Introduction to Immunology

Introduction to Immunology

• The immune system protects the body from harmful invaders, including:
  • Microorganisms (bacteria, viruses, fungi, parasites)
  • Microbial toxins
  • Foreign substances (pollens, pet danders, chemicals)
• There are two main types of immunity:
  • Innate (natural): This is the first line of defense and is always active.
  • Adaptive (acquired): This involves a specific immune response that develops after exposure to a particular pathogen.

Key Immune System Concepts

• Immunity: Resistance to disease, particularly infectious disease.
• Immune System: The collection of cells, tissues, and molecules that mediate this resistance.
• Immune Response: The coordinated reaction of these cells and molecules to infectious microbes.

Lymphocytes: The Main Players

• Lymphocytes are key components of the immune system. They are mononuclear cells classified as B and T cells, based on their origin:
  • B lymphocytes (B cells): Mature in the bone marrow.
  • T lymphocytes (T cells): Mature in the thymus.
• Lymphocytes have antigen receptors on their surface, enabling them to recognize specific antigens.

Other Important Immune Cells

• Basophils: These are granulocytes involved in inflammatory reactions, particularly those associated with allergic symptoms. They contain granules with:
  • Heparin: Inhibits blood clotting.
  • Histamine: Causes vasodilation and increased permeability, promoting inflammation.
• Eosinophils: These are multi-functional leukocytes involved in inflammatory responses, especially against:
  • Helminths (parasitic worms).
  • Allergies.
• Neutrophils: These are first responders to an infection. They engulf and destroy invading microorganisms through:
  • Phagocytosis: Engulfing and digesting foreign material.
  • Enzyme release: Killing microbes.
• Dendritic cells: These cells are located in the tissues, especially the skin. They present antigens to other immune cells, triggering an immune response. They are:
  • Phagocytes: Engulf and digest foreign matter.
  • Antigen-presenting cells (APCs): Show antigens to other immune cells.
• Macrophages: These are large phagocytic cells that engulf and destroy microbes, remove dead cells, and activate other immune system cells.
• Mast cells: These cells reside in connective tissue and release chemicals like histamine, tumor necrosis factor, and interleukins, triggering inflammation.

The Role of the Immune System

• Protection from invaders:
  • Viruses
  • Bacteria
  • Fungi
  • Parasites

The Main Components of the Immune System

• White blood cells: The primary cells involved in immunity.
• Antibodies: Proteins that bind to specific antigens, neutralizing them.
• Complement system: A series of proteins that work together to lyse cells, promote inflammation, and opsonize (tag) pathogens for phagocytosis.
• Lymphatic system: A network of vessels and nodes that transports lymph, a fluid that carries immune cells and substances.
  • Lymph nodes: Small organs that filter lymph and are sites of immune responses.
• Spleen: An organ that filters blood and removes old or damaged red blood cells. It also houses immune cells and plays a role in antibody production.
• Bone marrow: The site of blood cell production, including immune cells.
• Thymus: A gland that is essential for the development and maturation of T cells.
• T cells: Lymphocytes that are responsible for cell-mediated immunity.
• B cells: Lymphocytes that are responsible for humoral immunity (antibody production).

Immune System Components: Cells and Organs

• Lymphoid tissue: The tissues where immune cells are located and function.
  • Primary lymphoid tissue: Where lymphocytes differentiate, proliferate, and mature:
    • Bone marrow
    • Thymus
    • Fetal liver
  • Secondary lymphoid tissue: Where lymphocytes exert their effector function.
    • Spleen
    • Lymph nodes
    • Mucosal-associated lymphoid tissue (MALT)

T Cell Function

• Pre-T lymphocytes undergo maturation in the thymus.
• Mature T cells migrate to secondary lymphoid tissues.
  • Spleen: Found around central arterioles in the white pulp.
  • Lymph nodes: Found in the paracortex.

Types of T Cells

• T Helper (TH1-2) cells: Express CD4+ molecules on their surface. They:
  • Stimulate B cells to produce antibodies.
• Cytotoxic T (Tc) cells: Express CD8+ molecules on their surface. They:
  • Destroy target cells (infected cells, tumor cells).

B Cell Function

• Pre-B lymphocytes mature in the bone marrow.
• Mature B cells migrate to secondary lymphoid tissues:
  • Spleen: Located in the white pulp and form primary and secondary follicles.
  • Lymph nodes: Found in the cortex.
• B cells differentiate into:
  • Plasma cells: Produce antibodies.
  • Memory cells: Remember specific antigens and provide long-term immunity.

Leukocytes: Granulocytes and Agranulocytes

• Granulocytes: White blood cells with granules in their cytoplasm, which release signaling molecules. They are short-lived.
  • Neutrophils: The most abundant granulocytes.
  • Basophils: Involved in allergic responses.
  • Eosinophils: Involved in parasitic infections and allergic responses.
• Agranulocytes: White blood cells that lack granules. They are long-lived.
  • Lymphocytes: The main players in adaptive immunity.
  • Monocytes: Large phagocytic cells that mature into macrophages.

