Introduction

The Innate immune system is the body's first barrier of defence to infection. It relies on an older, more generic, and faster acting set of tools than the adaptive system. While the adaptive system is essential for a specific response to infection, it is ultimately the innate system that conquers foreign attackers through means of phagocytosis.

Non-specific protective mechanisms include such innate factors as:
- Physical barriers
  - Skin
  - Ciliated mucous membranes
  - Commensal organisms
- Humoral factors
  - Lysozyme
  - Complement
  - Interferons
- Cellular mechanisms
  - Phagocytosis
- Factors which regulate species specificity
  - Membrane receptors for pathogens
  - Nutritional requirements
  - Temperature
  - pH
Mechanisms of innate immunity are always present and generally unchanging
Adaptive immunity is acquired only on contact with the infectious agent (antigen) and therefore does not function before first contact with the antigen