Pack 6 - Cell recognition and immune system Part 1

Pack 6 - Cell recognition and immune system Part 1

1. Important definitions:

   • A pathogen is an organism that causes a disease
   • An antigen is the protein shape on the outside of a cell that stimulates an immune response
   • An antibody is a large y-shaped protein molecule that has a specific shape, complimentary to that of one particular antigen

2. Cell identification

   • All types of cells have specific molecules on their surface that can be used to identify them
     • This allows the immune system to identify cells as being self or non-self

3. Defence mechanisms:

   • Non-specific mechanisms
     • Primary defences (barriers to pathogen entry)
       • 1. The skin
         • The cells of the epidermis
       • 2. Mucous membranes
         • Goblet cells produce sticky mucus which traps pathogens
       • 3. HCl in the stomach
         • This denatures proteins/enzymes of pathogens and kills them
       • 4. Tears in the eyes
         • Contains lysozymes this hydrolyses peptidoglycan in bacterial cells
       • 5. Wax in ear canals
         • Traps pathogens
     • Phagocytosis
       • When physical barriers are breached by pathogens the non-specific immune response takes place
         • Pathogens or infected body cells release a chemical alarm causing the dilation of local blood vessels causing the area to become warm and red
           • The permeability of the capillaries also increases causing swelling
             • This means phagocytic white blood cells move to the area
       • REMEMBER
         • 1. Phagocytes are attracted by chemicals produced by pathogens
           • 2. Phagocyte attaches to pathogen
             • 3. It engulfs the pathogen to form vesicles called phagosomes
               • 4. Lysosomes in phagocytes fuse with phagosomes
                 • 5. Hydrolytic enzymes released from lysosomes hydrolyse pathogens
                   • 6. Soluble products of digestion are absorbed into the cytoplasm of phagocytes
   • Specific Immune Response
     • This is a response to detection of pathogens or antigens that involves white blood cells called B lymphocytes and T lymphocytes
       • B cells and T cells
         • B lymphocytes originate and mature in the Bone marrow
         • T lymphocytes originate in the bone marrow and mature in the thymus
         • B and T cells have specialised receptor molecules on their cell surface membranes
           • Each cell is capable of recognising one specific shape of antigen
             • Each has different protein receptor molecules on its surface complimentary to the shape of one specific antigen
               • There are not many lymphocytes specific to one antigen
         • Clonal Selection is:
           • The lymphocyte with complimentary protein receptors to those of the pathogen/antigen will be "selected" and stimulated to divide to build up numbers to a level where they can be effective in destroying the pathogen
     • Cell mediated immunity - T cells
       • T cells respond to an organisms own cells that have been infected by a pathogen
       • 1. Phagocytes and infected body cells place some of the pathogen's antigens on their cell surface - called antigen presentation
         • 2. The helper T cells with the specific receptor for the antigen come into contact with the infected cells and the receptor on the T helper cells binds with the antigen, this is the selection of the correct T lymphocyte
           • 3. This stimulates the selected T cell to divide rapidly by mitosis
             • 4. These cloned T helper cells will:
               • Form T memory cells
               • Stimulate phagocytosis
               • Stimulate B cells to divide and produce antibodies
               • Activate cytotoxic cells
               • 5. Activated cytotoxic T cells kill infected cells by producing a protein called perforin that makes holes in the cell surface membrane
                 • This makes the CSM permeable and water and solutes enter/exit freely
                 • Ideal for killing virus'
     • Humoral Immunity - B cells
       • B cells respond to infection by producing antibodies, antibodies are proteins that are soluble in the blood and tissue fluid of the body
       • 1. An antigen enters the blood
         • 2. There will be one B cell that has an antibody on its surface whose shape fits the antigen, the shapes of the antibody and antigen are complementary
           • 3. The specific B cells take in these antigens by endocytosis and present them on the surface of the cell
             • 4. Specific T helper cells attach and stimulate this B cell to divide by mitosis to form a clone of B cells that will produce antibodies to that specific antigen
               • This process is called clonal selection
               • 5. B cells develop into:
                 • Plasma cells
                   • These cells secrete antibodies into the blood plasma this is the primary response
                 • B memory cells
                   • These an enable an increased and more rapid response to a second exposure to infection (secondary response)

4. Multiplication of pathogens:

   • Bacteria multiply by binary fission
   • Virus multiplication:
     • They enter the host cell and use the host cell's DNA replication and protein production to produce new viral particles

5. Antibody structure

   • Made up of:
     • A constant region
     • Four polypeptide chains (two light two heavy) held together by disulphide bridges
     • A variable region, this has a specific shape due to its amino acid sequence.
       • The shape of the variable region and antigen are complementary to each other, this is the antigen binding site
     • A hinge region that enables antibodies to attach to more than one antigen
   • 4 polypeptide bonds = quaternary structure
     • Antigen binding site = tertiary structure
       • Primary structure determines how polypeptide chains fold

6. Agglutination

   • Antibodies bind to antigens at the ANTIGEN BINDING SITE at the tip of the variable region
     • This prepares pathogens for destruction
   • Pathogens clump together (loads of antibodies joining to pathogens which join together) and can no longer attack host cells
     • The clumps are engulfed by phagocytic cells

7. Responses

   • Primary Response:
     • This is the first time the immune system encounters a pathogen, the initial selection of the correct B cell and subsequent cloning take time as there is only one B cell specific to the antigen
     • Antibodies are secreted into the blood
       • Antibodies attach to and destroy pathogens and any antigens they produce
         • Individual plasma cells only survive for a few days and memory cells survive for a long time
   • Secondary response
     • This is a much greater and quicker response because of the memory cells
       • This is because the memory cells remaining in the cells increase the chance of the B cells of the correct type meeting the pathogen

8. Vaccination

   • Vaccination produces immunity to specific diseases by exposing someone to antigenic material that has been rendered harmless
   • Active immunity
     • This is produced by the stimulation of antibodies and memory cells by the body's immune system
     • Natural active immunity = response by the immune system to infection
     • Artificial active immunities = response by the immune system to vaccination
   • Passive immunity
     • Produced when antibodies are introduced from an outside source
       • Antibodies are not replaced by the body and no memory cells are produced
     • Maternal antibodies are passed to foetus from the mother across the placenta
       • Anti-venom given to snake-bite victims
   • Herd immunity
     • When a large proportion of the population is vaccinated it is hard for the pathogen to spread
   • Ethics
     • Development involves animals
     • Is it fair to make the vaccine compulsory in order to be effective?
     • Should expensive vaccines continue when a diseases is nearly eradicated

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