A-LEVEL BIOLOGY AQA NOTES
cell recognition and the immune system
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Components of the Immune System
- Antigens are any part of an organism/substance which is recognised as foreign by the immune system and goes on to trigger an immune response.
- Antibodies are a protein produced by lymphocytes in response to the presence of the corresponding antigen.
- Antibodies agglutinate pathogens by forming antigen-antibody complexes, leading to phagocytosis & neutralise toxins.
- Antigen from the pathogen is displayed on the cell surface of body cells or phagocytes after phagocytosis
- T cells with the correct specific receptor bind with the antigen and are activated
- They divide by mitosis (clonal expansion) and differentiate into T helper, cytotoxic and memory cells.
- The humoral response is best at fighting pathogens which are free in the bodily fluids
- Free antigen binds to a complementary B cell receptor, activating the B cell (clonal selection)
- The pathogen is endocytosed, and the antigen presented on the plasma membrane
- T helper cell binds to the presented antigen and stimulates the B cell to divide by mitosis (clonal expansion)
- The B cell differentiates to plasma and memory cells
Primary & Secondary Immune Response
- The primary immune response is when a pathogen infects the body for the first time the initial immune response is slow
- The secondary immune response is a more rapid and vigorous response caused by a second or subsequent infection by the same pathogens. This is due to the presence of memory cells.
- Vaccination is the introduction into the body of a vaccine containing disease antigens, by injection or mouth, in order to induce artificial immunity
- Vaccines work by injecting weakened/dead pathogens into the body to stimulate an immune response, to form memory cells against the specific antigen, which destroy the pathogen quickly upon infection.
- Herd immunity is when the vaccination of a significant proportion of the population provides protection for individuals who have not developed immunity
- Pathogen may mutate so that its antigens change suddenly (antigenic variability) So the vaccine is now ineffective to the new antigens.
- Ethical considerations: side effects, financial cost, right to choose, animal testing of vaccines, human trials
- Active immunity occurs when specific antibodies are produced by the individual’s own immune system
- Passive immunity occurs when specific antibodies are introduced to the individual from an outside source.
Direct contact with pathogen
Antibodies through breastmilk
Injection of antibodies
Human Immunodeficiency Virus (HIV)
- HIV replicates in T helper cells, causing the symptoms of AIDs due the to decreased cell count. The compromised immune system leads to the risk of serious infections
- Antibiotics kill bacteria by targeting bacteria specific enzymes or organelles. They are ineffective against viruses due to the virus using the host’s machinery.
Using Monoclonal Antibodies
- Drugs can be attached to monoclonal antibodies, in order to ensure the delivery of the drug to specific cell types e.g. cytotoxic drug to a cancer cell
- Disease diagnosis can occur by testing for the presence of specific pathogen antibodies in the blood.
- Monoclonal antibodies are also used for pregnancy testing
- Measurement & diagnosis of antigen occur in the ELISA test where different monoclonal antibodies are bound to the surface of a well. They attach to antigen present in a sample, allowing the attachment of a detection antibody. An enzyme attached to the detection antibody digests a substrate, which is added, causing a colour change. The colour intensity corresponds to the amount of the antigen present in the sample
- Ethical considerations: treatment may cause death (risky), use of animals for production may cause harm, human trials