primary defences against disease

• inflammation - swelling + redness of tissue caused by infection
• mucous membrane - specialised epithelial tissue that is covered by mucus
• primary defences - those that prevent pathogens entering the body
• Primary defences : Physical Barriers

  • Skin – this acts as a physical barrier all the way around the body to prevent the entry of bacteria. The skin produces sebum, an oily substance that inhibits pathogen growth.
  • Skin Flora – the skin naturally has a large population of natural health bacteria. These outcompete pathogens for surface space.
  • Mucous Membranes – gas exchange surfaces (e.g the trachea) have a layer of mucus that trap pathogens. The mucus contains the enzymes lysozymes which destroy bacteria
  • Lysozyme - in tears, urine and in stomach acid also destroys pathogens
• Blood clotting and skin repair 3

  1st stage - is the deposition of fibrous collagen under the scab

  →stem cells in the epidermis then divide by mitosis , which then migrate to edges of the cut and differentiate to form new skin

  • new blood vessels grow to supply oxygen and nutrients to new tissues

  → the tissues contract to help draw the edges of a cut together so that the repair can be completed

  as new skin is completed, the scab will be released
• Blood clotting and skin repair 2

  • blood clotting is a complex process, as it is important to prevent clots forming in the blood vessels where they aren’t needed

  →involves Ca 2+ and at least 12 factors - clotting factors

  many of the clotting factors are released from platelets and from the damaged tissue

  these factors activate an enzyme cascade

  • once clot has formed, it begins to dry out and form a scab; scab shrinks as it dries,drawing the sides of a cut together

  →this makes a temporary seal under which skin is repaired
• Blood clotting and skin repair

  skin is only protective if complete

  • abrasions or laceration damage the skin and open body to infection

  →when this happens the body must prevent excess blood loss by forming a clot, forming a temporary seal to prevent infection and repair skin
• Figure 1 blood clotting
• Mucous Membranes

  certain substances, such as oxygen and nutrients in our blood, must enter our blood

  • the exchange surfaces where this occurs is thinner and are less well protected from pathogens

  → the air and food that we take in from our environment may harbour microorganisms; therefore airways, lungs and digestive systems are at risk of infections

  • mucous membranes are also found in gut, genital areas, anus, ears and nose
• the skin

  the body’s main primary defence is the skin

  • the outer layer of the skin is called the epidermis, and it consists of layers of cells
  • most of the cells are called keratinocytes
  • these cells are produced by mitosis at the base of the epidermis ; then migrate out to skin’s surface
  • as they migrate they dry out and replaced by the protein keratin; this is called keratinisation, lasting about 30 days

  →by the time they reach the surface, they are no longer alive as the keratinised layer of dead cells acts as an effective barrier to pathogens and eventually the dead cells slough off
• mucous membranes (goblet cells)

  • the epithelial layer contains goblet cells which secrete mucus
  • there are also mucus-secreting glands under the epithelium

  → in the airways, the mucus lines the passages and traps any pathogens that may be in air
• mucous membrane (ciliated cells)

  • there are also ciliated cells , which are tiny hair-like organelles

  → they move in a coordinated fashion to waft the layer of mucus along and up the trachea which it can eneter the oesophagus

  →it is swallowed and passes down the digestive system (most pathogens are killed in the digestive system by the acidity of the stomach pH 1-2) ; this denatures the pathogen’s enzymes
• Coughing and sneezing

  areas that are prone to attack are sensitive

  • they respond to the irritation that may be caused by the presence of the microorganisms or toxins that they release
  • these reflexes include coughing, sneezing and vomiting

  → in a cough or sneeze the sudden expulsion of air will carry with it the microorganisms causing the irritation
• Inflammation

  one of the signs of an infected tissue is inflammation

  • the presence of microorganism in the tissue is detected by specialised cells called mast cells; these cells release a cell signalling subsance called histamine

  → histamine has a range of effects on surrounding tissue, which act to help combat the infection
• inflammation 2

  the main effect is to cause vasolidation and make the capillary walls more permeable to white blood cells and some protein

  → blood plasma and phagocytic white blood cells leave the blood and enter the tissue fluid ; leading to increased production of tissue fluid which causes swelling (oedema)

  excess tissue fluid is drained into the lymphatic system where lymphocytes are stored; leading to pathogens coming into contact with the lymphocytes and initiating specifc immune responses
• Other primary defences

  • the eyes are protected by antibodies and enzymes in the tear fluid
  • the ear canal is lined by wax, which traps pathogens
  • the female reproductive system is protected by a mucus plug in the cervix and by maintaining relatively acidic conditions in the vagina