9.2 Organs of the Immune System

Introduction

The immune system is a complex network of organs and tissues that work together to protect the body from infections and diseases. These organs are called lymphoid organs and are connected with the lymphatic system. This topic explains the organs of the immune system and their functions.

Definition

Organs of the immune system are the specialized organs and tissues that are important for the proper functioning of the immune system. They include bone marrow, thymus, lymph nodes and vessels, spleen, liver, tonsils, adenoids, skin, and mucous membranes.

Main Idea

Organs and tissues important to the proper functioning of the immune system are:

• Bone marrow
• Thymus
• Lymph nodes and vessels
• Spleen
• Liver
• Tonsils and adenoids
• Skin
• Mucous membranes

Core Concept

Organs of the Immune System and Their Functions

Detailed Explanation

1. Bone Marrow

What it is: Red blood cells, many types of white blood cells, and platelets are produced from stem cells found in the spongy interior of the bones.

Function:

• Every day, the bone marrow produces billions of new blood cells and releases them into the blood.
• B-cells and T-cells are also produced in bone marrow.
• The B-cell maturation takes place in the bone marrow.

Simple explanation: Bone marrow is the factory where all blood cells are made. It also produces B cells and T cells.

Why this is important:

• All immune cells originate from bone marrow.
• Without bone marrow, the body cannot produce immune cells.

2. Thymus

What it is: This tiny organ is located in the upper chest below the breast bone.

Function:

- It is responsible for T-cell maturation.

Simple explanation: The thymus is where T cells mature. T cells are important for fighting infections.

Why this is important:

• T cells are essential for adaptive immunity.
• Without the thymus, T cells cannot mature and function properly.

3. Lymph Nodes and Vessels

What they are: The lymphatic system in our body contains lymph vessels and lymph nodes. The networks of lymph vessels are connected by lymph nodes.

Function:

• These tiny nodes filter and kill pathogens to prevent germs from spreading to other body areas.
• Lymph nodes have collections of B cells and T cells throughout the body.
• Cells collect in lymph nodes to communicate with each other.
• Lymph nodes can become swollen when they are fighting an infection.

Simple explanation: Lymph nodes are like filters that trap and kill germs. They also contain B cells and T cells that fight infections.

Why this is important:

• Lymph nodes are important for filtering pathogens.
• Swollen lymph nodes are a sign of infection.

4. Spleen

What it is: Spleen is a lymphoid organ located near the stomach.

Function:

• Spleen filters blood, captures pathogens in blood, and fights blood infections.
• The spleen has a collection of B cells, T cells, and monocytes.

Simple explanation: The spleen filters blood and fights infections in the blood.

Why this is important:

• The spleen is important for fighting blood-borne infections.
• It also helps remove old red blood cells.

5. Liver

What it is: The liver is the major organ responsible for producing proteins of the complement system.

Function:

• The liver produces complement proteins.
• It contains large numbers of phagocytic cells that eat bacteria in the blood as it passes through the liver.

Simple explanation: The liver produces proteins that help the immune system and contains cells that eat bacteria.

Why this is important:

• Complement proteins are important for immune defence.
• Phagocytic cells in the liver help remove bacteria from the blood.

6. Tonsils and Adenoids

What they are: The adenoids and tonsils function as a part of the body's immune system.

Function:

• Acting as our first line of defence, they trap and kill harmful pathogens like bacteria and viruses that enter our mouth and nose.
• These are made up of lymphoid tissue which is densely filled with lymphocytes.

Simple explanation: Tonsils and adenoids are the first line of defence against germs entering through the mouth and nose.

Why this is important:

• They help prevent infections from entering the body.
• They are part of the lymphatic system.

7. Skin

What it is: The human skin serves as a barrier against microbial invasion, protecting the body from potential pathogens.

Primary defensive features:

• The outermost layer of the skin is called epidermis. It is composed of tightly packed cells that provide a physical barrier.
• Skin cells contain a tough protein keratin, making them resistant to abrasion and water loss.
• The skin surface has a slightly acidic pH (around 5.5), which inhibits the growth of many pathogens.
• The process of shedding dead skin cells helps remove attached microbes.
• Dermis of skin contains oil glands, sweat glands, hair follicles, receptors, nerves, and blood vessels.
• Oil and sweat play an important role in inhibiting and killing microorganisms.

Simple explanation: The skin is a physical barrier that prevents germs from entering the body. It also produces oil and sweat that kill microorganisms.

Why this is important:

• The skin is the largest organ of the body.
• It is the first line of defence against infections.

8. Mucous Membranes

What they are: Mucous membranes line the digestive and respiratory tracts.

Function:

• The digestive tract mucous membrane kills bacteria present in food by producing stomach acid and digestive enzymes.
• The respiratory system mucous membrane can trap and remove airborne microbes.
• It has ciliated epithelium and produces mucus.
• Mucus traps dust, microbes, and other particles, and cilia remove them from the respiratory system.

Simple explanation: Mucous membranes line the digestive and respiratory tracts and trap and remove pathogens.

Why this is important:

• They are part of the first line of defence.
• They prevent pathogens from entering the body.

Do You Know?

• Bone marrow, thymus, lymph nodes and vessels, spleen, liver, tonsils and adenoids are lymphoid organs.
• All lymphoid organs are connected with the lymphatic system.
• Skin is the largest organ of the body and plays a key role in the first line of defence.

