AQA AS Biology Unit 1 - The Immune System

Antigens are molecules (usually proteins or polysaccharides) 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 4 main stages in the immune response:

A phagocyte (e.g. a 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 fuses with the phagocytic vacuole. The lysosomal enzymes break down the pathogen.

5. The phagocyte then presents the pathogen's antigens - it sticks the antigens on its surface to activate other immune system cells.

A T-cell is another type of white blood cell. It has proteins on its surface that bind to the antigens presented to it by phagocytes. This activates the T-cell. Different types of T-cell respond in different ways:

1. Some release substances to activate B-cells.

2. Some attach to antigens on a pathogen and kill the cell.

B-cells are also a type of white blood cell. They're covered with antibodies - proteins that 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.

Plasma cells are identical to the B-cell. They secrete a lot of the antibody that is 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.

1. Cellular - The T-cells and other immune system cells that they interact with, e.g. phagocytes, form the cellular response.

2. Humoral - B-cells and the production of antibodies form the humoral response.

Both types of response are needed to remove a pathogen from the body and the responses interact with each other, e.g. T-cells help to activate B-cells, and antibodies coat pathogens making it easier for phagocytes to engulf them.

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

2. The primary response is slow because there aren't many B-cells that can make the antibody needed to bind to it.

3. Eventually the body will produce enough of the right antibody to overcome the infection. Meanwhile the infected person will show symptoms of the disease.

4. After being exposed to an antigen, both T- and B- cells produce memory cells. These memory cells remain in the body for a long time. Memory T-cells remember the specific antigen and will recognise it a second time round. Memory B-cells record the specific antibodies needed to bind the antigen.

5. The person is now immune - their immune system has the ability to respond quickly to a 2nd infection.

1. If the same pathogen enters the body again, the immune system will produce a quicker, stronger immune response - the secondary response.

2. 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-cells to kill the cell carrying the antigen.

3. The secondary response often gets rid of the pathogen before you begin to show any symptoms.