Immunology Lesson : What is Immune System, Types & Disorders

Lesson Overview

• What Is Immunology?
• Why Is the Immune System Important? 
• Components of the Immune System
• Major Functions of the Immune System
• What Are the Types of Immunity?
• Common Immune System Disorders

Immunology explores the body's defense system, protecting against diseases and infections. This lesson covers the immune system's structure, types, and common disorders. Understanding immunity is essential for recognizing how the body fights harmful agents, maintains health, and adapts to various threats, ensuring overall protection and long-term well-being.

What Is Immunology?

Immunology is the branch of biology that studies the immune system, which protects the body from harmful pathogens like bacteria, viruses, and toxins. It tells us how the body detects and fights infections, the role of immune cells, and how immunity develops through natural defenses and medical interventions like vaccines.

Why Is the Immune System Important? 

The importance of the immune system lies in its crucial role in protecting the body from harmful invaders, maintaining overall health, and ensuring the body functions properly. Let's understand in more detail. 

1. Protects Against Infections – It identifies and fights harmful bacteria, viruses, fungi, and parasites.
2. Prevents Diseases – It reduces the risk of infections and chronic illnesses.
3. Creates Immunological Memory – It remembers past infections for faster responses in the future.
4. Produces Antibodies – It neutralizes toxins and prevents pathogens from spreading.
5. Regulates Immune Responses – It prevents excessive immune reactions and autoimmunity.
6. Removes Damaged Cells – It eliminates dead, infected, or cancerous cells to maintain health.
7. Supports Wound Healing – It aids in tissue repair and recovery after injuries.
8. Essential for Survival – Without a strong immune system, the body cannot defend itself against infections.

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Components of the Immune System

The components of the immune system work together to protect the body from harmful invaders. This complex network of organs, cells, and molecules identifies threats, neutralizes them, and remembers previous infections for faster responses in the future. 

1. Primary Organs of the Immune System

The immune system relies on specialized organs where immune cells are produced, matured, and stored. These primary organs are essential for immune function.

a. Bone Marrow

• The bone marrow is a soft, spongy tissue found inside bones.
• It produces all blood cells, including immune cells such as white blood cells (WBCs), also called leukocytes.
• It is responsible for the formation of B-cells, a type of lymphocyte essential for antibody production.

b. Thymus

• The thymus is a small gland located behind the sternum, near the heart.
• It plays a crucial role in the development and maturation of T-cells, which are responsible for attacking infected or abnormal cells.
• It is most active during childhood and shrinks as a person ages, but it continues to influence immune function.

2. Secondary Organs of the Immune System

Secondary immune organs help immune cells mature, activate, and interact with pathogens. These include the spleen, lymph nodes, and mucosal-associated lymphoid tissue (MALT).

a. Spleen

• The spleen is an organ located in the upper left abdomen.
• It filters blood, removing old or damaged red blood cells and detecting infections.
• It stores white blood cells, including B-cells and T-cells, that help fight infections.

b. Lymph Nodes

• Lymph nodes are small, bean-shaped structures found throughout the body.
• They act as filtration sites where immune cells detect and respond to pathogens.
• They contain macrophages, B-cells, and T-cells that help identify and destroy harmful invaders.

c. Mucosal-Associated Lymphoid Tissue (MALT)

• MALT is a collection of immune tissue found in mucosal linings, including the respiratory, digestive, and urogenital tracts.
• It includes the tonsils, adenoids, and Peyer's patches (found in the intestines).
• It helps protect against pathogens entering through the mouth, nose, or digestive system.

Fig: A diagram of immune systems organs

3. Major Types of Immune Cells

White blood cells (WBCs), or leukocytes, are the primary immune cells that defend the body. They are divided into two main categories: innate immune cells and adaptive immune cells.

a. Innate Immune Cells (Non-Specific Defenders)

These cells provide a first line of defense against infections by recognizing general patterns found in pathogens.

• Macrophages: Large cells that engulf and digest pathogens and dead cells.
• Neutrophils: The most abundant WBCs; they attack bacteria and fungi.
• Dendritic Cells: They act as messengers, alerting other immune cells to the presence of invaders.
• Natural Killer (NK) Cells: These cells detect and destroy virus-infected and cancerous cells.

b. Adaptive Immune Cells (Specific Defenders)

These cells target specific pathogens and create long-term immunity.

• B-Cells: Produced in the bone marrow, they create antibodies that bind to and neutralize pathogens.
• T-Cells: Mature in the thymus, they directly attack infected cells or help other immune cells function.
  • Helper T-Cells (CD4+): Assist other immune cells in coordinating responses.
  • Cytotoxic T-Cells (CD8+): Destroy virus-infected and cancerous cells.
• Memory Cells: These long-lived cells "remember" past infections, providing quicker responses to future exposures.

Fig: A diagram of the immune system showing innate cells (dendritic, macrophage, epithelial, NK cells) and adaptive cells (T-cells, B-cells, antibodies, plasma cells).

