defence against pathogens

• Pathogens need to enter an organism in order to cause disease. Animals and plants have evolved defences to protect themselves from pathogens gaining entry.
• The skin acts as a physical barrier, blocking pathogens from entering the body.

  It also acts as a chemical barrier by producing chemicals that are antimicrobial (which destroy or slow the growth of microorganisms) and can lower pH, inhibiting the growth of pathogens.
• Skin cells secrete fatty acids, such as oleic acid, that can kill some bacteria. Fatty acids also lower the pH of the skin, creating an acidic environment that is difficult for pathogens to colonise.

  Skin cells also secrete lysozyme, an enzyme which catalyses the breakdown of carbohydrates in the cell walls of some bacteria.
• Mucous membranes protect body openings that are exposed to the environment (such as the mouth, nostrils, ears, genitals and anus). Some membranes secrete mucus - a sticky substance that traps pathogens and contains antimicrobial enzymes.
• If you breathe in air that contains pathogens, most of them will be trapped in mucus lining the lung epithelium (the outer layer of cells in the passages to the lungs). These cells also have cilia (hair-like structures) that beat and move the mucus up the trachea to the throat and mouth, where it's removed
• A blood clot is a mesh of protein (fibrin) fires. Blood clots plug wounds to prevent pathogen entry and blood loss. They're formed by a series of chemical reactions that take place when platelets (fragments of cells in the blood) are exposed to damaged blood vessels.
• The signs of inflammation include swelling, pain, heat and redness. It can be triggered by tissue damage - the damaged tissue releases molecules which increase the permeability of the blood vessels, so they start to leak fluid into the surrounding area. This causes swelling and helps to isolate any pathogens that may have entered the damaged tissue. The molecules also cause vasodilation (widening of the blood vessels), which increases blood flow to the affected area. This makes the area hot and brings white blood cells to the area to fight off any pathogens that may be present.
• The skin is able to repair itself in the event of injury and re-form a barrier against pathogen entry. The surface is repaired by the outer layer of skin cells dividing and migrating to the edges of the wound. The tissue below the wound then contracts to bring the edges of the wound closer together. It is repaired using collagen fibres - too many collagen fibres and you'll end up with a scar.
• Expulsive reflexes include coughing and sneezing. A sneeze happens when the mucous membranes in the nostrils are irritated by things such as dust or dirt. A cough stems from irritation in the respiratory tract. Both coughing and sneezing are an attempt to expel foreign objects, including pathogens, from the body. They happen automatically.
• Most plant leaves and stems have a waxy cuticle, which provides a physical barrier against pathogen entry. It may also stop water collecting on the leaf, which could reduce the risk of infection by pathogens that are transferred between plants in water.
• Plant cells themselves are surrounded by cell walls. These form

  a physical barrier against pathogens that make it past the waxy cuticle.

  Plants also produce a polysaccharide called callose. Callose gets deposited between plant cell walls and plasma membranes during times of stress, e.g. pathogen invasion. Callose deposition may make it harder for pathogens to enter cells. Callose deposition at the plasmodesmata (small channels in the cell walls) may limit the spread of viruses between cells
• physical plant defences

  A) callose deposition

  B) plasmodesmata

  C) cell wall

  
• Plants don't just rely on physical defences. They also produce antimicrobial chemicals (including antibiotics) which kill pathogens or inhibit their growth.

  Other chemicals secreted by plants are toxic to insects - this reduces the amount of insect-feeding on plants and therefore reduces the risk of infection by plant viruses carried by insect vectors.
• e.g

  Some plants produce chemicals called saponins. These are thought to destroy the cell membranes of fungi and other pathogens.

  Plants also produce chemicals called phytoalexins, which inhibit the growth of fungi and other pathogens.