3.5 Non-specific Body Defences

Question 1

Pathogens

Answer 1

Bacterium, virus or other organism that can cause disease.

Question 2

Physical Barriers
(First Line of Defence)

Answer 2

The body uses PHYSICAL barriers to prevent entry of pathogens.
These include:
1. Skin – skin surface is composed of epithelial cells to provide
physical protection against bacteria and viruses.
2. Mucous membranes - linings of digestive and respiratory
tracts are also composed of epithelial cells forming another physical barrier.

Question 3

Chemical Barriers
(First Line of Defence)

Answer 3

The body also uses CHEMICAL barriers including:
1. Lysozymes -Secretions from tears and saliva containing the enzyme lysozyme which digests bacterial walls.
2. Secretions from sweat and sebaceous glands- these keep skin
at a pH too low for microbes to thrive.
3.Acid - hydrochloric acid is secreted by the lining of the stomach to destroy many ingested pathogens.

Question 4

Secondary Repsonse

Answer 4

Should the potential pathogens survive these physical and chemical
barriers, the body then has several second lines of defence. These
include:
1. Inflammatory response
2. Phagocytosis

Question 5

Inflammatory Response

Answer 5

This is a localised defence mechanism at an affected site.
Mast cells (specialised immune cells) are present in connective tissue. They release a substance called histamine which causes:
a) vasodilation of local capillaries, causing them to become swollen
with blood. The area then becomes red and inflamed.
b) Increased permeability of the capillary walls become more permeable so more fluid leaks out into surrounding tissues, causing them to swell.
The increased blood flow also leads to an accumulation of phagocytes and clotting elements at the site of infection.

Question 6

Phagocytosis

Answer 6

Invading pathogens are engulfed and destroyed by a process called PHAGOCYTOSIS. Cells able to undergo
phagocytosis are called PHAGOCYTES (or sometimes macrophages); a specialised type of white blood cell.
They detect a chemical released by bacteria and move up a concentration gradient towards it. They then engulf the
pathogen and digest it using digestive enzymes (usually a combination of lysozymes, proteases and nucleases)
contained in a vesicle inside them.