Defence Mechanisms
Antigens - unique proteins on the cell surface membrane which have a specific shape
OR any part of substance or organism that is recognised by the immune system as non-self and
stimulate an immune response
Proteins are extremely varied and have highly specific 3D structures - these molecules allow the
immune system to identify:
1. Pathogens - e.g. the Human Immunodeficiency virus
2. Non-self-material - such as cells from other organisms of the same species
3. Toxin - including those produced by certain pathogens
4. Abnormal body cells - such as cancer
All of these are potentially harmful, and identification is the first stage in removing them
Although this response is advantageous to an organism, it has implications for humans with
organ or tissue transplants - the immune system recognises these as non-self and begin to
attach the transplanted tissue
There are two main types of white blood cells that defend us from pathogens:
Lymphocytes - produce antibodies
Phagocytes - engulf pathogens
How does the body deal with these so that the body will only recognise non-self in the
future?
In adults, lymphocytes produced in the bone marrow initially only encounter self-antigens
Any lymphocytes that show an immune response to these self-antigens undergo
programmed cell death (apoptosis) before they can differentiate into mature lymphocytes
No clones of these anti-self-lymphocytes will appear in the blood, leaving only those that
might respond to non-self-antigens
To defend the body from invasion by foreign material, lymphocytes must be able to
distinguish the body's own cells and molecules (self) from those that are foreign
Self-cells - the body's own cells and molecules
Non-self - cells or molecules that are foreign
How do lymphocytes recognise cells belonging to the body?
There are probably around 10 million different lymphocytes present at any time - each capable of
recognising a different chemical shape
In the fetus, lymphocytes are constantly colliding with other cells and so will collide almost
exclusively with the body's own material
(infection in the fetus is rare because it is protected from the outside world by the mother
and the placenta)
Some of the lymphocytes will have receptors that exactly fit those of the body's cells - these
lymphocytes either die or are suppressed
The only remaining lymphocytes are those that might fit foreign material (non-self) and
therefore only respond to foreign material
If the lymphocytes couldn't do this, then the lymphocytes would destroy the organism's own tissues
, Phagocytosis
There are two types of white blood cells:
Phagocytes - ingest and destroy pathogens by engulfing them in a process called
phagocytosis, preventing them causing harm
None specific - phagocytes will engulf any bacteria
Lymphocytes - involved in immune responses
The process of phagocytosis
1. Chemical products of pathogens or dead, damaged and abnormal cells act as attractants,
causing phagocytes to move towards the pathogen by chemotaxis
2. Phagocytes have several receptors on their cell-surface membrane that recognise and attach
to chemicals on the surface of the pathogen
3. They engulf the pathogen to form a vesicles known as a phagosome
4. A lysosome moves towards the vesicle and fuse with it - the enzyme lysozyme inside the
lysosome destroy the ingested bacteria by hydrolysis of their cell walls
This process hydrolyses larger, insoluble molecules into smaller, soluble ones
5. The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm
of the phagocyte
6. Antigens from the pathogen are displayed on the surface of the phagocyte
Lymphocytes and Cell Mediated Immunity
Lymphocytes:
1. T lymphocytes (T cells)
Mature in the thymus gland
Linked to cell mediated immunity (i.e. immunity involving body cells)
2. B lymphocytes (B cells)
Mature in the bone marrow
Linked to humoral immunity (i.e. antibodies present in body fluid or plasma)
Non-specific Immunity: e.g. physical barriers and phagocytes
Specific Immunity:
Specific response for each pathogen
Takes time to enable
Long lasting
Cell mediated response (T)
Humoral response (B)
T lymphocytes
Cell-mediated immunity (i.e. immunity requiring the cell to be present) - T lymphocytes only
respond to antigens that are presented on a body cell (not floating in body fluid)
Receptor proteins on the surface of T lymphocytes detect and attach to complementary
antigens on the surface of other cells to activate the T cell into helper T-cells
There are 4 main sources of these antigens:
1. Phagocytes - antigens from hydrolysed pathogens become presented on their cellular
membrane (antigen presenting cells - cells that display foreign antigens)
Antigens - unique proteins on the cell surface membrane which have a specific shape
OR any part of substance or organism that is recognised by the immune system as non-self and
stimulate an immune response
Proteins are extremely varied and have highly specific 3D structures - these molecules allow the
immune system to identify:
1. Pathogens - e.g. the Human Immunodeficiency virus
2. Non-self-material - such as cells from other organisms of the same species
3. Toxin - including those produced by certain pathogens
4. Abnormal body cells - such as cancer
All of these are potentially harmful, and identification is the first stage in removing them
Although this response is advantageous to an organism, it has implications for humans with
organ or tissue transplants - the immune system recognises these as non-self and begin to
attach the transplanted tissue
There are two main types of white blood cells that defend us from pathogens:
Lymphocytes - produce antibodies
Phagocytes - engulf pathogens
How does the body deal with these so that the body will only recognise non-self in the
future?
