4.1.5 Phagocytes Flashcards
What are 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
What are 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
What are 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
What are 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
What is 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
