9.2.2 Components of the Immune System

Introduction

The immune system is the body's defence network against infections and diseases. It is composed of two basic parts: the innate immune system and the adaptive immune system. Each part has a unique role in protecting the body. This topic explains the components of the immune system.

Definition

The immune system is composed of two basic parts:

• The innate immune system: Individuals are born with this immune system.
• The adaptive immune system: Individuals develop this immunity when exposed to pathogens or chemicals released by pathogens.

Main Idea

The immune system is composed of two basic parts:

• The innate immune system: Individuals are born with this immune system.
• The adaptive immune system: Individuals develop this immunity when exposed to pathogens or chemicals released by pathogens.

Core Concept

Components of the Immune System

Detailed Explanation

1. Innate Immune System

What it is: Individuals are born with this immune system.

Characteristics:

• Present at birth.
• 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:
• These defences are nonspecific because they target a large range of germs rather than specific microbes.

Simple explanation: The innate immune system is the body's first and second lines of defence. It is present at birth and fights all germs in a general way.

2. Adaptive Immune System

What it is: Individuals develop this immunity when exposed to pathogens or chemicals released by pathogens.

Characteristics:

• Also called acquired immune system.
• It consists of immune cells that target specific antigens.
• The immune cells that play a role in the third line of defence are B-cells and T-cells.
• The B-cells produce antibodies.
• The T-cells help identify pathogenic cells and destroy targeted cells.

Third Line of Defence:

• The third line of defence is also called adaptive or acquired immune system.
• It consists of immune cells that target specific antigens.
• The immune cells that play a role in the third line of defence are B-cells and T-cells.
• The B-cells produce antibodies.
• The T-cells help identify pathogenic cells and destroy targeted cells.

Memory:

• This acquired ability to defend against infections improves over time.
• The body produces a variety of antibodies to various pathogens due to immunizations and exposure to various diseases.
• When the body creates an antibody, it stores its memory cells so that it can respond more quickly the next time the same pathogen attacks.
• In general, exposure to many pathogens strengthens the immune system.
• By adulthood, most people will have been exposed to several pathogens and strengthened their immune systems.

Simple explanation: The adaptive immune system is the body's third line of defence. It develops after exposure to pathogens and targets specific germs. It also remembers germs for future attacks.

3. Strengthening Immunity

Making healthy food and exercise choices, quitting smoking and drinking, and receiving the necessary vaccinations are all ways to increase immunity.

Simple explanation: You can strengthen your immune system by eating healthy, exercising, not smoking, not drinking alcohol, and getting vaccinated.

Summary Table

Lines of Defence

Intermediate Level

The immune system is composed of two basic parts:

Lines of defence:

Key points:

• Innate immunity is present at birth.
• Adaptive immunity develops after exposure.
• First line of defence = skin and mucous membranes.
• Second line of defence = non-specific internal defences.
• Third line of defence = specific adaptive immunity.
• B-cells produce antibodies.
• T-cells destroy infected cells.
• Memory cells provide long-lasting immunity.
• Vaccination strengthens immunity.

Important Points

• - The immune system is composed of two basic parts:
• 1. The innate immune system: Individuals are born with this immune system.
• 2. The adaptive immune system: Individuals develop this immunity when exposed to pathogens or chemicals released by pathogens.
• - The innate immune system 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.
• - 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.
• - The third line of defence is also called adaptive or acquired immune system.
• - It consists of immune cells that target specific antigens.
• - The immune cells that play a role in the third line of defence are B-cells and T-cells.
• - The B-cells produce antibodies.
• - The T-cells help identify pathogenic cells and destroy targeted cells.
• - This acquired ability to defend against infections improves over time.
• - The body produces a variety of antibodies to various pathogens due to immunizations and exposure to various diseases.
• - When the body creates an antibody, it stores its memory cells so that it can respond more quickly the next time the same pathogen attacks.
• - In general, exposure to many pathogens strengthens the immune system.
• - By adulthood, most people will have been exposed to several pathogens and strengthened their immune systems.
• - Making healthy food and exercise choices, quitting smoking and drinking, and receiving the necessary vaccinations are all ways to increase immunity.

Remember

• - Immune system = innate + adaptive
• - Innate = present at birth, non-specific
• - Adaptive = develops after exposure, specific, memory
• - First line of defence = skin, mucous membranes
• - Second line of defence = phagocytes, inflammation, fever, protective proteins
• - Third line of defence = B-cells, T-cells
• - B-cells = produce antibodies
• - T-cells = destroy infected cells
• - Memory cells = long-lasting immunity
• - Vaccination = strengthens immunity

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.
• - Third line = B-cells, T-cells.
• 3. Forgetting that innate immunity has no memory.
• - Only adaptive immunity has memory.
• 4. Thinking B-cells and T-cells are part of innate immunity.
• - B-cells and T-cells are part of adaptive immunity.
• 5. Believing all immune responses are specific.
• - Innate immunity is non-specific.
• 6. Confusing memory B cells and memory T cells.
• - Memory B cells = produce antibodies on re-exposure.
• - Memory T cells = rapid response on re-exposure.
• 7. Ignoring the importance of vaccination.
• - Vaccination strengthens adaptive immunity and provides protection.

Quick Revision

60-Second Revision

• Immune system = innate + adaptive
• Innate = present at birth, non-specific
• Adaptive = develops after exposure, specific, memory
• First line of defence = skin, mucous membranes
• Second line of defence = phagocytes, inflammation, fever, protective proteins
• Third line of defence = B-cells, T-cells
• B-cells = produce antibodies
• T-cells = destroy infected cells
• Memory cells = long-lasting immunity
• Vaccination = strengthens immunity
• Warning: Do not confuse innate (present at birth, non-specific) with adaptive (develops after exposure, specific) immunity. Only adaptive immunity has memory.