IMMUNITY

IMMUNITY

1. Defence mechanisms

   • Non-specific
     • Physical barrier
       • Skin, mucus, HCl
     • Phagocytosis
   • Specific
     • Cell-mediated
       • T-Cells
     • Humoral
       • B-Cells

2. Specific and non-specific mechanisms

   • Non-specific
     • Don't distinguish between pathogens
       • Act immediately
   • Specific
     • Do distinguish between pathogens
       • Slower but provide long-lasting immunity

3. How lymphocytes recognise their own cells

   • There are around 10 million different lymphocytes,each capable of recognising a different chemical shape
     • ie having a protein on it's surface that is complementary to one of the proteins on the pathogen
     • 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 be the mother and in particular the placenta
         • Lymphocytes will therefore collide almost exclusively with the body's own cells
           • Some of the lymphocytes will have receptors that exactly fit those of the body's own cells
             • These lymphocytes either die or are supressed
               • The only remaining lymphocytes are those that fit foreign material and therefore only respond to foreign material

4. Phagocytosis

   • Large particles - such as bacteria - are too big to cross cell-surface membranes by diffusion or active transport
     • Instead they are engulfed by cells in the form of vesicles
   • The phagocyte is attracted to the pathogen by chemoattractants. It moves towards the pathogen along a concentration gradient
     • The phagocyte binds to the pathogen
       • Lysosomes within the phagocyte migrate towards the phagosome formed by engulfing the bacterium
         • The lysosomes release their lytic enzymes into the phagosome, where they break down the bacterium
           • The breakdown products of the bacterium are absorbed by the phagocyte
   • Causes inflammation at the site of infection
     • The swollen area contains dead pathogens and phagocytes (pus)
     • Inflammation is the result of the release of histamine - causes dilation of the blood vessels
       • This speeds up the delivery of phagocytes to the site of infection

5. Cell-mediated immunity (T cells)

   • T lymphocytes mature in the thymus gland
     • They can distinguish invader cells from normal cells because:
       • After phagocytosis, the phagocytes present some of the antigens of the pathogen on their own cell surface membrane
         • Body cells invaded bt a virus present some of the viral antigens on their own cell-surface membranes as a sign of distress
           • Cancer cells also present antigens on their cell surface membrane
             • These cells are ANTIGEN-PRESENTING CELLS
   • T lymphocytes will only respond to antigens attached to a body cell
     • Pathogens invade body cells or are taken in by phagocytes
       • The phagocyte 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 cloned T cells:
               • Develop into memory cells that enable a rapid response to future infections
               • Stimulate phagocytes to engulf pathogens by phagocytosis
               • Stimulate B cells to divide
               • Kill infected cells
             • The role of the receptors on the T cells is important
               • The receptors on each T cell respond to a single antigen
                 • There are a vast number of different types of T cell, all responding to a different antigen
   • How T cells kill infected cells
     • Produce a protein that makes a hole in the cell surface membrane
       • The holes mean the cell becomes freely permeable to all substances and dies as a result
         • The action of T cells is most effective against viruses - viruses live and reproduce inside living cells

6. Humoral immunity (B cells(

   • Involves antibodies (soluble in tissue fluid and blood aka humor)
   • There are 10 million types of B cells, each type producing a different antibody that responds to one specific antigen
     • The specific antibody attaches to the complementary antigen and the B cell that produces this antibody divides by mitosis
       • This forms clones of identical B cells that all produce the antibody that is specific to the antigen of the foreign cell
         • One pathogen has many different proteins on its surface - all of which act as antigens
           • Toxins produced by a pathogen also act as antigens
             • Therefore many different B cells make clones. The clones develop into one of two types of cell:
               • Memory cells
                 • Live for decades
                 • Circulate in the blood and tissue fluid
                 • When they encounter the same antigen again, they divide rapidly and develop into plasma cells and more memory cells
                   • Plasma cells produce the antibodies needed to destroy the pathogen
                     • SECONDARY IMMUNE RESPONSE
                       • More rapid and of greater intensity than primary response
                       • New infection is repulsed before it can cause any harm
                         • Role of B cells
                           • Surface antigens on the invading pathogen are taken up by B cells
                             • The B cells process the antigens and present them on their surfaces
                               • T helper cells attach to the processed antigens on the B cells and activate them
                                 • B cells are now activated to divide by mitosis to give a clone of plasma cells
                                   • The cloned plasma cells produce antibodies that exactly fir the antigens on the pathogens surface
                                     • The antibodies attach to the antigens on the pathogen and destroy them
                                       • Some B cells develop into memory cells that can quickly respond to future infections
               • Plasma cells
                 • Secrete antibodies
                   • Antibodies destroy pathogens and toxins
                 • Last for a few days
                 • Responsible for the immediate defense of the body against infection
                   • PRIMARY IMMUNE RESPONSE
   • Antigenic variability
     • Some pathogens have many different strains, each with different antigens. These strains, and so the anitgens, are always changing
       • Any subsequent infections are therefore likely to be caused by a different variety of the pathogen
         • Their antigens will not correspond to the antibodies of the memory cells formed in the first infection
           • The body acts as if each infection is a new one and the reaction is much slower - in which time we develop symptoms