6.1-6.4 IMMUNOLOGY

6.1-6.4 IMMUNOLOGY

1. Non-specific mechanisms

   • Physical barriers
     • Protective covering
       • The skin
     • Epithelia covered in mucus
       • Lungs
       • Pathogens stick to mucus
         • Transported away by cilia
           • Swallowed into stomach
     • Hydrochloric acid in stomach
       • low pH
         • Most of the enzymes of the pathogen denature
   • Phagocytosis
     • The phagocyte is attracted to the pathogen by chemoattractants
       • It moves towards the pathogen along a concentration gradient
         • The phagocyte binds to the pathogen
           • They engulf the pathogen to form a phagosome
             • Lysosomes move towards the vesicle and fuse with it
               • The lysosomes release lytic enzymes into the phagosome
                 • The enzymes break down the pathogen
                   • The breakdown products are absorbed by the phagocyte
   • Do not distinguish between one type of pathogen and another
     • They respond to all pathogens in the same way
       • They act immediatley

2. Specific mechanisms

   • Cell-mediated response
     • T cells
       • Mature in the Thymus gland
       • Respond to foreign material inside body cells
       • Respond to own cells altered by viruses or cancer and to transplanted tissues
     • Stages in T lymphocyte response
       • Pathogens invade body cells or are taken in by phagocytes
         • The phagocyte/cancer cell/infected body cell places antigens from the pathogen on its cell-surface membrane
           • Receptors on certain T helper cells fit exactly onto these antigens
             • This activates other T cells to divide rapidly by mitosis and form a clone
               • The clones T cells
                 • develop into memory cells that enable a rapid response to future infections by the same pathogen
                 • stimulate phagocytes to engulf pathogens by phagocytosis
                 • stimulate B cells to divide
                 • kill infected cells
     • How T cell kill infected cells
       • Produce a protein that makes holes in the cell surface membrane
         • The cell becomes freely permeable to all substances and dies as a result
   • Humoral response
     • B cells
       • Mature in the bone marrow
       • Produce antibodies
       • Respond to foreign material outside body cells
       • Respond to bacteria and viruses
     • B cell immunity
       • The surface antigens of the invading pathogens are taken up by B cells
       • The B cells process the antigens and present them on their surfaces
       • T helped cells attach to the processed antigens on the B cells thereby activating them
       • he B cells are activated and divide by mitosis to give a clone of plasma cells
       • The clones plasma cells produce antibodies that exactly fit the antigens on the pathogen's surface
       • The antibodies attach to antigens on the pathogen and destroy them - this is a primary immune response
       • Some B cells develop into memory cells. They respond to future infections by dividing rapidly and developing into plasma cells.
         • This is a secondary immune response.
       • Plasma cells
         • Short life span
           • Primary response
       • Memory cells
         • Long life span
           • Secondary response
   • Distinguish between different pathogens
   • Less rapid
   • Provide long lasting immunity
   • Involve lymphocytes

3. How lymphocytes recognise their own cells

   • There are around 10 million different lymphocytes each capable of recognising a different chemical shape
     • In the fetus, these lymphocytes are constantly colliding with other cells
       • Infection in the fetus is rare because it is protected from the outside world by the mother and placenta
         • Lymphocytes collide almost exclusively with self material
           • Some lymphocytes will have receptors that exactly fit the body's cells
             • These lymphocytes die or are supressed
               • The only remaining lymphocytes fit non-self material

4. Antigens

   • Any part of an organism that is recognised as non-self
     • Stimulates an immune response
   • Usually proteins
     • Part of cell-surface membranes or walls
       • Micro-organisms
       • Diseased cells

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