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Internal Defences
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Syllabus Content
Specific resistance:
* role of B cells, T cells, memory cells and plasma cells * antibody and cell-mediated defence
* primary and secondary immune response
* passive and active immunity
* natural and artificial immunity
* role of antibiotics and antivirals.

Risks, ethical concerns and benefits:
* production and use of vaccines

Review of Body’s Defences
Pathogens – living organisms or agents that cause disease (e.g. bacteria, fungi, parasites, viruses and prions)
Non-self – any pathogen or foreign substance not recognised as part of an individual
Non-specific – a line of defence that prevents the entry or effect of any non-self material
Macrophage – mobile white blood cells that engulf and destroy pathogens through the process of phagocytosis

The Lymphatic System
The lymphatic system is found throughout the body and has its own network of capillaries and vessels. It contains a clear fluid known as lymph that can circulate between the blood vessels, tissue fluid and lymphatic vessels. As the fluid enters the lymphatic vessels it encounters white blood cells that monitor the body for ‘non-self’ substances. This primarily happens at the lymph nodes.

Specific Defences
Our body has the ability to target and resist a particular non-self substance (now known as an antigen) if it penetrates the non-specific defences. This is known as specific immunity. Specific immunity is achieved through the immune response – a very complex process that involves non-specific cells like macrophages as well as cells known as lymphocytes.

Lymphocytes
Lymphocytes are made in the bone marrow in the hollow of long bones. From here, two types of lymphocytes start – B cells and T cells.
The B cells continue the maturation process in the bone marrow. T Cells, however, mature in the thymus.
Once mature, B and T cells migrate to lymphoid tissue throughout the body but become mainly concentrated in the lymph nodes.
Immune Response
When we are exposed to an antigen for the first time our immune system can deal with it in two ways: 1. Humoral immunity / Antibody mediated immunity – involves B cells primarily 2. Cell mediated immunity – involves T cells

Both of these processes are interdependent.

Antibody-mediated Immunity
If an antigen penetrates the non-specific defences it can be directly identified by a specific B cells in lymphoid tissue. This particular B cell is sensitised and enlarges, dividing to form many cells that are identical to it. These copies are called a clone.

Most of the clone then become plasma cells. The role of plasma cells is to manufacture proteins that can target the antigen. The other name for these proteins is antibodies. Antibodies are very small and able to travel through the bloodstream to every corner of the body to seek out the antigen.

DIAGRAM
Once the antibodies locate an antigen, they attach to its surface and inactivate it (antigen-antibody complex). Once here they help to destroy the antigen by: * Agglutination – clumping together of the pathogen to enhance phagocytosis * Making soluble substances insoluble so it can be readily identified by macrophages * Dissolving the pathogen’s membrane

DIAGRAM

Primary and Secondary Responses
The first time our body is exposed to an antigen it takes a while for the immune response to occur. As a result an individual often fall sicks and shows major symptoms. As part of the primary response, some of the sensitised B cells become memory cells. These remain in the body for a period of time.

Should an individual encounter the antigen later on, these memory cells and any antibodies still circulating will ensure that the immune response occurs faster. The antibodies are produced in greater quantities at a faster rate. The individual often does not even know that they have been exposed as they may experience few if...
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