9.1 First Line of Defence

The human body has three lines of defence against microbial attack. The first line of defence comprises external barriers that keep microbes out of the body. The human body has two surfaces exposed to the environment: the skin and the mucous membranes of the digestive and respiratory tracts. These surfaces are external barriers to microbial invasion.

Definition

First Line of Defence: The first line of defence comprises external barriers that keep microbes out of the body. It includes the skin and the mucous membranes of the digestive and respiratory tracts.

Innate Immunity: First line and second line of defence together make innate immunity.

Main Idea

• The human body has three lines of defence against microbial attack.
• First line of defence comprises external barriers that keep microbes out of the body.
• Second line of defence consists of nonspecific internal defence that combats all invading microbes.
• First line and second line of defence together make innate immunity.
• Third line of defence includes the specific internal defence also called adaptive/acquired immunity.
• The human body has two surfaces exposed to the environment:
• These surfaces are external barriers to microbial invasion.
• Since these barriers inhibit generally all kind of microbial invasion, first line of defence is supposed to be a non-specific defence.

Core Concept

• First line of defence = external barriers
• Second line of defence = nonspecific internal defence
• Third line of defence = specific internal defence
• Innate immunity = first line + second line
• Acquired immunity = third line
• Skin = external barrier
• Mucous membranes = external barrier
• Non-specific defence = inhibits all kinds of microbial invasion

Detailed Explanation

First Line of Defence

• The human body has three lines of defence against microbial attack.
• First line of defence comprises external barriers that keep microbes out of the body.
• Second line of defence consists of nonspecific internal defence that combats all invading microbes.
• First line and second line of defence together make innate immunity.
• Third line of defence includes the specific internal defence also called adaptive/acquired immunity.

External Barriers

The first and obviously best, defence is to keep microbes out in the first place. The human body has two surfaces exposed to the environment:

• The skin
• The mucous membranes of the digestive and respiratory tracts

These surfaces are external barriers to microbial invasion.

- Since these barriers inhibit generally all kind of microbial invasion thus, first line of defence is supposed to be a non-specific defence.

Summary Table of Lines of Defence

Summary of Immunity

Intermediate Level

Lines of Defence

Immunity

External Barriers

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Human body has three lines of defence.
• 2. First line of defence comprises external barriers.
• 3. Second line of defence is nonspecific internal defence.
• 4. Third line of defence is specific internal defence.
• 5. First and second lines together make innate immunity.
• 6. Third line is adaptive/acquired immunity.
• 7. Two surfaces are exposed to the environment.
• 8. These are skin and mucous membranes.
• 9. These surfaces are external barriers.
• 10. First line of defence is non-specific.
• 11. It inhibits all kinds of microbial invasion.
• 12. Skin is a physical barrier.
• 13. Mucous membranes line digestive and respiratory tracts.
• 14. First line of defence keeps microbes out.
• 15. It is the best defence.

Remember

• - First line of defence = external barriers
• - Second line of defence = nonspecific internal defence
• - Third line of defence = specific internal defence
• - Innate immunity = first line + second line
• - Acquired immunity = third line
• - Skin = external barrier
• - Mucous membranes = external barrier
• - Non-specific defence = inhibits all kinds of microbial invasion

Common Mistakes

• 1. Confusing the three lines of defence — First = external barriers; second = nonspecific internal; third = specific internal.
• 2. Mixing up innate and acquired immunity — Innate = first + second; acquired = third.
• 3. Forgetting the role of skin — Physical barrier.
• 4. Forgetting the role of mucous membranes — Line digestive and respiratory tracts.
• 5. Thinking the first line of defence is specific — It is non-specific.
• 6. Mixing up the surfaces exposed to the environment — Skin and mucous membranes.
• 7. Believing the first line of defence is not important — It is the best defence.
• 8. Mixing up the types of defence — Non-specific vs. specific.
• 9. Forgetting the role of cilia — Sweep trapped microbes away.
• 10. Thinking the first line of defence only includes skin — It also includes mucous membranes.

Quick Revision

The human body has three lines of defence against microbial attack. First line of defence comprises external barriers that keep microbes out of the body. Second line of defence consists of nonspecific internal defence that combats all invading microbes. First line and second line of defence together make innate immunity. Third line of defence includes the specific internal defence also called adaptive/acquired immunity. The first and obviously best, defence is to keep microbes out in the first place. The human body has two surfaces exposed to the environment: the skin and the mucous membranes of the digestive and respiratory tracts. These surfaces are external barriers to microbial invasion. Since these barriers inhibit generally all kind of microbial invasion thus, first line of defence is supposed to be a non-specific defence.

60-Second Revision

• First line of defence = external barriers
• Second line of defence = nonspecific internal defence
• Third line of defence = specific internal defence
• Innate immunity = first line + second line
• Acquired immunity = third line
• Skin = external barrier
• Mucous membranes = external barrier
• Non-specific defence = inhibits all kinds of microbial invasion
• Warning: First line of defence is non-specific; it inhibits all kinds of microbial invasion

NEXT TOPIC

• Chapter:
• 9. Immunity

• Topic:
• 9.1.1 SKIN: AN IMPENETRABLE BARRIER AGAINST MICROBIAL INVASION

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