An Introduction to Immunity

Question 1

WHAT IS IMMUNITY?

Answer 1

• 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)

Question 2

HOST STRATEGIES AND RESPONSES

Answer 2

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

Question 3

BARRIERS TO INFECTION: SKIN

Answer 3

• 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

Question 4

BARRIERS TO INFECTION: MUCOUS MEMBRANES

Answer 4

• 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

Question 5

NEUTRALISATION

Answer 5

• 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

Question 6

DESTRUCTION: KILLING OF FOREIGN CELLS

Answer 6

E.g. phagocytosis and lysis of infected cells

Question 7

INTERNAL DEFENCES

Answer 7

In cases where barriers to infection are overcome

• Fever
• The acute phase reaction
• The immune system