The Body's Defences

go back

Pathogens

• Pathogens are microbes 
• Micro-organism or other agent that causes disease
• Not all microbes are pathogens
• Many types of pathogens that are beneficial to organisms

Vaccination

• Acts to increase the host’s resistance to an pathogen
• Administration of a material that stimulates adaptive immunity to a disease

External Non-Specific Defences

Skin

• Provides physical barrier to prevent pathogens from entering the body
• Dead skin cells are constantly sloughed off, making it difficult for invading bacteria to colonise
• Has low pH between 4.7-5.7, making it difficult for many pathogens to grow
• Chemical secretions e.g. sebum (oil) and antimicrobial peptides hamper growth of bacteria and fungi

Mucous Membrane

• Lines cavities of the respiratory (lungs, nose, mouth), urinary, reproductive and digestive tracts
• Excrete mucus that protects the underlying cell layers
• Traps particles including pathogens
• Contains anti-microbial peptides
• Mucus in respiratory tract can be swallowed and destroyed in the stomach, coughed up or sneezed out

Hairs and Cilia

• Hairs line the
  • Nose cavity
  • Ears
• Trap bacteria in mucous on the hairs/cilia
• Cilia
  • Tiny hair-like projections
  • Located on the mucous membranes of respiratory tract including the nose cavity, trachea, lungs etc
  • Beating motion moves the mucus with the trapped particles towards the throat to be swallowed or coughed up

Acids

• Stomach juices are highly acidic
  • Kills ingested bacteria and many pathogens in swallowed mucus 
  • pH: 1-2
• Vaginal acids
  • Reduces growth of micro-organisms
• Sweat is slightly acidic
  • Contains lysosome(bacteria-killing enzyme)
• Earwax is slightly acidic
  • Contains lysosome
  • Protects outer-ear from some pathogenic bacteria

4 Protective Reflexes

Sneezing

• Stimulus = when the walls of the nasal cavity are irritated by:
  • Noxious fumes
  • Dust particles that may carry organisms
• Result of stimulus = forceful expulsion of air from the lungs out through the nose and mouth
• The air expelled may carry mucus, foreign particles, irritating gases

Coughing

• Stimulus = irritation in lower respiratory tract (bronchi and bronchioles)
• Result of stimulus = expulsion of air from lungs
• The expelled air drives mucus and foreign matter up the trachea towards throat and mouth

Vomiting

• Induced by either
  • Psychological stimuli (pain, sight, smell, emotion)
  • Excess stretching of stomach (gasses)
  • Bacterial toxins
• Expulsion of bile, acid and digested/undigested matter from stomach
• Expels waste, and potentially pathogens from stomach

Diarrhoea 

• Results due to irritation of small and large intestines by either
  • Viruses
  • Bacteria
  • Protozoa
• Irritation causes increased contractions of muscles leading to rapid removal or irritant
• Faeces is watery as material does not remain in large intestine long enough for water absorption

Internal Non-Specific Defences

Phagocytosis

⁃ Process where a cell binds to the item it wants to engulf on the cell surface and draws the item inward while engulfing around it

Phagocytes

⁃ Type of cell ⁃ Phagocytes are attracted by an inflammatory response of damaged cells ⁃ Engulfs microorganisms that penetrate our external defences

Phagocytic Leukocytes

⁃ Neutrophils (Neutrophilic [stained only by neutral dyes] white blood cell) ⁃ Macrophages (develop from monocytes)

Inflammatory Response (Inflammation)

• Response to any tissue damage
• Reduces spread of pathogens, destroys them and prevents the entry of additional pathogens
• Removes damaged tissue and cell debris
• Begins repair of damaged tissue
• Signs of inflammation: REDNESS, SWELLING, HEAT AND PAIN
• Steps
  1. Mast cells release histamine, heparin and other substances into the tissue fluid. Mast cells stimulate and co-ordinate inflammation by releasing chemicals. Mast cells are present in most cells.
  2. Histamine increases blood flow to the area and causes blood capillaries to become more permeable so that fluid is filtered from the blood. This increased blood flow causes the heat and redness, the escape of fluid from the body causes the swelling
  3. Heparin is an anticoagulant therefore prevents clotting in the immediate area of the injury. A clot of build around the damaged area forms, and this slows the spread of pathogens to healthy tissue
  4. Chemicals released by mast cells attract phagocytes. Macrophages and leucocytes actively consume micro-organisms and debris by phagocytosis
  5. Pain receptors are stimulated
  6. The phagocytes filled with bacteria, cell debris and dead cells, begin to die. The dead phagocytes and tissue fluid form a yellow fluid called pus
  7. New cells are produced by mitosis and repair of damaged tissues takes place

Fever

• Increases in body temperature due to resetting of the body’s thermostat (controlled by the hypothalamus) to a higher level
• The individual feels cold. Vasoconstrictions of the vessels of the skin and shivering occur to conserve heat and increase heat production. The body temperature increases rapidly.
• When the fever breaks, the individual feels very hot and appears fused due to vasodilation of the vessels of the skin and profuse sweating occurs
• High temperature inhibits growth of some bacteria and viruses AND speeds up rate of chemical reactions which may help in cell repair

Specific Defences

Parts of the Immune System

• White blood cells
  • Protect body from infection
• Lymph nodes
  • Glands that filter pathogens to make sure that they don’t spread further
  • They then activate, replicate and send the lymphocytes to fight the invader
• Spleen
  • Stores white blood cells
  • Filters blood, destroys old and damaged red-blood cells
• Tonsils and adenoids
  • Located in nasal passage 
  • Trap foreign invaders
  • Immune cells produce antibodies to prevent infection
• Thymus
  • Organ in upper chest
  • Learns to recognise and remember an invader, so that next time the invader comes, it can be swiftly neutralised
  • Also helps develop certain white blood cells
• Bone Marrow
  • Stem cells in the marrow develop into red blood cells, white blood cells, and other types of immune cells
  • Generate blood cells and release them into bloodstream
• External Non-specific defences
• Stomach and bowel
  • Acids may kill pathogens after they enter your body
  • Also contain beneficial bacteria that kill harmful bacteria

Major Blood Components

• Red blood cells (erythrocytes)
  • Transport oxygen to your tissue from your lungs
  • Contain haemoglobin, which allows them to carry oxygen
• White Blood cells (leukocytes)
  • Protect body from infection
  • Circulate in blood and respond to infection and illness
  • T-cells grow in thymus gland and lymph nodes
  • B-cells grow in spleen
• Plasma
  • Liquid component of blood that contains all the proteins in the blood
• Platelets (Thrombocytes)
  • Smallest component of blood that seals bleeding
  • Platelets will cluster at the site of bleeding and seal the vessel to clot bleeding

Leucocytes

• White blood cells that are able to leave the capillaries and migrate through the tissues to places of infection or injury
• Some secrete substances that destroy bacteria before engulfing them
• Others engulf live bacteria and digest them

Lymphocytes

• Type of leucocytes
• Part of the cellular immune system
• 3 types:
  • T-lymphocytes
    • Can recognise antigens
    • Can destroy pathogens
  • B-lymphocytes
    • Produce antigens to combat infection
  • Natural Killer (NK) Cells
    • Kill infected cells

Macrophages

• Large phagocytic cells that develop from leucocytes
• Some are wandering cells that move through tissues looking for and destroying pathogens
• Some are in fixed position
• Either engulf and digest microorganisms or release substances that destroy them
• These processes eliminate many pathogens before an infection has a chance to take hold