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 DEFENCE AGAINST DISEASE

DEFENCE AGAINST INFECTIOUS DISEASE

Skin and Mucous Membranes

  • Skin consists of the epidermis (an outer later of dead, cornified cells on top of a layer of living cells).
  • These are produced continually from a layer of dividing cells at the
  • Below this the dermis made up of blood vessels, glands, hair follicles and connective tissues.
  • The dead cells are impregnated with keratin which makes them waterproof (that is why they die).
  • The waterproof layer prevent microbes from entering the skin
  • The sweat glands sweat and oils which permit the skin flora to grow (a mixture of bacteria and yeast)
  • The skin flora occupies a niche on the skin that prevents pathogens from growing and invading.
  • Mucus membranes cover the inner compartments of the body (digestive tract, lungs, and genital tracts).
  • It also consists of cells that are being constantly renewed from below but they are not waterproof.
  • These membranes secrete mucus (mucopolysaccharide), and sugars.
  • A commensal flora grows there too feeding on the secretions.
  • Mucus also traps materials in the airways.
  • HCl is secreted from the stomach mucosa, which acts as an anti-microbial agent.
  • The humidity permits the leucocytes (white blood cells) to leave the blood capillaries and patrol over the surface of the membrane engulfing pathogens by phagocytosis.
  • Secretions of lysozyme (an enzyme which digests bacterial cell walls) are also produced by glands such as tear and salivary glands.

Phagocytic leucocytes ingest disease-causing organisms in the blood and tissues

  • Revise phagocytosis
  • Leucocytes can act directly upon microbes by PHAGOCYTOSIS (Cell Mediated Immunity)
  • Phagocytes recognise and engulf foreign (non-self) material
  • Pus forms when large numbers of phagocytes die from engulfing microbes
  • The most common phagocytic cells are neutrophils
  • Macrophages also engulf and digest non-self but they are involved in activating the antibody production in lymphocytes
  • Macrophages present antigens from non-self to the lymphocyte T-cells

What is the difference between antigen and antibody?

  • Antibodies are proteins produced by B-cell lymphocytes of the immune system
  • They recognise non-self molecules called antigens
  • They call up phagocytes
  • They have two or more antigen binding sites
  • They are very specific: One antibody recognises one antigen molecule
  • The different binding sites can bind to two or more cells clumping them together = AGGLUTINATION.
  • Antigens are molecules found on the surface of non-self cells (e.g. microbes infecting the body)
  • They are usually complex molecules (e.g. glycoproteins)

Antibody production

  • Each type of antibody is produced by a specific lymphocyte B-cell
  • B-cells are produced by the red bone marrow
  • The body produces a vast range of B-cells capable of producing different antibody molecules
  • Early on in development the body learns to recognise the difference between self (belonging to the body) and non-self (foreign material)
  • Only non-self recognising lymphocytes are retained
  • Macrophages capture pathogens and present their antigens to helper T-cell lymphocytes
  • Helper T-cell lymphocytes stimulate the appropriate B-cells to multiply forming a clone
  • These B-cells produce the antibodies to fight that particular microbe
  • Antibodies are found in the blood plasma (Humoral Immunity)
  • When the infection is overcome memory cells remain so the immune response is faster the second time the body is infected (natural immunity)
  • The body can be stimulated into producing the memory cells by vaccination/immunisation (artificial immunity)

The effects of HIV on the immune system

  • HIV (Human Immunodeficiency Virus) is the virus that causes AIDS (Acquired Immune Deficiency Syndrome)
  • HIV infects the body through transfer of body fluids (blood, blood products, semen) or across the placenta
  • HIV infects one type of the T-cell lymphocyte, helper T-cells.
  • Antibody production cannot be stimulated
  • The immune system fails to respond to an infection by certain bacteria (e.g. pneumonia) and fungi (e.g. Candida) which are normally easily resisted
  • The disease may take 8-10 years to reveal itself
  • This gives time for several cross infections
  • There is no vaccine yet
  • Drugs may stop the disease progressing but do not cure it

 

© Paul Billiet 2004