Phagocytes (OCR A Level Biology)

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Phagocytes: Structure & Mode of Action

  • Phagocytes are white blood cells that are produced continuously in the bone marrow
  • They are stored in the bone marrow before being distributed around the body in the blood
  • They are responsible for removing dead cells and invasive microorganisms
  • They carry out what is known as a non-specific immune response
  • There are three main types of phagocyte, each with a specific mode of action. The three types are:
    • Neutrophils
    • Macrophages
    • Dendritic cells
  • As they are all phagocytes, they carry out phagocytosis (the process of recognising and engulfing a pathogen) but the process is slightly different for each type of phagocyte

Neutrophils

  • Neutrophils are short-lived cells that often leave the blood by squeezing through capillary walls to ‘patrol’ the body tissues
  • During an infection they are released in large numbers from their stores
  • They have a lobed nucleus which can be used to identify them in blood smears
  • Mode of action:
    • Chemicals released by pathogens, as well as chemicals released by the body cells under attack (e.g. histamine), attract neutrophils to the site where the pathogens are located
      • This response to chemical stimuli is known as chemotaxis
    • Neutrophils move towards pathogens, which may have antibodies attached to their surface antigens
      • Neutrophils have receptor proteins on their surfaces that recognise antibody molecules and attach to them
    • Once attached to a pathogen the cell surface membrane of a neutrophil extends out and around the pathogen, engulfing it and trapping the pathogen within a phagocytic vacuole
      • This part of the process is known as endocytosis
    • The neutrophil then secretes digestive enzymes into the vacuole
      • The enzymes are released from lysosomes which fuse with the phagocytic vacuole
    • These digestive enzymes destroy the pathogen
    • After killing and digesting the pathogens, the neutrophils die
      • Pus is a sign of dead neutrophils

Neutrophil actionNeutrophil action part 2

Neutrophils carry out phagocytosis, after which they digest the pathogen

Macrophages

  • Macrophages are larger than neutrophils and are long-lived cells
  • After being produced in the bone marrow, macrophages travel in the blood as monocytes, which then develop into macrophages once they leave the blood
    • After leaving the blood macrophages settle in the lungs, liver, spleen, kidney and lymph nodes
  • Mode of action:
    • Macrophages play an important role in initiating the specific immune response
    • They carry out phagocytosis in a similar way to neutrophils but they do not destroy pathogens completely; instead they cut the pathogens up so that they can display the antigens of the pathogens on their surface
      • Antigens are displayed as part of a structure called a major histocompatibility complex (MHC)
    • The cell is now called an antigen-presenting cell and can be recognised by lymphocytes, another type of white blood cell

Dendritic cells

  • Dendritic cells are large phagocytic cells with lengthy extensions
    • These extensions give them a large surface area to interact with pathogens and lymphocytes
  • These cells can be found throughout the body
  • Once they have ingested foreign material they transport it to the lymph nodes

The role of antigen-presenting cells

  • T-lymphocytes produce an immune response when they are exposed to a specific antigen
  • T cells will only bind to an antigen if it is present on the surface of an antigen-presenting cell
    • These cells present the antigens from toxins, foreign cells and ingested pathogens
    • They help to recruit other cells of the immune system to produce a specific immune response
  • An antigen-presenting cell is one of the host's cells
    • It 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
  • Once the surface receptor of the T cell binds to the specific complementary antigen it becomes sensitised and starts dividing to produce a clone of cells

Examiner Tip

The vacuole formed around a bacterium once it has been engulfed by a phagocyte is called a phagosome. A lysosome fuses with the membrane of the phagosome (to form a phagolysosome) and releases lysozymes (digestive enzymes) to digest the pathogen.

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.