IMMUNITY

IMMUNITY

Defence mechanisms
1. Non-specific
  1. Physical barrier
    1. Skin, mucus, HCl
  2. Phagocytosis
2. Specific
  1. Cell-mediated
    1. T-Cells
  2. Humoral
    1. B-Cells
Specific and non-specific mechanisms
1. Non-specific
  1. Don't distinguish between pathogens
    1. Act immediately
2. Specific
  1. Do distinguish between pathogens
    1. Slower but provide long-lasting immunity
How lymphocytes recognise their own cells
1. There are around 10 million different lymphocytes,each capable of recognising a different chemical shape
  1. ie having a protein on it's surface that is complementary to one of the proteins on the pathogen
  2. In the fetus, these lymphocytes are constantly colliding with other cells
    1. Infection in the fetus is rare because it is protected from the outside world be the mother and in particular the placenta
      1. 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
Phagocytosis
1. Large particles - such as bacteria - are too big to cross cell-surface membranes by diffusion or active transport
  1. Instead they are engulfed by cells in the form of vesicles
2. The phagocyte is attracted to the pathogen by chemoattractants. It moves towards the pathogen along a concentration gradient
  1. The phagocyte binds to the pathogen
    1. Lysosomes within the phagocyte migrate towards the phagosome formed by engulfing the bacterium
      1. 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
3. Causes inflammation at the site of infection
  1. The swollen area contains dead pathogens and phagocytes (pus)
  2. Inflammation is the result of the release of histamine - causes dilation of the blood vessels
    1. This speeds up the delivery of phagocytes to the site of infection
Cell-mediated immunity (T cells)
1. T lymphocytes mature in the thymus gland
  1. They can distinguish invader cells from normal cells because:
    1. After phagocytosis, the phagocytes present some of the antigens of the pathogen on their own cell surface membrane
      1. 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
2. T lymphocytes will only respond to antigens attached to a body cell
  1. Pathogens invade body cells or are taken in by phagocytes
    1. The phagocyte places antigens from the pathogen on its cell surface membrane
      1. 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
3. How T cells kill infected cells
  1. Produce a protein that makes a hole in the cell surface membrane
    1. The holes mean the cell becomes freely permeable to all substances and dies as a result
      1. The action of T cells is most effective against viruses - viruses live and reproduce inside living cells
Humoral immunity (B cells(
1. Involves antibodies (soluble in tissue fluid and blood aka humor)
2. There are 10 million types of B cells, each type producing a different antibody that responds to one specific antigen
  1. The specific antibody attaches to the complementary antigen and the B cell that produces this antibody divides by mitosis
    1. This forms clones of identical B cells that all produce the antibody that is specific to the antigen of the foreign cell
      1. 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
3. Antigenic variability
  1. Some pathogens have many different strains, each with different antigens. These strains, and so the anitgens, are always changing
    1. Any subsequent infections are therefore likely to be caused by a different variety of the pathogen
      1. 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

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	Created by Ashiana Fraser about 9 years ago