Non-specific Animal Defences Against Pathogens Flashcards
What are non-specific defences
Primary non-specific defences against pathogens are always present or activated very rapidly. This system defends against all pathogens in the same way. Mammals have a specific immune response, which is specific to each pathogen but is slower to respond.
Barriers to entry of pathogens
-skin covers the body and prevents the entry of pathogens. It has a skin flora of healthy microorganisms that outcompete pathogens for space on the body surface. The skin also produces sebum, an oily substance that inhibits the growth of pathogens.
-Many of the body tracts, including the airways of the gas exchange system, are lined by mucous membranes that secrete sticky mucus. This traps microorganisms and contains lysozymes, which destroy bacterial and fungal cell walls. Mucus also contains phagocytes, which remove remaining pathogens.
-Lysozymes in tears and urine, and the acid in the stomach, also help to prevent pathogens getting into our bodies.
-We also have expulsive reflexes. Coughs and sneezes eject pathogen-laden mucus from the gas exchange system, while vomiting and diarrhoea expel the contents of the gut along with any infective pathogens.
Blood clotting and wound repair
If you cut yourself, the skin is breached and pathogens can enter the body. The blood clots rapidly to seal the wound. When platelets come into contact with collagen in skin or the wall of the damaged blood vessel, they adhere and begin secreting several substances. Most important are:
-thromboplastin, an enzyme that triggers a cascade of reactions resulting in the formation of a blood clot.
-serotonin, which makes makes the smooth muscle in the walls of the blood vessels contract, so they narrow and reduce the supply of blood to the area.
The clot dries out, forming a hard, tough scab that keeps pathogens out, this is the first stage of wound repair. Epidermal cells below the scab start to grow sealing the wound permanently, while damaged blood vessels grow. Collagen fibres are deposited to give new tissue strength. Once the new epidermis reaches normal thickness, the scab sloughs off and the wound is healed.
Inflammatory response
Localised response to pathogens (or damage or irritants) resulting in inflammation at the sight of the wound. Inflammation is characterised by pain, heat, redness and swelling of tissue.
Mast cells are activated in damaged tissue and release chemicals called histamines and cytokines.
-Histamines make the blood vessels dilate, causing localised heat and redness. The raised temperature helps prevent pathogens reproducing.
-Histamines make blood vessel walls more leaky so blood plasma is forced out, once forced out of the blood it is known as tissue fluid. Tissue fluid causes swelling (oedema) and pain.
-cytokines attract white blood cells (phagocytes) to the site. They dispose pathogens by phagocytosis.
If an infection is widespread, the inflammatory response can cause a whole-body rash.
Non-specific defences-getting rid of pathogens
If the pathogens get into the body, the next lines of defence are adaptations to prevent them growing or destroy them
Fevers
Normal body temperature of around 37 degree Celsius is maintained by the hypothalamus in your brain. When a pathogen invades your body, cytokines stimulate your hypothalamus to reset the thermostat and your temperature goes up. This is a useful adaptation because:
-most pathogens reproduce best at or below 37 degrees. Higher temperatures inhibit pathogen reproduction
-The specific immune system works faster at higher temperatures
Phagocytosis
Phagocytes are specialised white cells that engulf and destroy pathogens. There are two main types of phagocytes-neutrophils and macrophages
Phagocytes build up at the sight of infection and attack pathogens. Sometimes you can see pus in a spot, cut or wound. Pus consists of dead neutrophils and pathogens
The stages of phagocytosis
- Pathogens produce chemicals that attract phagocytes
2.Phagocytes recognise non-human proteins on the pathogen. This is a response not to a specific type of pathogen, but simply a cell or organism that is non-self.
3.The phagocyte engulfs the pathogen and encloses it in a vacuole called a phagosome
4.The phagosome combines with a lysosome to form a phagolysosome
- Enzymes from the lysosome digest and destroy the pathogen
It usually takes a human neutrophil under 10 minutes to engulf and destroy a bacterium. Macrophages take longer but they undergo a more complex process. When a macrophage has digested a pathogen, it combines antigens from the pathogen surface membrane with special glycoproteins in the cytoplasm called the major histocompatibility complex (MHC). The MHC complex moves these pathogen antigens to the macrophages own surface membrane, becoming an antigen-presenting cell (APC). These antigens now stimulate other cells involved in the specific immune system response.
Counting blood cells
Blood smears made by spreading a single drop of blood very thinly across a slide. They are often stained to show up the nuclei of the lymphocytes , making them easier to identify. Identifying the numbers of different types of lymphocytes in a blood smear indicates if a non-specific or specific immune response is taking place.
Helpful chemicals
Phagocytes that have engulfed a pathogen produce chemicals called cytokins. Cytokinins act as cell signalling molecules, informing other phagocytes that the body is under attack and stimulating them to move to the site of infection or inflammation. Cytokines can also increase body temperature and stimulate the specific immune system.
Opsonins are chemicals that bind to pathogens and ‘tag’ them so they can be more easily recognised by phagocytes. Phagocytes have receptors on their cell membranes that bind to common opsonins, and the phagocyte then engulfs the pathogen.
