4.1.5 Phagocytes

• 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

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