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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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.