4. Chemical Components of the Immune System

In addition to immune cells, the body uses chemical messengers and proteins to detect and neutralize threats.

a. Antibodies (Immunoglobulins, Ig)

• Antibodies are Y-shaped proteins produced by B-cells.
• They specifically bind to pathogens, marking them for destruction by other immune cells.

b. Cytokines

• Cytokines are chemical messengers that help regulate immune responses.
• They include interleukins (ILs), interferons (IFNs), and tumor necrosis factors (TNFs).
• They help activate immune cells, promote inflammation, and coordinate immune activity.

c. Complement System

• The complement system is a group of proteins that enhance the ability of antibodies and immune cells to clear infections.
• It helps destroy pathogens by forming a membrane attack complex (MAC) that punctures bacterial cell membranes.

5. The Lymphatic System's Role in Immunity

The lymphatic system plays a crucial role in immune defense by transporting white blood cells and filtering harmful substances.

• Lymph: A clear fluid that circulates throughout the body, carrying immune cells.
• Lymphatic Vessels: Channels that transport lymph and immune cells.
• Lymphoid Organs: Include the lymph nodes, spleen, and thymus, which store and activate immune cells.

The lymphatic system helps remove toxins, waste, and unwanted materials, ensuring a well-functioning immune response.

Fig: A diagram of lymphatic system highlighting lymph, lymphatic vessels, and lymphoid organs (lymph nodes, spleen, thymus) in immune defense.

6. Barriers of the Immune System (First Line of Defense)

Before pathogens even enter the body, physical, chemical, and biological barriers act as the first defense.

a. Physical Barriers

• Skin: A protective outer layer that prevents microbes from entering.
• Mucous Membranes: Found in the respiratory and digestive tracts; trap pathogens.

b. Chemical Barriers

• Stomach Acid (HCl): Destroys bacteria and viruses in food.
• Enzymes (Lysozymes): Found in saliva, tears, and mucus; break down bacterial cell walls.

c. Biological Barriers

• Normal Flora (Beneficial Bacteria): Found in the gut and skin, these microbes outcompete harmful pathogens.
  
  

Major Functions of the Immune System

The major functions of the immune system include recognizing pathogens, fighting infections, preventing diseases, and maintaining overall health. Let's understand in detail: 

1. Recognizing and Identifying Pathogens

The immune system's first function is to detect harmful invaders such as bacteria, viruses, fungi, and parasites.

a. Self vs. Non-Self Recognition

• The immune system distinguishes self (body's own cells) from non-self (foreign invaders).
• It uses major histocompatibility complex (MHC) proteins to identify and differentiate between healthy cells and threats.

b. Pathogen Recognition

• The body recognizes pathogens using pattern recognition receptors (PRRs) found in immune cells.
• These receptors detect unique molecules called pathogen-associated molecular patterns (PAMPs) found on harmful microbes.

Without this recognition ability, the body would be unable to react appropriately to infections.

2. Defending the Body Against Infections

Once a pathogen is detected, the immune system activates a defense response to eliminate the threat.

a. First Line of Defense

• This is a non-specific response that acts quickly to stop infections.
• Key components include:
  • Skin and mucous membranes (act as physical barriers).
  • Inflammation (swelling and redness caused by increased blood flow and immune activity).
  • Phagocytes (such as macrophages and neutrophils) that engulf and destroy microbes.
  • Natural Killer (NK) Cells that attack virus-infected and cancerous cells.

b. Second Line of Defense

• This response is specific and takes longer to activate but is more effective.
• Key components include:
  • B-Cells: Produce antibodies that neutralize pathogens.
  • T-Cells: Directly attack infected cells or assist in immune coordination.
  • Memory Cells: Store information about past infections to create faster responses in the future.

This function ensures that infections are controlled and eliminated before they cause severe damage.

3. Producing Antibodies and Neutralizing Harmful Substances

The immune system produces antibodies to neutralize infections and toxins.

a. How Antibodies Work

• B-cells recognize harmful substances and create antibodies (immunoglobulins, Ig) to fight them.
• Antibodies attach to antigens (foreign molecules) and:
  • Neutralize toxins (prevent them from harming the body).
  • Tag pathogens for destruction by immune cells.
  • Block viruses and bacteria from entering healthy cells.

b. Types of Antibodies

• IgG: Provides long-term immunity.
• IgA: Protects mucosal surfaces (respiratory and digestive tracts).
• IgM: First antibody produced during infections.
• IgE: Involved in allergic reactions.
• IgD: Assists in immune responses.

Antibodies are essential in preventing the spread of infections and neutralizing harmful substances before they cause harm.

Fig: Diagram showing types of antibodies: IgG (long-term immunity), IgA (mucosal protection), IgM (first response), IgE (allergic reactions), and IgD (immune response assistance).

4. Creating Immunological Memory

One of the immune system's most important functions is remembering past infections and responding faster if the same pathogen attacks again.

a. Memory B-Cells and T-Cells

• After an infection, some B-cells and T-cells remain in the body as memory cells.
• If the same pathogen reappears, memory cells allow the immune system to react quickly and prevent reinfection.

b. Vaccination and Immunological Memory

• Vaccines use weakened or inactive forms of pathogens to stimulate the immune system without causing disease.
• This helps the body develop memory cells so it can fight the real infection more effectively in the future.

