cells
      • Remain in the blood and enable a faster specific immune response if the same pathogen is encountered again in the future

B cells

  • B cells, also known as B lymphocytes, are a second type of white blood cell in the specific immune response
    • B cells remain in the bone marrow as they mature, hence the B in their name
  • B cells have many specific receptors on their cell surface membrane
    • The receptors are in fact antibodies, and are known as antibody receptors
    • Each B cell has a different type of antibody receptor, meaning that each B cell can bind to a different type of antigen

The maturation of B lymphocytes

Mature B cells each have different types of antibody receptors on their cell surface membrane

  • If the corresponding antigen enters the body, B cells with the correct cell surface antibodies will be able to recognise it and bind to it 
    • When the B cell binds to an antigen it forms an antigen-antibody complex
  • The binding of the B cell to its specific antigen, along with the cell signalling molecules produced by T helper cells, activates the B cell
  • Once activated the B cells divide repeatedly by mitosis, producing many clones of the original activated B cell
  • There are two main types of B cell
    • Effector cells, which differentiate into plasma cells
      • Plasma cells produce specific antibodies to combat non-self antigens
    • Memory cells
      • Remain in the blood to allow a faster immune response to the same pathogen in the future

The function of B-lymphocytes during a primary immune response

During a primary immune response B cells divide by mitosis to form plasma cells and memory cells. Note that a primary response occurs the first time an individual comes into contact with a particular pathogen

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