9.2.2 Innate Immune System

Introduction

The innate immune system is the body's first line of defence against infections. It is present at birth and provides immediate, non-specific protection against a wide range of pathogens. Unlike the adaptive immune system, it does not have memory. This topic explains the innate immune system and its two lines of defence.

Definition

Innate immune system: The immune system that individuals are born with. It provides non-specific, general protection against pathogens.

Main Idea

People have some immunity from birth, ready to fight against invaders immediately. This response is non-specific and general. It has first and second line of defence.

Core Concept

Lines of Defence in Innate Immunity

Detailed Explanation

1. First Line of Defence

What it is: The external barriers of our body serve as the first line of defence against pathogens.

Components:

• Skin
• Mucous membranes of digestive and respiratory system

How it works:

• The multiple defences including chemical, physical and biological barriers make the first line of defence extremely effective to prevent microbial entry.
• Physical barriers: Skin, mucous membranes.
• Chemical barriers: Acidic pH of skin, enzymes in tears and saliva, stomach acid.
• Biological barriers: Normal flora (beneficial bacteria) compete with pathogens.

Simple explanation: The first line of defence is like a wall that keeps germs out of the body.

Why this is important:

• It prevents most pathogens from entering the body.
• It is the body's first and most important defence.

2. Second Line of Defence

What it is: If pathogens successfully enter the skin or mucous membranes, a second line of defence is activated to combat these foreign invaders.

Components:

It consists of four nonspecific internal defences:

(1) Phagocytes, neutrophils, and natural killer cells:

- The body keeps a constant army of phagocytes, neutrophils and natural killer cells which kill pathogens by phagocytosis.

(2) Inflammation:

- The invasion of bacteria produces inflammation which removes pathogens and initiates healing process.

(3) Fever:

- The body develops fever to make the environment less favourable for pathogen growth.

(4) Protective proteins:

- Many protective proteins including complement proteins and interferons are produced which kill bacteria and viruses.

Simple explanation: The second line of defence is like a security team that fights germs that have already entered the body.

Why this is important:

• It provides immediate, non-specific defence against pathogens.
• It helps contain and eliminate infections.

Important point: These defences are nonspecific because they target a large range of germs rather than specific microbes.

3. Characteristics of Innate Immunity

Simple explanation: Innate immunity is fast, general, and present from birth. It does not remember specific germs.

Summary Table

Intermediate Level

The innate immune system is present at birth and provides non-specific, general protection.

Key points:

• First line of defence = skin and mucous membranes.
• Second line of defence = phagocytes, neutrophils, NK cells, inflammation, fever, protective proteins.
• Innate immunity is non-specific.
• It has no memory.
• It is fast and present from birth.

Important Points

• - People have some immunity from birth, ready to fight against invaders immediately.
• - This response is non-specific and general.
• - It has first and second line of defence.
• - First Line of Defence: The external barriers of our body serve as the first line of defence against pathogens. It includes skin and mucous membranes of digestive and respiratory system. The multiple defences including chemical, physical and biological barriers make the first line of defence extremely effective to prevent microbial entry.
• - Second Line of Defence: If pathogens successfully enter the skin or mucous membranes, a second line of defence is activated to combat these foreign invaders. It consists of four nonspecific internal defences.
• 1. The body keeps a constant army of phagocytes, neutrophils and natural killer cells which kill pathogens by phagocytosis.
• 2. The invasion of bacteria produces inflammation which removes pathogens and initiates healing process.
• 3. The body develops fever to make the environment less favourable for pathogen growth.
• 4. Many protective proteins including complement proteins and interferons are produced which kill bacteria and viruses.
• - These defences are nonspecific because they target a large range of germs rather than specific microbes.

Remember

• - Innate immune system = present at birth
• - Non-specific, general
• - First line of defence = skin, mucous membranes
• - Second line of defence = phagocytes, neutrophils, NK cells, inflammation, fever, protective proteins
• - No memory
• - Fast response
• - Targets a large range of germs

Common Mistakes

• 1. Confusing innate and adaptive immunity.
• - Innate = present at birth, non-specific.
• - Adaptive = develops after exposure, specific.
• 2. Mixing up the lines of defence.
• - First line = skin, mucous membranes.
• - Second line = phagocytes, inflammation, fever, protective proteins.
• 3. Thinking innate immunity has memory.
• - Innate immunity has no memory.
• 4. Forgetting that innate immunity is non-specific.
• - Innate immunity targets a large range of germs, not specific microbes.
• 5. Believing the first line of defence is only physical.
• - The first line of defence includes physical, chemical, and biological barriers.
• 6. Confusing inflammation and fever.
• - Inflammation = local response (redness, heat, swelling, pain).
• - Fever = systemic response (elevated body temperature).
• 7. Thinking NK cells are specific.
• - NK cells are non-specific; they destroy infected and cancerous cells.

Quick Revision

60-Second Revision

• Innate immune system = present at birth
• Non-specific, general
• First line of defence = skin, mucous membranes
• Second line of defence = phagocytes, neutrophils, NK cells, inflammation, fever, protective proteins
• No memory
• Fast response
• Targets a large range of germs
• Warning: Do not confuse innate (present at birth, non-specific) with adaptive (develops after exposure, specific) immunity. Innate immunity has no memory.