Chapter 16 Host defence
The body is under continual threat from injury from a wide range of causes (Ch. 9). A wide range of mechanisms exists to counter these: behavioural changes, physical barriers and the presence of an immune system. Innate immunity is the first line of defence; it is present at birth and has no specificity or memory and comprises physicochemical barriers, humoral components (e.g. complement) and cells (e.g. mast cells). Acquired immunity describes the adaptive responses acquired during development through exposure (Ch. 17).
Behavioural protective mechanisms
Humans have developed the ability to influence their environment to an extraordinary degree. These generally allow the avoidance of extremes of temperature and potentially the minimization of the risk of trauma. Protective reflexes include the cough reflex and the blink reflex. Therefore, individuals who are unable to cough because of neurological disease or physical disease involving the chest or abdomen are especially prone to the development of pneumonia. Conditions that impair the ability to swallow will lead to an increased risk of aspiration of gastric contents and pneumonia. Pneumonia is, therefore, a very common terminal condition in elderly patients or those with severe coexistent diseases such as severe strokes or advanced malignancy (Ch. 42).
Physical barriers to infection
The skin and the epithelia lining the hollow viscera provide a physical barrier. The skin is a complex collection of tissues that, when intact, protects the body from a wide range of physical and microbial threats (Fig. 3.16.1):
stratified squamous epithelium provides a tough multilayered physical defence against trauma, temperature excesses and microorganisms in the surface skin, the oesophagus and the anal canal
< div class='tao-gold-member'>
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
