- Antigens are molecules that generate an immune response. They are usually found on the surface of cells. Antigens that are not normally found in the body are called foreign antigens.
- Pathogens are organisms that cause disease(bacteria, viruses, fungi). All pathogens are foreign antigens.
- Abnormal body cells are cancerous or pathogen-infected cells that have abnormal antigens on there surface which trigger an immune response
- Toxins are poisons and are molecules not cells. Some toxins are made by bacteria and the immune system can respond to toxins.
- During blood transfusions the most important antigens are the ABO blood group antigens, if the donated blood contains a group that is not recognised then this will generate and immune repsonse
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The immune response
- A phagocyte recognises the foreign antigens on a pathogen. The cytoplasm of the phagocyte englufs the pathogen. The pathogen is contained in the phagocytic vacuole. A lysosome which contains the enzyme lysozyme fuses with the phagocytic vacuole and breaks down the pathogen. The anitgens of the pathogen is now displayed on the outside of the phagocyte to activate other immune system cells.
- A T-cell has receptor proteins on its surface which bind to complementary antigens presented to it by phagocytes. This activates the T-cell. There are different types of T-cell, helper T-cells release chemicals that activate and stimulate phagocytes and cytotoxic T-cells which kill abnormal and foreign cells
- B-cells are covered in antibodies and bind to antigens to form a antigen-antibody complex. Each B-cell has a different shaped antibody so different ones bind to different shaped antigens. Together with substances released from helper T-cells, activates the B-cell. This process is called clonal selection. The activated B-cell divides into plasma cells
- Plasma cells are identical to B-cells and secrete lots of antibodies specific to the antigen. These are called monoclonal antibodies. They bind to the antigens on the surface of the pathogen to form lots of antigen-antibody complexes
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The structure of an antibody
- The specifity of an antibody depends on its variable region, which form antigen binding sites
- Each antibody has a variable region with a unique tertiary structure thats complementary to one specific antigen
- All antibodies have the same constant region
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Cellular and humoral response
- Cellular- The T-cells and other immune system cells that they interact with
- Humoral- B-cells, clonal selection and the production of monoclonal 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
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Primary and secondary immune responses
- The primary response- When an antigen enters the body it activates the immune response. The primary response is slow because there are not that many B-cells that can make the antibody needed. While the body produces enough antibodies the person will show symptoms of the disease. After being exposed to the antigen both B and T cells produce memory cells. This means if the person is infected with the foreign antigen again the immune system can make lots of specific antibodies needed very quickly meaning the person probably does not know they are infected. The person is now immune
- The secondary response- If the same pathogen enters the body again then the immune system will produce a quicker and stronger response. Memory B-cells are activated and divide into the plasma cells that produce the antibody to fight the antigen. Memory T-cells are activated and divide into the right type of cell to kill the cell carrying the infection.
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Active and passive immunity and vaccines
- Active immunity- Natural immunity is when you become immune after catching the disease (chickenpox). Artificial is when you become immune after being given a harmless vaccination
- Passive immunity- Natural immunity is when a baby becomes immune after receiving antibodies from the mother through breastmilk. Artificial is when you become immune after being injected with antibodies from someone else
- Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing the disease.
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- Some pathogens can change their surface antigens and this is called antigenetic variation.
- When your infected for a second time the memory cells from the first infection will not work as they will not recognise the new antigens.
- So the immune system has to start again and make new antibodies to fight the foreign antigens
- This takes time to happen thats why you get ill again
- An example of this is ithe influenza virus which is constantly changing meaning there are new strands of the flue every year
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Antibodies in medicine
- Monoclonal antibodies are antibodies produced from a single group of genetically identical B-cells
- Monoclonal antibodies can be made to bind to tumour markers
- You can attach anti-cancer drugs to the antibodies and when the antibodies come into contact with the cancer cells they will only bind to the tumour markers meaning that the drug will only accumulate in the body where the cancer cells are.
- Pregnancy tests detect the hormone human chorionic gonadotropin (hCG)
- The application area contains anitbodies that are complementary to the hCG protein bound to a blue coloured gene.
- If there is hCG present in the womans urine then an antigen-antibody complex will form.
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Direct and indirect elisa test
- The enzyme-linked immunosorbent assay (ELISA) allows you to see if a patient has any antibodies to a certain antigen or any antigen to a certain antibody
- A direct ELISA uses a single antibody that is complementary to the antigen your testing for. Antigens from a patient sample is bound to the inside of a well and then a detection protein with an attached enzyme that is complementary to the antigen of interest is added. If the antigen of interest is present then it will bind to the antigen. The well is then washed out to remove any unbound antibody and a substrate solution is added and will produce a colour change if the detection antibody is still present and hasn't been washed out.
- An indirect ELISA is different as it uses 2 different antibodies and is used to see if a patient possesses the anitbodies for HIV. The HIV antigen is bound to the bottom of the well. A sample of the patients blodd plasma is added to the well if there are HIV antibodies they with bind to the antigen. The well is washed out to remove any unbound antibodies. A secondary antibody with an enzyme is added and the secondary antibody can bind to the HIV antibody. The well is washed out again. A substrate solution is added which will produce a colour change if the antibody with the enzyme is still attached.
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HIV and AIDS
- Human immunodeficiency virus is a virus that affects the human immune system and eventually leads to acquired immune deficiency syndrome. AIDS means that a person is vulnerable to infections because there immune system has deteriorated.
- HIV infects and kills helper T-cells which act as the host cell. Without helper T-cells other cells in the immune response cannot be activated.
- HIV replicates quickly and the infected may experience sever flu-like symptoms, after the symptoms have subsided the replication drops.
- People with HIV are classed with having AIDS when there helper T-cell count drops below a certain level and this time period is usually around 10 years.
- The intial symptoms of AIDS include minor infections of mucous membranes and recurring respiratory infections.
- During the later stages of AIDS patients can develop infections such as toxoplasmosis of the brain and candidiasis of the respiratory system and its these infections that kill AIDS patients, not the HIV itself
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- The virus particle has a spherical strucuture and is made up of a core containing genetic material (RNA) and some proteins (including the enzyme reverse transcriptase), it has an outer coating protein called a capsid and a extra outer layer which is called an envelope. Sticking out of the envelope is lots of attachment proteins which allow the HIV to attach to the helper T-cell
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- HIV can only reproduce inside the cells of the organism it has infected. It uses the organelles such as ribosomes inside the helper T-cell to replicate itself
- The attachment protein attaches to a receptor molecule on the cell. The capsid is then released into the cell, where it uncoats and releases the RNA. Inside the cell reverse transcriptase is used to make a complementary strand of DNA from the RNA. The host cell enzymes are used to make viral proteins from the viral DNA found within the human DNA. The viral proteins are made into new virus which can go and infect other cells.
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