Immune System Flashcards
(43 cards)
What are antigens?
Antigens are molecules (usually foreign proteins) found on the surface of cells/viruses, which may cause an immune response when detected by the body.
Do all antigens cause an immune response?
No, as all of your body cells also have antigens. The immune system will only respond to antigens which are foreign (except in the case of autoimmune diseases).
What are pathogens?
Pathogens are microorganisms which cause disease (e.g. bacteria, viruses, fungi).
Which types of antigens will the immune system respond to?
The immune system will only respond to foreign antigens (i.e. those found on pathogens, abnormal body cells (e.g. cancer or transplanted cells), or those which make up toxins, which are sometimes released by bacteria (the toxins themselves are antigens, they don’t have antigens on their surface).
What is antigenic variation?
Antigenic variation is when a pathogen changes the antigens on its surface.
What is the effect of antigenic variation on immunity?
Antigenic variation means that if you are infected with the same pathogen multiple times, a primary response (which is slower and weaker), may still be triggered rather than a more effective secondary response. This is because the antigens on the surface of the pathogen have changed since first exposure, so the memory B and T cells produced will no longer be specific to the pathogen, so won’t work on it. This is why new flu vaccines have to be made every year because the influenza virus shows antigenic variation.
Describe the process of phagocytosis.
Phagocytosis is carried out by phagocytes (macrophages or neutrophils). When a phagocyte recognises a foreign antigen, the cytoplasm of the phagocyte moves around the pathogen, engulfing it. The pathogen becomes contained within a phagocytic vesicle and the membranes of the phagocytic vesicle and the lysosome fuse, allowing lysozymes to destroy the pathogen, which is digested by the cell. The phagocyte then presents the antigens on its surface to activate other immune system cells.
Describe the cellular response (cell-mediated immune response).
A phagocyte engulfs and destroys a pathogen. The antigens from that pathogen form complexes with self proteins of the phagocyte, forming non-self complexes. These non-self complexes, containing the antigens of the pathogen, are presented on the surface of the antigen-presenting cells. This triggers T-helper cells with specific receptors complementary to the antigens on the APC to bind to the APC. The helper T cell then releases cytokines, which stimulate/activate other B and T cells. T cells divide to form more helper T cells and cytotoxic T cells, and some memory T cells. Cytokines trigger the clonal selection of B cells, leading to a humoral response.
What do cytotoxic T cells do?
Cytotoxic T cells specific to a particular antigen bind to infected cells and release Perforin, a chemical which makes holes in infected cells’ membranes, causing the lysis and death of the infected cell.
Describe the process of the humoral response.
B cells contain many different protein receptors on them, one of which will be complementary to an antigen. An antigen binds to the B cell and the B cell processes and presents the antigens on its surface (it becomes an antigen presenting cell). The B cell is then activated by cytokines from T helper cells, and the B cell divides rapidly by mitosis (clonal selection) to give many clones of plasma cells. The cloned plasma cells produce and secrete the specific antibody that exactly fits the antigen on the pathogen surface. The antibodies can then bind to antigens on pathogens and signals other immune cells to destroy them, also causes agglutination. Some B cells remain in the blood as memory cells, which cause a secondary response upon another infection of the same strain of pathogen.
What are the main roles of antibodies?
Antibodies can bind to the antigens of pathogens, acting as markers to signal phagocytes to engulf the pathogen. Antibodies can also cause the agglutination of pathogens (they bind to multiple pathogens, causing them to clump together, rendering the pathogen inactive). This triggers phagocytosis. Antibodies can also neutralise toxins produced by bacteria (they are antitoxins). They can also neutralise pathogens by binding to all of their antigens and preventing the pathogen from infecting any cells.
What are antibodies?
Antibodies are proteins/immunoglobulins with specific tertiary structures, giving specific binding sites complementary to a particular antigen, which are synthesised by B-cells and secreted by plasma cells.
Describe the structure of an antibody?
Antibodies are globular glycoproteins, consisting of four polypeptide chains (two light and two heavy chains). Antibodies are generally a Y shape and are held together by two disulphide bridges between the heavy chains and one between each light chain and the heavy chains. The top end of each polypeptide chain is the variable region, which is different in different antibodies, while the remainder of the antibody is the same for all (constant region). At the tips of the Y are the antigen-binding sites, which are specific to a particular antigen. When a complementary antigen binds to an antibody, an antigen-antibody complex is formed.
What is the difference between a primary and secondary response?
