Cell Recognition and the Immune System
Defence Mechanisms
Any infection is an interaction between the pathogen and the body’s Immunity - the body's
various defence mechanisms. defences seem to be better
- If a pathogen overwhelms the body’s defence mechanisms the prepared for a second infection
individual dies from the same pathogen and
- If the body’s defence mechanisms overwhelm the pathogen the can kill it before it can cause
individual recovers from the disease any harm
The body’s reaction to infections much The human body has a range of defences:
depends on the overall state of health of an - Some are general and immediate:
individual: - Skin forms a barrier for entry of pathogens
- A t, healthy adult will rarely die of an - Phagocytosis
infection - Others are more speci c, longer lasting but less rapid:
- Those in ill health, the young and elderly - Cell-mediated responses involving T lymphocytes
are usually more vulnerable to infections - Humoral responses involving B lymphocytes
• Lymphocytes need to be able to Protein molecules on the surface of cells allows the immune
distinguish the body’s own cells to foreign system to recognise:
cells (non-self), otherwise lymphocytes will - Pathogens (any organism that causes disease)
destroy the organism’s own tissues. - Non-self material (cells from other organisms of the same
• Each cell has speci c molecules on its species)
surface that identify it. - Toxins (a poison produced by a microorganism)
• This is made out of proteins because they - Abnormal body cells (such as cancer cells)
have enormous variety in their tertiary
structure
Identi cation of potentially harmful cells also has implications
• Speci c lymphocytes already exist for those who have had tissue or organ transplants.
beforehand and there is a high probability 1. The immune system recognises these as non-self cells
that one will be complementary to an 2. Then attempts to destroy the transplant
invading pathogen (1 of ten million types) To reduce this reaction of the immune system:
• Due to there being so many di erent - Donor tissues for transplant are matched as closely as
types there are few of each possible to those of the recipient (best often relatives)
• When an infection occurs, the one type - Also immunosuppressant drugs may often be administered
already present that has complementary
proteins to the pathogen’s
• This lymphocyte is stimulated to divide to
- Ten million di erent lymphocytes present are at any time,
build up its numbers to a level where it is each capable of recognising a di erent chemical shape
be e ective in destroying it. (Clonal 1. In the fetus, these lymphocytes are constantly colliding
selection) with other cells
• This is why there is a time lag between - Infection in fetus is rare as it protected from the outside
exposure to the pathogen and the body’s world by the mother and the placenta
defences bringing it under control 2. Lymphocytes will therefore collide almost exclusively
with self material
3. Some lymphocytes will have receptors that t self cells,
these either die or are suppressed
- The remaining lymphocytes must t foreign material
1. In adults, lymphocytes produced in the bone marrow
initially only encounter self-antigens
2. Those that show immune response to self-antigens
undergo programmed death (apoptosis) before they can
di erentiate into mature lymphocytes
3. No clones of these anti-self cells will appear in the blood
leaving only those that respond to non-self antigens
1
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, Cell Recognition and the Immune System
Phagocytosis
• If the rst line of defence fails (physical barrier), the next line of defence is white blood cells.
• Phagocytes ingest and destroy the pathogen by the process called phagocytosis
•
• Another way some types of bacteria can be engulfed is by vesicles formed from the cell membrane
• Attractants cause phagocytes to move towards the pathogen
• Examples of attractants are chemical products of pathogens or dead, damaged and abnormal cells
1. Chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing the
phagocyte to move toward the pathogen
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 vesicle (this is known as a phagosome)
4. Lysosomes move toward the vesicle and fuse with it
5. Lysozymes destroy ingested bacteria by hydrolysis of their cell walls
6. Soluble products from the breakdown of a pathogen are absorbed into the cytoplasm of the phagocyte
2
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Defence Mechanisms
Any infection is an interaction between the pathogen and the body’s Immunity - the body's
various defence mechanisms. defences seem to be better
- If a pathogen overwhelms the body’s defence mechanisms the prepared for a second infection
individual dies from the same pathogen and
- If the body’s defence mechanisms overwhelm the pathogen the can kill it before it can cause
individual recovers from the disease any harm
The body’s reaction to infections much The human body has a range of defences:
depends on the overall state of health of an - Some are general and immediate:
individual: - Skin forms a barrier for entry of pathogens
- A t, healthy adult will rarely die of an - Phagocytosis
infection - Others are more speci c, longer lasting but less rapid:
- Those in ill health, the young and elderly - Cell-mediated responses involving T lymphocytes
are usually more vulnerable to infections - Humoral responses involving B lymphocytes
• Lymphocytes need to be able to Protein molecules on the surface of cells allows the immune
distinguish the body’s own cells to foreign system to recognise:
cells (non-self), otherwise lymphocytes will - Pathogens (any organism that causes disease)
destroy the organism’s own tissues. - Non-self material (cells from other organisms of the same
• Each cell has speci c molecules on its species)
surface that identify it. - Toxins (a poison produced by a microorganism)
• This is made out of proteins because they - Abnormal body cells (such as cancer cells)
have enormous variety in their tertiary
structure
Identi cation of potentially harmful cells also has implications
• Speci c lymphocytes already exist for those who have had tissue or organ transplants.
beforehand and there is a high probability 1. The immune system recognises these as non-self cells
that one will be complementary to an 2. Then attempts to destroy the transplant
invading pathogen (1 of ten million types) To reduce this reaction of the immune system:
• Due to there being so many di erent - Donor tissues for transplant are matched as closely as
types there are few of each possible to those of the recipient (best often relatives)
• When an infection occurs, the one type - Also immunosuppressant drugs may often be administered
already present that has complementary
proteins to the pathogen’s
• This lymphocyte is stimulated to divide to
- Ten million di erent lymphocytes present are at any time,
build up its numbers to a level where it is each capable of recognising a di erent chemical shape
be e ective in destroying it. (Clonal 1. In the fetus, these lymphocytes are constantly colliding
selection) with other cells
• This is why there is a time lag between - Infection in fetus is rare as it protected from the outside
exposure to the pathogen and the body’s world by the mother and the placenta
defences bringing it under control 2. Lymphocytes will therefore collide almost exclusively
with self material
3. Some lymphocytes will have receptors that t self cells,
these either die or are suppressed
- The remaining lymphocytes must t foreign material
1. In adults, lymphocytes produced in the bone marrow
initially only encounter self-antigens
2. Those that show immune response to self-antigens
undergo programmed death (apoptosis) before they can
di erentiate into mature lymphocytes
3. No clones of these anti-self cells will appear in the blood
leaving only those that respond to non-self antigens
1
fffi fffi ff fi fi ff fffi fi
, Cell Recognition and the Immune System
Phagocytosis
• If the rst line of defence fails (physical barrier), the next line of defence is white blood cells.
• Phagocytes ingest and destroy the pathogen by the process called phagocytosis
•
• Another way some types of bacteria can be engulfed is by vesicles formed from the cell membrane
• Attractants cause phagocytes to move towards the pathogen
• Examples of attractants are chemical products of pathogens or dead, damaged and abnormal cells
1. Chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing the
phagocyte to move toward the pathogen
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 vesicle (this is known as a phagosome)
4. Lysosomes move toward the vesicle and fuse with it
5. Lysozymes destroy ingested bacteria by hydrolysis of their cell walls
6. Soluble products from the breakdown of a pathogen are absorbed into the cytoplasm of the phagocyte
2
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