This function ensures long-term immunity and protects individuals from recurring infections.

5. Regulating Immune Responses and Preventing Autoimmunity

The immune system must balance attacking harmful invaders while preventing unnecessary damage to the body's own cells.

a. Immune Regulation

• Regulatory T-cells (Tregs) control immune responses to prevent excessive reactions.
• This ensures that the immune system fights infections without harming the body.

b. Preventing Autoimmune Diseases

• Sometimes, the immune system mistakenly attacks the body's own cells, leading to autoimmune disorders like:
  • Rheumatoid arthritis (attacks joints).
  • Type 1 diabetes (attacks insulin-producing cells).
  • Multiple sclerosis (attacks nerve cells).
• The immune system has built-in mechanisms to prevent self-attack, but if these fail, autoimmune diseases occur.

Proper regulation ensures that the immune system functions correctly without harming the body.

6. Removing Damaged Cells and Preventing Cancer

The immune system helps remove damaged, infected, or cancerous cells before they cause problems.

a. Detecting and Eliminating Cancer Cells

• The immune system identifies and destroys abnormal cells before they develop into tumors.
• Natural Killer (NK) Cells and Cytotoxic T-Cells target and kill cancerous cells.

b. Removing Dead and Damaged Cells

• Macrophages clean up dead and damaged cells to keep tissues healthy.
• This function prevents harmful buildup and ensures the body maintains proper balance.

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What Are the Types of Immunity?

The immune system protects the body using different mechanisms, categorized into various immune types based on how immunity is acquired and how it functions. The immune types are classified into two main categories: innate immunity and adaptive immunity.

1. Innate Immunity (Natural or Non-Specific Immunity)

• This is the body's first line of defense present from birth.
• It acts quickly against pathogens without targeting specific ones.
• Components include skin, mucous membranes, white blood cells (phagocytes), and natural killer (NK) cells.
• It provides a generalized defense, blocking or destroying harmful microorganisms.

2. Adaptive Immunity (Acquired or Specific Immunity)

• This develops over time as the body is exposed to different pathogens.
• It provides long-term protection by recognizing and remembering past infections.
• It involves lymphocytes (B cells and T cells) that produce antibodies and destroy infected cells.
• Adaptive immunity is further divided into:
  • Active Immunity: The body produces its own antibodies after an infection or vaccination.
  • Passive Immunity: Antibodies are transferred from another source, such as from mother to baby through breast milk or via antibody injections.

3. Artificial Immunity

• This is a man-made method of developing immunity through medical intervention.
• It is achieved by vaccination (active artificial immunity) or antibody injections (passive artificial immunity).
• It helps in preventing severe infections by training the immune system to recognize harmful pathogens.
  
  

Common Immune System Disorders

Several immunology-significant diseases occur when the immune system fails to function properly. These disorders can result in either overactive immune responses or underactive immune defenses, leading to various health complications. Below are some of the most common immune system disorders:

1. Autoimmune Diseases

• The immune system mistakenly attacks the body's own tissues as if they were foreign invaders.
• Examples of autoimmune diseases include:
  • Rheumatoid Arthritis: The immune system attacks joints, leading to pain and swelling.
  • Lupus: A systemic disease where the immune system damages various organs, including skin, kidneys, and heart.
  • Multiple Sclerosis: The immune system attacks the protective covering of nerve fibers in the brain and spinal cord.

2. Immunodeficiency Disorders

• These occur when the immune system is weakened or unable to mount a proper defense against pathogens.
• Primary immunodeficiency is often genetic and present from birth.
• Acquired immunodeficiency can result from infections like HIV.
• Examples include:
  • HIV/AIDS: Human Immunodeficiency Virus (HIV) attacks T-cells, weakening the immune system and leading to Acquired Immunodeficiency Syndrome (AIDS).
  • Severe Combined Immunodeficiency (SCID): A genetic disorder resulting in a severely weakened immune system from birth, making individuals highly susceptible to infections.

3. Allergies

• Allergic reactions occur when the immune system overreacts to harmless substances (allergens).
• Common allergens include pollen, food, dust mites, and pet dander.
• Examples of allergic diseases:
  • Asthma: Airway inflammation triggered by allergens.
  • Hay Fever: Allergic reaction to pollen causing sneezing and itchy eyes.
  • Food Allergies: Immune responses to certain foods like peanuts, shellfish, or dairy.

4. Chronic Inflammatory Diseases

• Chronic inflammation occurs when the immune system continues to fight an infection or injury even after it has healed.
• Examples include:
  • Inflammatory Bowel Disease (IBD): Chronic inflammation of the digestive tract, including Crohn's disease and ulcerative colitis.
  • Psoriasis: Skin inflammation leading to red, scaly patches.

5. Graft-Versus-Host Disease (GVHD)

• This occurs after organ transplants, particularly bone marrow or stem cell transplants.
• The transplanted immune cells attack the recipient's tissues, causing inflammation and tissue damage.