Protection Against Disease Natural Defence Mechanisms

Learning Objectives How the natural defence mechanisms prevent the entry of pathogens

Starter List all the adaptations that the body has to prevent entry of pathogens

Preventing Entry The most effective way for humans to avoid infection is to prevent them from gaining access Skin is an effective barrier because it is impermeable Only a few pathogens (mainly viruses) are able to penetrate undamaged skin The skin secretes sweat and sebum which contain substances that destroy bacteria We also have our own flora of harmless bacteria called COMMENSALS They compete more successfully than pathogens for the small amount of nutrients on the skin They are also found in the mouth, respiratory tract, vagina and digestive system

Preventing Infection Most pathogenic bacteria however cannot survive in these places-partly because of the commensals and partly because of the lactic acid and fatty acids secreted from sweat and sebaceous gland which provide a pH too low for them

Candida and Thrush The normal bacterial flora can help to prevent infection by other microorganisms e.g. the bacteria that normally live in the vagina keep the pH low by secreting lactic acid If a person takes antibiotics these normal bacteria may be killed and the pH of the vagina then rises and other organisms like Candida can multiply too much greater population densities causing thrush

Methods of prevention Moist body surfaces e.g. the surface of the eyes and mouth are bathed with fluids which have some BACTERICIDAL ACTION An enzyme called LYSOZYME is present in saliva and tears and this enzyme can damage and destroy many bacteria Semen contains a bactericide called spermine, milk contains a bacteriacidal enzyme called lactoperoxidase

Other methods The HYDROCHOLORIC ACID secreted by the stomach is very effective in destroying bacteria in ingested food MUCUS helps to protect the digestive and respiratory tracts from infection Mucus contains LYSOZYME an enzyme which destroys bacteria and it also acts as a barrier so that bacteria cannot make contact with the epithelia cells lining the walls of the tubes

Mucus In the trachea and bronchi the mucus is swept upwards to the back of the throat by cilia and then swallowed Coughing and sneezing help to expel the mucus containing microorganisms from the trachea and bronchi Once the mucus is swallowed the acid and enzymes in the stomach destroy any bacteria trapped in it. Any bacteria which manages to get throught he defence mechanims may then enter the blood stream and by ingested by phagocytic white blood cells

Tasks Complete the summary table Answer ppq

Plenary Codeword