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1

WHAT IS IMMUNITY?

- Immunity is the response of the host to foreign substance

Examples of foreign substances
- Infectious microorganisms
- Foreign compounds (components of venoms or drugs)
- Foreign cells or tissues (e.g. transplanted organ or a blood transfusion)

2

HOST STRATEGIES AND RESPONSES

The type of physiological responses that are manifested as “immunity” include
- Barriers to invasion
- Expulsion/ removal of foreign material
- Neutralisation of the pathological effects of foreign material
- Destruction/ killing of foreign cells

3

BARRIERS TO INFECTION: SKIN

- The uppermost layer (the epidermis) is a stratified squamous epithelium
- The surface layers of the epidermis are keratinised
- Most of the skin is dry and salty
- Antimicrobial secretions: sebum and sweat
- Commensal organisms at sites where bacteria and microorganisms can survive

4

BARRIERS TO INFECTION: MUCOUS MEMBRANES

- Line the lumen of tracts which have one or two openings to the external environment
- E.g. mucous secreted on the surface: inhibits microbial adhesion
- Secretions into the tract also include anti-microbials (e.g. acids, bile, salts and digestive enzymes)
- In the respiratory tract: the mucous works in combination with surface cilia to remove foreign material: the muco-ciliary escalator

5

NEUTRALISATION

- Refers to preventing the foreign material from causing damage
- Coating bacteria so they don't stick to tissue surfaces
- Anti-venoms for spider or snake bites: preventing a toxin binding to a receptor on a cell

6

DESTRUCTION: KILLING OF FOREIGN CELLS

E.g. phagocytosis and lysis of infected cells

7

INTERNAL DEFENCES

In cases where barriers to infection are overcome
- Fever
- The acute phase reaction
- The immune system