Immunity and Immune System Overview

Study Notes

Immunity is derived from the Latin stem immunis, meaning safe or free from burden.
The immune system is a complex system that gives us immunity.
The immune system regulates our interaction with microbes.
The immune system is the body’s defense system.

Our immune system functions at two levels:
- Nonspecific or nonselective mechanisms: This is called natural or innate immunity.
- Specific or selective mechanisms: This is called acquired or adaptive or specific immunity.
The immune system is divided into two branches:
- Innate
- Adaptive.
Innate and adaptive branches can function independently or together.
There are three lines of defense in the immune system:
- First line: Anatomical and biochemical barriers
- Second line: Phagocytes and natural killing cells
- Third line: Acquired immunity (B and T lymphocytes).

Innate immunity is the bedrock of immunity in all organisms.
Innate immunity regulates microbial interactions through nonspecific mechanisms.
Nonspecific mechanisms are fully functional at birth without previous contact with pathogens.
Innate immunity is characterized by a rapid response against a broad range of microbes.

The three main mediators that microbes encounter are:
- Anatomical or physical barriers
- Antimicrobial biochemical barriers
- Cellular immune defense.

Skin: Provides protection when intact.
Ciliated epithelial cells in the respiratory tract: Remove microbes by sweeping them up and out.

Mucus: Traps dust and microbes.
Fatty acids: Act as a chemical barrier.
High Acidic environments: Found in gastric juice and the adult vagina.
Lysozyme: Present in most tissue fluids, degrades bacterial cell walls.
Complement: Serum protein system that can be activated through innate pathways.
Interferon: Biochemical signals for neighboring cells to start synthesizing antiviral proteins.

The skin acts as a physical barrier and a chemical barrier due to:
- Saturated & unsaturated fatty acids secreted by sebaceous glands and resident skin microbiota
- High salt concentration in sweat.

Mucous membranes in the respiratory tract secrete mucus, which traps dust and microbes.
Ciliated epithelial cells sweep the trapped dust and microbes into the throat, allowing them to be swallowed or coughed out.

Lysozyme is present in many fluids including tears, saliva, and nasal secretions.
It degrades peptidoglycan, weakening bacterial cell walls.
Lysozyme is more effective against Gram-positive bacteria because their peptidoglycan layer is more accessible.

There are two types of interferon, α and β, involved in innate immunity.
Interferon does not directly stop a viral infection.
It signals neighboring cells to produce antiviral proteins (AVPs), which can interfere with viral assembly and maturation.