A primary response occurs on first exposure to an antigen, whereas the secondary response is triggered during subsequent infection. During a primary response, the immune system must undergo clonal selection and recognise the antigen for the first time. This is a fairly slow process, and so you feel sick during the response. Whereas, with a secondary response, you already have memory cells against the antigen, so your B and T cells can start dividing rapidly and producing antibodies more quickly and at a greater volume. You usually don’t feel sick when a secondary response occurs as it is quicker and stronger.
What are some examples of non-specific defences the body has?
- skin and membranes act as barriers to pathogens
- ciliates epithelial cells and goblet cells work together in the trachea to trap and destroy microbes in inhaled air
- hydrochloric acid is found in the stomach, which destroys pathogens ingested in food/drink
- sweat contains anti microbial agents
- blood clots at sites of bleeding, stopping pathogens from entering through the broken skin
- lysozymes are found in tears, which are hydrolytic enzymes which break down bacterial cell walls
What are monoclonal antibodies?
Monoclonal antibodies are antibodies with the same tertiary structure, which are produced from cloned plasma cells.
What are some uses of monoclonal antibodies?
- targeting specific cell types
- pregnancy / disease testing
- medical diagnosis - linked to ELISA tests (identify presence and concentration of an antigen or protein).
What are some ethical issues associated with the use of monoclonal antibodies?
Animals are used to produce the monoclonal antibodies, which raises animal rights issues as they are subjected to disease and can’t consent to this. Some people disagree with this.
How can monoclonal antibodies be used to target specific cell types?
Direct monoclonal antibody therapy (e.g. Herceptin as a treatment for breast cancer) uses monoclonal antibodies specific to antigens on cancerous/diseased cells and the antibodies attach to antigens on the surface of a cancer cells, preventing their uncontrolled growth. Indirect monoclonal antibody therapy attached cytotoxic drugs to monoclonal antibodies, causing them to attach to the antigens on the cancer cells. The cytotoxic drugs are released and they kill the tumour cells. This means smaller doses can be used and fewer side effects for the patient.
How can monoclonal antibodies be used in pregnancy testing?
The test strip contains three sites, the reaction site, the test site and the control site. To test for pregnancy, the end of the test strip is placed in a sample of urine, which will contain hCG if pregnant. The urine carries hCG molecules to the reaction site, where they bind to the complementary monoclonal antibodies on the test strip. These antigen-antibody complexes and unbound antibodies move up the strip by capillary action, and reach the test site. Here, they bind to fixed antibodies. This concentrated a dye bound to the monoclonal antibodies in a line, causing a positive result. To ensure the test has worked, a control site contains antibodies which bind to the unbound monoclonal antibodies, causing a second ‘control’ line. If the result is negative, then antigen-antibody complexes won’t form, but the unbound antibodies will still bind at the control site, so you will only get one line. This principle is the same regardless of what you are testing for (it can be used to test for COVID, malaria etc).
How does a direct ELISA test work?
A direct ELISA uses a single antibody that is complementary to the antigen you’re testing for. Antigens from a patient sample are bound to the inside of a well from a well plate. A detection antibody (linked to an enzyme) is added that is complementary to the antigen of interest. Any complementary antibodies will bind to the antigens on the surface of the well and be immobilised. The well is washed to remove unbound antibodies and a substrate solution is added. The enzyme breaks down the substrate and a colour change occurs, showing a positive result. The intensity of the colour change indicates the concentration of the antigen.
How does an indirect ELISA test work?
An antigen of interest in bound to the surface of a well in a well plate. Wash to remove unbound antigens. A sample of a patients blood plasma, which may contain antibodies against the antigen, is added. If there are antibodies specific to the antigens in the well plate, they will bind, forming antigen-antibody complexes which stick to the surface. Wash to remove any unbound antibodies. A secondary antibody, which has a specific enzyme attached, is added, which binds to the primary antibody. Wash to remove unbound secondary antibodies. Add a substrate solution, and the enzyme will react with the complementary substrates, causing a colour change, indicating the presence of the antibodies in the patient’s blood.
What is a primary response?
A primary immune response is triggered when an antigen enters the body for the first time. The primary response is slow because there aren’t vary many specific B-cells to make the antibodies needed, or helper-T cells to activate them. Clonal selection has to happen, which is a slow process. During this time, an infected person will show symptoms of disease. The primary response is also less strong than a secondary response (concentration of antibodies in the blood is lower). Memory B-cells and T-helper cells are also produced.
What is a secondary response?
A secondary response is triggered when a foreign antigen enters the body for a subsequent time, for example, upon a second infection or a first infection after vaccination. In this response, memory B and T-helper cells already exist against this specific antigen, so the secondary response is much faster and stronger. Infected people often won’t have symptoms when they have a secondary response, as the pathogens are destroyed before symptoms can develop.