In adults, lymphocytes produced in the bone marrow initially only encounter self-antigens
Any lymphocytes that show an immune response to these self-antigens undergo
programmed cell death (apoptosis) before they can differentiate into mature lymphocytes
No clones of these anti-self-lymphocytes will appear in the blood, leaving only those that
might respond to non-self-antigens
To defend the body from invasion by foreign material, lymphocytes must be able to
distinguish the body's own cells and molecules (self) from those that are foreign
Self-cells - the body's own cells and molecules
Non-self - cells or molecules that are foreign
How do lymphocytes recognise cells belonging to the body?
There are probably around 10 million different lymphocytes present at any time - each capable of
recognising a different chemical shape
In the fetus, lymphocytes are constantly colliding with other cells and so will collide almost
exclusively with the body's own material
(infection in the fetus is rare because it is protected from the outside world by the mother
and the placenta)
Some of the lymphocytes will have receptors that exactly fit those of the body's cells - these
lymphocytes either die or are suppressed
The only remaining lymphocytes are those that might fit foreign material (non-self) and
therefore only respond to foreign material
If the lymphocytes couldn't do this, then the lymphocytes would destroy the organism's own tissues
, Phagocytosis
There are two types of white blood cells:
Phagocytes - ingest and destroy pathogens by engulfing them in a process called
phagocytosis, preventing them causing harm
None specific - phagocytes will engulf any bacteria
Lymphocytes - involved in immune responses
The process of phagocytosis
1. Chemical products of pathogens or dead, damaged and abnormal cells act as attractants,
causing phagocytes to move towards the pathogen by chemotaxis
2. Phagocytes have several receptors on their cell-surface membrane that recognise and attach
to chemicals on the surface of the pathogen
3. They engulf the pathogen to form a vesicles known as a phagosome
4. A lysosome moves towards the vesicle and fuse with it - the enzyme lysozyme inside the
lysosome destroy the ingested bacteria by hydrolysis of their cell walls
This process hydrolyses larger, insoluble molecules into smaller, soluble ones
5. The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm
of the phagocyte
6. Antigens from the pathogen are displayed on the surface of the phagocyte
Lymphocytes and Cell Mediated Immunity
Lymphocytes:
1. T lymphocytes (T cells)
Mature in the thymus gland
Linked to cell mediated immunity (i.e. immunity involving body cells)
2. B lymphocytes (B cells)
Mature in the bone marrow
Linked to humoral immunity (i.e. antibodies present in body fluid or plasma)
Non-specific Immunity: e.g. physical barriers and phagocytes
Specific Immunity:
Specific response for each pathogen
Takes time to enable
Long lasting
Cell mediated response (T)
Humoral response (B)
T lymphocytes
Cell-mediated immunity (i.e. immunity requiring the cell to be present) - T lymphocytes only
respond to antigens that are presented on a body cell (not floating in body fluid)
Receptor proteins on the surface of T lymphocytes detect and attach to complementary
antigens on the surface of other cells to activate the T cell into helper T-cells
There are 4 main sources of these antigens:
1. Phagocytes - antigens from hydrolysed pathogens become presented on their cellular
membrane (antigen presenting cells - cells that display foreign antigens)
