Phagocytes: Origin & 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 nonspecific immune response.
- There are two main types of phagocyte,each with a specific mode of action:
Neutrophils and macrophages
- Both carry out phagocytosis (the process of recognising and engulfing a
pathogen) but the process is slightly different for each type of phagocyte
Neutrophils
- Neutrophils travel throughout the body and 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 are short lived cells. Mode of action:
1. Chemicals released by pathogens, as well as chemicals released by the body
cells under attack attract neutrophils to the site where the pathogens are
located (this response is chemotaxis)
2. Neutrophils move towards pathogens which may be covered in antibodies ( is
a proteins with specific binding sites produced by B cells in response to the
presence of an appropriate antigen)
3. The antibodies are another trigger to stimulate neutrophils to attack the
pathogens (neutrophils have receptor proteins on their surfaces that
recognise antibody molecules and attach to them)
4. 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 phagocytosis vacuole. This is endocytosis.
5. The neutrophil then secretes digestive enzymes into the vacuole (the
enzymes are released from lysosomes which fuse with the phagocytic
vacuole)
6. These digestive enzymes destroy the pathogen
7. After killing and digesting the pathogens, the neutrophils die
8. Pus is a sign dead neutrophils
,Macrophages:
- Macrophages are larger than neutrophils and are long lived cells, rather than
remaining in the blood, they move into organs including the lungs, liver,
spleen, kidney and lymph nodes
, - AFter being produced in the bone marrow, macrophages travel in the blood as
monocytes, which then develop into macrophages once they leave the blood
to settle in the various organs listed above.
- Mode of action:
1. Macrophages play a very important role in initiating an immune response.
2. Although they still carry out phagocytosis in a similar way to neutrophils, they
do not destroy pathogens completely.
3. They cut the pathogens up so that they can display the antigens (an antigen is
a molecule that triggers an immune response by lymphocytes) of the
pathogens on their surface through the structure called the major
histocompatibility complex.
4. These displayed antigens can then be recognised by lymphocytes.
Antigens:
- Every cell in the human body has markers that identify it
- Microorganisms such as bacteria and viruses, also have their own unique
markers
- These markers are called antigens and they allow cell to cell recognition
- Antigens are found on cell surface membranes, bacterial cell walls, or the
surfaces of viruses. SOme glycolipids and glycoproteins on the outer surface
of cell surface membranes act as antigens
- Antigens can be either self antigens or non self antigens
- Self antigens are antigens produced by the organism's own body cells and
they do not stimulate an immune response
- Non self antigens are antigens not produced by the organism's own body
cells; they stimulate an immune response.
Primary Immune Response
- Lymphocytes are another type of white blood cell
- They play an important part in the specific immune response
- They are smaller than phagocytes and they have a larger nucleus that fills
most of the cell
- They are produced in the bone marrow before birth. These are two types of
lymphocytes (with different modes of action). The two types are
B-lymphocytes (B cells), T-lymphocytes (T cells).
B-lymphocytes
- B cells remain in the bone marrow until they are mature and then spread
through the body, concentrating in lymph nodes and the spleen
- Millions of types of B cells are produced within us because as they mature the
genes coding for antibodies are changed to code for different antibodies
- Once mature, each type of B cells can make one type of antibody molecule.
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