Summary Table

Intermediate Level

Organs and tissues important to the proper functioning of the immune system are:

Key points:

• Bone marrow produces all blood cells.
• Thymus is where T cells mature.
• Lymph nodes filter pathogens.
• Spleen filters blood.
• Liver produces complement proteins.
• Tonsils and adenoids are the first line of defence.
• Skin is the largest organ and a physical barrier.
• Mucous membranes line the digestive and respiratory tracts.

Important Points

• - Organs and tissues important to the proper functioning of the immune system are following.
• - Bone marrow, thymus, lymph nodes and vessels, spleen, liver, tonsils, adenoids, skin and mucous membranes.
• - Bone Marrow: Red blood cells, many types of white blood cells, and platelets are produced from stem cells found in the spongy interior of the bones. Every day, the bone marrow produces billions of new blood cells and releases them into the blood. B-cells and T-cells are also produced in bone marrow. The B-cell maturation takes place in the bone marrow.
• - Thymus: This tiny organ, located in the upper chest below the breast bone, is responsible for T-cell maturation.
• - Lymph Nodes and Vessels: The lymphatic system in our body contains lymph vessels and lymph nodes. The networks of lymph vessels are connected by lymph nodes. These tiny nodes filter and kill pathogens to prevent germs from spreading to other body areas. Lymph nodes have collections of B cells and T cells throughout the body. Cells collect in lymph nodes to communicate with each other. Lymph nodes can become swollen when they are fighting an infection.
• - Spleen: Spleen is a lymphoid organ located near stomach. Spleen filter blood, capture pathogens in blood, and fight blood infections. The spleen has a collection of B cells, T cells, and monocytes.
• - Liver: The liver is the major organ responsible for producing proteins of the complement system. In addition, it contains large numbers of phagocytic cells that eat bacteria in the blood as it passes through the liver.
• - Tonsils and adenoids: The adenoids and tonsils function as a part of the body's immune system. Acting as our first line of defence, they trap and kill harmful pathogens like bacteria and viruses that enter our mouth and nose. These are made up of lymphoid tissue which is densely filled with lymphocytes.
• - Skin: The human skin serves as a barrier against microbial invasion, protecting the body from potential pathogens. The skin's primary defensive features include: The outermost layer of the skin is called epidermis. It is composed of tightly packed cells that provide a physical barrier. Skin cells contain a tough protein Keratin, making them resistant to abrasion and water loss. The skin surface has a slightly acidic pH (around 5.5), which inhibits the growth of many pathogens. The process of shedding dead skin cells helps remove attached microbes. Dermis of skin contains oil glands, sweat glands, hair follicles, receptors, nerves and blood vessels. Oil and sweat play important role in inhibiting and killing microorganisms.
• - Mucous membrane: Mucous membranes line digestive and respiratory tracts. The digestive tract mucous membrane kills bacteria present in food by producing stomach acid and digestive enzymes. The respiratory system mucous membrane can trap and remove airborne microbes. It has ciliated epithelium and produces mucus. Mucus traps dust, microbes, and other particles and cilia remove them from respiratory system.
• - Do you know that bone marrow, thymus, lymph nodes and vessels, spleen, liver, tonsils and adenoids are lymphoid organs? All lymphoid organs are connected with lymphatic system.
• - Do you know that skin is the largest organ of the body and plays a key role in the first line of defence?

Remember

• - Organs of the immune system:
• 1. Bone marrow
• 2. Thymus
• 3. Lymph nodes and vessels
• 4. Spleen
• 5. Liver
• 6. Tonsils and adenoids
• 7. Skin
• 8. Mucous membranes
• - Bone marrow = produces blood cells; B cell maturation
• - Thymus = T cell maturation
• - Lymph nodes = filter and kill pathogens
• - Spleen = filters blood; fights blood infections
• - Liver = produces complement proteins; phagocytic cells
• - Tonsils and adenoids = first line of defence
• - Skin = physical barrier; largest organ
• - Mucous membranes = trap and remove pathogens

Common Mistakes

• 1. Confusing bone marrow and thymus.
• - Bone marrow = produces all blood cells; B cell maturation.
• - Thymus = T cell maturation.
• 2. Thinking lymph nodes only filter lymph.
• - Lymph nodes also contain B cells and T cells that fight infections.
• 3. Forgetting the role of the spleen.
• - Spleen filters blood and fights blood infections.
• 4. Mixing up the liver's immune function.
• - Liver produces complement proteins and contains phagocytic cells.
• 5. Believing tonsils and adenoids are useless.
• - Tonsils and adenoids are the first line of defence against pathogens entering the mouth and nose.
• 6. Thinking skin is not part of the immune system.
• - Skin is a physical barrier and part of the first line of defence.
• 7. Confusing mucous membranes and skin.
• - Skin = physical barrier on the outside.
• - Mucous membranes = line internal tracts.

Quick Revision

60-Second Revision

• Bone marrow — produces blood cells; B cell maturation
• Thymus — T cell maturation
• Lymph nodes and vessels — filter and kill pathogens
• Spleen — filters blood; fights blood infections
• Liver — produces complement proteins; phagocytic cells
• Tonsils and adenoids — first line of defence
• Skin — physical barrier; largest organ
• Mucous membranes — trap and remove pathogens