Aqa Biology - Cell Recognition And The Immune System

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Defence mechanisms

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The human body has a range of defences to protect itself from pathogens. Some are general and immediate defences like the skin forming a barrier to the entry of pathogens and phagocytosis. In addition, there is a white blood cell called a lymphocyte in two forms:

1) cell-mediated responses involving T lymphycytes

2) humoral responses involving B lymphocytes

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Recognising your own cells

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Lymphocytes must be able to distinguish the body's own cells and molecules to ensure they can defend the body form foreign materials. If they could not do this, the lymphocytes would destroy the organism's own tissues.

Each type of cell has specific molecules on its surface to identify it. Proteins are highly important, this is because proteins have a enormous variety and they have a highly specific tertiary structure. This structure distinguishes one cell from another. The protein molecules usually allow the immune system to identify:

- pathogens e.g. the human immunodeficiency virus

- non-self materials such as cells from other organisms of the same species

- toxins including those produced by certain pathogens like the bacterium that causes cholera

- abnormal body cells such as cancer cells.

All of these are potentially harmful and their indication in the first stage in removing the threat they pose. Despite, this usually being advantageous for organisms, it can cause issues for humans who have had tissue or organ transplants. The immune system recognises them as non-self even though they have come from individuals of the same species. Therefore, it attempts to destroy the transplant. To avoid this donor transplants are normally matched as closely as possible to those of the recipient, best matches being from relatives which are genetically close. In addition, immunosuppressant drugs are administered to reduce the level of the immune response that still occur.

Specific lymphocytes are not produced in response to an infection as they already exist. There is a high chance that when a pathogen gets into the body, one of the ten million different types have a protein on its surface that is complementary to one of the proteins on the pathogen. When an infection occurs, the one type has complementary proteins to those of the pathogen is stimulated to divide to build up its numbers to a level where it is effective at destroying the pathogen. This is called clonal selection.

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How lymphocytes recognise cells belonging to the body.

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- Each lymphocyte is capable of recognising a different chemical shape.

- In a fetus, lymphocytes are constantly colliding with each other. Infection in the fetus is extremely rare because it is protected from the outside world by its mother and the placenta.

- Lymphocytes collide almost exclusively wih the body's own material.

- Some of the lymphocytes have receptors that exactly fit those of the body's cells. The lymphocytes either die or are suppressed.

- The remaining lymphocytes are those that might fit foreign material, therefore only respond to foreign material.

- 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.

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What are the two types of white blood cells?

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Phagocytes: ingest and destroy the pathogen by a process called phagocytosis before it can cause harm.

Lymphocytes: are involved in immune responses.

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Phagocytosis

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1) Chemical products of pathogens or dead, damaged and abnormal cells act as attractants, causing phagocytes on the surface of the pathogen.

2) Pathogens have several receptors on their cell-surface membrane that recognise chemicals on the surface of the pathogen.

3) They engulf the pathogen to form a vesicle, known as a phagosome.

4) Lysosomes move towards the vesicle and fuse with it.

5) Enzymes called lysozymes are present within the lysosome. The lysozymes destroy ingested bacteria by hydrolysis of their cell walls. The soluble products from the breakdown of the pathogen are absorbed into the cytoplasm of the phagocyte.

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Antigens

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Antigen: any part of an organism or substance that is recognised as non-self by the immune system and stimulated an immune response. Antigens are usually part of the cell-surface membranes or cell walls, such as cancer cells. The presence of an antigen triggers the production of an antibody as part of the body's defence system.

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Lymphocytes

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Immune responses such as phagocytosis are non-specific and occur whatever the infection. The body also has specific responses that react to specific antigens. To begin with, these are slower, but can provide long-term immunity. This specific immune response depends on a type of white blood called a lymphocyte. Lymphocytes are produced by stem cells in the bone marrow. There are two types:

- B lymphocytes (B cells) mature in the bone marrow. These are associated with humoral immunity - immunity involving antibodies that are present in body fluids.

- T lymphocytes (T cells) mature in the thymus gland. They are associated with cell-mediated immunity - immunity involving body cells.

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Cell-mediated immunity

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Lymphocytes respond to an organism's own cells that have been infected by non-self material from a different species e.g. a virus. They also respond to cells from other individuals of the same species because these are genetically different. Therefore, these have different antigens on their cell-surface membrane from the antigens on the organism's own cells. T lymphocytes can distinguish these invader cells from normal cells because:

- phagocytes that have engulfed and hydrolysed a pathogen present some of a pathogen's antigens on their own cell-surface membrane

- body cells invaded by a virus present some of the viral antigens on their own cell-surface membrane

- transplanted cells from individuals of the same species have different antigens on their cell-surface membrane

- cancer cells are different from normal body cells and present antigens on their cell-surface membranes.

Cells that display foreign antigens on their surface are called antigen-presenting cells because they can present antigens of other cells on their own cell-surface membrane.

T lymphocytes will only respond to antigens that are presented on a body cell. This type of response is called cell-mediated immunity or the cellular response. The receptors on each T cell respond to a single antigen.

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What are the stages in response of T lymphocytes to infection by a pathogen?

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1) Pathogens invade body cells or are taken in by phagocytes.

2) The phagocyte place antigens from the pathogen on its cell-surface membrane.

3) Receptors on a specific helper T cell fit exactly onto these antigens.

4) This attachment activates the T cell to divide rapidly by mitosis and form a clone of genetically identical cells.

5) The cloned cells:

a) develop into memory cells that enable a rapid response to future infections by the same pathogen

b) stimulate phagocytes to engulf pathogens by phagocytosis

c) stimulate B cells to divide and secrete their antibody

d) activate cytotoxic T cells

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How cytotoxic T cells kill infected cells

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Cytotoxic T cells kill abnormal cells and body cells that are infected by pathogens by producing a protein called perforin which makes holes in the cell-surface membrane. These holes mean that the cell membrane becomes freely permeable to all substance and the cell dies as a result. The action of the T cell is most effective against viruses because viruses replicate inside cells.