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
- 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
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.