Cell Recognition and the Immune System

Cell Recognition and the Immune System

1. Defence Mechanisms

   • The body recognises own cells from foreign cells due to proteins on the cell surface that are specific to each cell
     • Implications for transplants as the body recognises it as non-self
       • immunosuppressant drugs and closely matched donor tissue are used
     • A part of an organism that stimulates an immune response = antigen
     • When feotus is developing, lymphocytes only collide with body material, so recognise as own
   • Non-Specific
     • Physical barrier
       • e.g skin, earwax, eyelashes
     • Phagocytosis
       • Phagocytes (white blood cells) destroy pathogens by engulfing them
       • 1) Chemical products from pathogens attract phagocytes
         • 2) Phagocytes membrane receptors attach to surface of pathogen
           • 3) Engulf the pathogen to form vesicle called phagosome
             • 4) Lysosomes fuse with vesicle releasing lysozymes that hydrolyse the pathogens cell walls
               • 5) Soluble products are absorbed into the cytoplasm
   • Specific
     • Cell-Mediated response
       • Involves T lymphocytes that mature in thymus gland
       • T lymphocytes recognise infected cells as they will present the antigens on their surface (cancer cells will also have different antigens)
         • (Antigen presenting cells)
         • Receptors on T-cells are specific to one antigen only so there are many different versions
         • 1) Specific T helper cell fits exactly into the antigen displayed on phagocytes membrane
           • 2) Attachment activates T cell to :
             • Stimulate phagocytes to engulf pathogens
             • Activate cytotoxic T cells
               • Kill infected cells by producing a protein (perforin) making holes in cell membrane
                 • Cells become fully permeable and die
             • Stimulate B cells to divide and secrete antibody
             • Stimulate B cells to divide into memory cells
     • Humoral response
       • Involves B lymphocytes that mature in the bone marrow
       • 10 million different B cells all specific to one antigen each, producing one complementary antibody
       • T helper cells stimulate B cells to divide to produce the specific antibody (clonal selection)
         • B cells divide into Plasma cells (secrete antibodies) that only survive a couple of days
         • B cells divide into memory cells that live longer and when encountering same antigen can divide into plasma cells in the future for a rapid secondary response
     • Can provide long - term immunity
     • Involves lymphocytes produced by stem cells in the bone marrow

2. An infection is an interaction between a pathogen and body's defence mechanisms

3. Immunity is the ability of organisms to resist infection, by recognising foreign material

4. Antibodies

   • Proteins with specific binding sites synthesised by B cells
   • When the body is infected antibodies bind to antigens on its surface
     • Due to a complementary shape
   • Made up of 4 polypeptide chains
   • Binding site is different on each antibody so called the variable region
   • Antibodies cause agglutination making clumps of cells so phagocytes can engulf them easily
     • they also act as markers to attract phagocytes
   • Monoclonal Antibodies
     • Antibodies are isolated and cloned outside of the body
     • Used in medicine and science due to the specific shape
     • Target medication to specific cells by attaching drug to antibody e.g cancer cells
     • Non-toxic and highly specific
     • Indirect monoclonal antibody therapy = attaching a radio-active / toxic drug to monoclonal antibody which then kills cells
     • Can be used in smaller doses and so cheaper
     • Less side effects
     • Medical Diagnosis
       • Some diseases produce a protein and so antibodies interact with them
       • Can compare levels of protein in blood to a person without the disease
       • Pregnancy testing (ELISA test)
         • 1) Placenta produces hCG hormone found in a mothers urine
         • 2) Monoclonal antibodies present on the test strip are linked to coloured particles
           • Amount of colour present is proportional to the amount of antigen present
         • 3) The complex moves down the strip until strapped by another antibody, creating a coloured line
         • When doing proper ELISA tests you should wash between each attachment to wash away any non-bound substance and therefore avoid a false positive result
     • Production of monoclonal antibodies against cancer involves the use of mice and deliberately inducing cancer on them
     • They have been associated with the deaths in some treatments (multiple sclerosis)
     • Drug trials have been known to cause multiple organ failure as T cells over produce chemicals

5. Vaccination

   • Passive Immunity
     • Introduction of antibodies into human from outside
     • No antibodies, memory cells are being produced
     • Not long lasting immuntiy
   • Active immunity
     • Stimulating the production of antibodies by the individuals own immune system
     • Natural Active Immunity
       • individuals become infected under normal circumstances
     • Artificial Active Immunity
       • Inducing an immune response without suffering the symptoms
       • Forms the basis of vaccination
   • Introduction of the appropriate disease antigens into the body stimulating an immune response
     • Whole bacteria (dead) or certain parts containing the antigens are inserted
     • Memory cells are produced ensuring a more immediate response to future infection
   • used as a precautionary measure to prevent diseases, not treatment
   • Herd Immunity
     • Large amount of the population have been vaccinated so makes it difficult for pathogen to spread
     • When most of the population are immune it makes it highly unlikely that an infected person will meet a person thats not immune
   • Vaccination may not eliminate a disease
     • Fails to induce immunity in some individuals
     • Pathogen may mutate frequently
       • Antigenic variability = new antigens are no longer recognised
     • Cannot vaccinate against all varieties
     • Religious / ethical / medical reasons mean individuals cannot be vaccinated
   • Vaccines have harmful side effects
   • Often involves the use of animals
   • To be effective most of the population should be vaccinated

6. HIV

   • Causes accquired immune deficiency syndrome (AIDS)
     • Only leads to AIDS many years after infection
   • Lipid envelope on the outside has peg-like structures
   • Capsid inside contains RNA and enzymes
   • HIV Replication
     • 1) HIV cannot replicate itself and so uses a host cell
     • 2) HIV enters via the bloodstream
     • 3) A protein on HIV molecule binds to CD4 protein on T helper cells
     • 4) The protein capsid and the cell membrane fuse together allowing RNA and viral enzymes to enter
     • 5) HIV transcriptase converts the RNA into DNA
     • 6) Newly made DNA is combined with the T helper cells DNA
     • 7) The T - helper cell DNA creates viral mRNA to make viral proteins
     • 8) newly formed HIV molecule buds off forming a lipid envelope from the cell surface membrane
   • HIV kills T-helper cells
     • B-cells won't be stimulated to produce antibodies
     • T toxic cells aren't stimulated to kill infected cells
     • Cells will be susceptible to other pathogens
       • People die from this
   • Antibiotics are ineffective as they cannot reach the viruses due to replication within the host cell

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