Disease and Immunity



A pathogen is an organism which causes disease. For example, a virus, bacteria or fungi.

They get in through the bodies interface (surface of contact with the environment). There are three main interfaces:
-the digestive system
-the skin
-the gas-exchange system

Pathogens cause disease by:
-Producing toxins (they release toxins into the body)
-Damaging the cell (by rupturing them to release nutrients inside them, breaking the nutrients inside of them for their own use which starves and kills the cell, or by replicating inside the cell and bursting them when released).

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The Immune System

Antigens are proteins found on the surface of cells.

When a pathogen invades the body, the antigens on its cell surface are identified as foreign, which activates cells in the immune system.

There are four main stages in the immune response
-Phagocytes engulf pathogens
-Phagocytes activate T-cells
-T-cells activate B-cells, which divide into plasma cells
-Plasma cells make more antibodies to a specific antigen

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Stage 1 Phagocytes engulf pathogens

A phagocyte (e.g. macrophage) is a type of white blood cell that carries out phagocytosis (engulfment of pathogens). They're found in the blood and in tissues and are the first cells to respond to a pathogen inside the body.

1. A phagocyte recognises the antigens on a pathogen.
2. The cytoplasm of the phagocyte moves round the pathogen, engulfing it.
3. The pathogen is now contained in a phagocytic vacuole in the cytoplasm of the phagocyte.
4. A lysosome (contains lysosomal enzymes) fuses with the phagocytic vacuole. The lysosomal enzymes break down the pathogen.
5. The phagocyte then presents the pathogens antigens - it sticks the antigens on its surface to activate other immune system cells.

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Stage 2 Phagocytes activate T-cells

A T-cell is another type of white blood cell. It has proteins on its surface which bind to the antigens presented to it by phagocytes. This activates the T-cell.
Different types of T-cells respond in dfferent ways
1. Some release substances to activate B-cells
2. Some attach to antigens on a pathogen and kill the cell.

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Stage 3 T-cells activate B-cells which divide in t

B-cells are also a type of white blood cell. They're covered with antibodies (proteins which bind antigens to form an antigen-antibody complex).

Each B-cell has a different shaped antibody on its membrane, so different ones bind to different shaped antigens.

1. When the antibody on the surface of a B-cell meets a complementary shaped antigen, it binds to it.
2.This, together with substances released from T-cells, activates the B-cell.
3. The activated B-cell divides into plasma cells.

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Stage 4 Plasma cells make more antibodies to a spe

Plasma cells are clones of B-cells.
They secrete loads of the antibody specific to the antigen.

Antibody functions include:
1. Coating the pathogen to make it easier for a phagocyte to engulf it
2. Coating the pathogen to prevent it from entering host cells
3. Binding to and neutralising (inactivating) toxins produced by the pathogen.

Antibodies are proteins made up of chains of amino acid monomers linked by peptide bonds. The specificity of the antibody depends on its variable regions. Each antibody has a different shaped variable region (due to different amino acid sequences) thats complemantary to one specific antigen. The constant regions are all the same in all antibodies.

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Cellular or Humoral

The Cellular Response
The T-cells and other imune system cells that they interact with. For example, phagocytes form the cellular response.

The Humoral Response
B-cells and the production of antibodies form the humoral response

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The Primary Response

When an antigen enters the body for the first time it activates the immune system. This is called the primary response.

The primary response is slow because there arn't many B-cells that can make the antibody needed to bind to it.
Eventually the body will produce enough of the right antibody to overcome the infection. Meanwhile the infected person will show symptoms of the disease.

After being exposed to an antigen, both T-cells and B-cells prooduce memory cells. These memory cells remain in the body for a long time. Memory T-cells remember the specific antigen and will recognise it the second time round. Memory B-cells record the specific antibodies needed to bind the antigen.
The person is now immune so their immune system has the ability to respond quickly to a second infection.

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The Secondary Response

If the same pathogen enters the body again, the immune system will produce a quicker and stronger immune response (the secondary response).

Memory B-cells divide into plasma cells that produce the right antibody to the antigen.
Memory T-cells divide into the correct type of T-cell to kill the cell carrying the antigen.
The secondary response often gets rid of the pathogen before you begin to show any symptoms.

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While your B-cells are busy dividing to build up their numbers to deal with the pathogen, you suffer from the disease. Vaccination can help avoid this.
Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease. This means you become immune without getting any symptoms.
Vaccines protect individuals that have them and, because they reduce the occurance of the disease , those not vaccinated are also less likely to catch the disease. This is called herd immunity.
Vaccines always contain antigens, these may be free or attached to a dead or weakened pathogen.
Vaccines may be injected or taken orally. The disadvantages of taking a vaccine orally are that it could be broken down by enzymes in the gut or the molecules of the vaccine may be too large to be absorbed into the blood.
Sometimes booster vaccines are given later on (e.g. after several years) to make sure that memory cells are prduced.

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Antigenic Variation

Antigens on the surface of pathogens activate the immune response.
When you're infected a second time with the same pathogen, they activate the secondary response and you dont get ill.
However some pathogens can change their surface antigens. This is called antigenic variation.
Antigenic Variation: Different antigens are formed due to the changes in the genes of a pathogen.
This means when you're infected the second time, the memory cells produced from the first infection will recognise the different antigens. So the immune system has to start from the primary response again.
This means you get ill again as the primary response takes a while.
Antigenic variation also makes it difficult to develop vaccines against some pathogens.

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Monoclonal Antibodies

Monoclonal Antibodies:
Antibodies produced from a single group of genetically identical B-cells (plasma cells). This means they are all identical in structure.

You can make monoclonal antibodies that clone to whatever you want, and they will only bind to the target molecule.

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Lifestyle Factors and Data

Risk: A measure of the probablility that damage to health will occur as a result of a given hazard.

Increasing the risk of cancer
-Physical activity

Correlation does not mean cause. There could be other factors which affect the results too.

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