Answer:
The skin
Digestive tract
Vagina
Mucous membrane
Step-by-step explanation:
The skin prevents microbes from entering the body
Digestive tract e.g. stomach acid is a natural defence mechanism
The vagina is acidic and this prevents harmful bacteria from growing with it's own flora.
Mucous membranes e.g. lining of the mouth, nose, eyelids are all defense mechanisms.