Natural Killer (NK) Cells

• NK cells: Large granular lymphocytes that belong to the innate immune system. They are cytotoxic (cell-killing) for:
  • Tumor cells: Immune surveillance.
  • Virally infected cells: Fight viral infections.
  • Bacteria, fungi, and parasites: Innate defense.
• Mechanisms of action:
  • Perforin: An extracellular protein that perforates cell membranes, leading to cell lysis.
  • Interferon (IFN): Proteins that inhibit viral replication.

Antigen-Presenting Cells (APCs)

• APCs: A heterogeneous population of leukocytes that present antigens to other immune cells.
• Location: Found primarily in the skin, lymph nodes, thymus, and mucosal epithelia.
• Types:
  • Macrophages: Large phagocytic cells that engulf and destroy microbes and present antigens.
  • Dendritic cells: Specialized APCs that present antigens to T cells.
    • Interdigitating dendritic cells (ID): Present antigens to T cells.
    • Follicular dendritic cells (FD): Present antigens to B cells.

The Immune System

• The immune system is responsible for defending the body against invading organisms and substances.
• These invaders include living organisms like viruses, bacteria, fungi, and parasites, as well as non-living substances like toxins, pollens, pet danders, and chemicals.
• Immunity is the body's resistance to disease, specifically infectious disease.
• The immune system comprises cells, tissues, and molecules that work together to prevent or eradicate infections.

Types of Immunity

• Innate (natural) immunity: This is the body's immediate, non-specific defense system. It is always active and does not need prior exposure to an invader.
• Adaptive (acquired) immunity: This system is specific to each pathogen it encounters and involves the development of memory cells that can quickly respond to repeat infections.

Immune System Cells and Organs

• Leukocytes (white blood cells): These are the primary cells of the immune system. They can be further categorized as:

  • Granulocytes: These cells contain granules in their cytoplasm. They are short-lived, lasting only 2–3 days. Examples include neutrophils, basophils, and eosinophils.
  • Agranulocytes: These cells lack granules. They live longer, for months. Examples include lymphocytes and monocytes.

• Lymphocytes: These are crucial for adaptive immunity. Two main types exist:

  • T cells: They are derived from the thymus gland and migrate to secondary lymphoid tissues.
    • T Helper (TH1-2) Cells (CD4+): These cells stimulate B cells to produce antibodies.
    • Cytotoxic T (Tc) Cells (CD8+): These cells directly destroy target cells infected with viruses or cancer.
  • B cells: They mature in the bone marrow and migrate to secondary lymphoid tissues.
    • They differentiate into plasma cells that produce antibodies and memory cells that provide long-term immunity.

• Natural Killer (NK) Cells: These are large granular lymphocytes that are part of innate immunity. They are cytotoxic and can kill:

  • Tumor cells
  • Virus-infected cells
  • Cells infected with bacteria, fungi, or parasites

• Antigen-Presenting Cells (APCs): These cells present antigens to T cells, initiating adaptive immune responses. Examples include:

  • Macrophages: These cells engulf and destroy microorganisms.
  • Dendritic cells: These cells capture antigens in tissues and migrate to lymph nodes to present them to T cells.

• Lymphoid Tissue: This tissue houses lymphocytes and facilitates immune responses.

  • Primary lymphoid tissue: This is where lymphocytes differentiate, proliferate, and mature. Examples include the bone marrow and fetal liver.
  • Secondary lymphoid tissues: This is where lymphocytes are activated and exert their effector functions. Examples include the spleen, lymph nodes, and mucosal-associated lymphoid tissue (MALT).

Other Immune System Components

• Antibodies: These are proteins produced by B cells. They bind to specific antigens, neutralizing them and marking them for destruction by other immune cells.
• Complement system: This is a series of proteins that work together to augment the immune response. They can directly kill bacteria, attract immune cells, and enhance antibody binding.
• Lymphatic system: This system is a network of vessels and nodes that transports lymph fluid, filtering it and carrying immune cells throughout the body.
• Spleen: This organ filters blood and removes old red blood cells. It also houses lymphocytes and plays a role in antibody production.
• Bone marrow: This is where all blood cells, including immune cells, are produced.
• Thymus: This is where T cells mature.

Introduction to Immunology

• The immune system protects against foreign organisms and substances, including microbes, toxins, pollens, pet danders, and chemicals.
• Immunity is the resistance to disease, specifically infectious disease.
• The immune system consists of cells, tissues, and molecules that mediate resistance to infections.
• The immune response is the coordinated reaction of these cells and molecules to infectious microbes.

Immune System Components

• Lymphocytes: Mononuclear cells that are part of the leukocyte cell lineage. Subdivided into B (Bone marrow-derived) and T (Thymus-derived) lymphocytes.
• Basophils: Granulocytes that participate in inflammatory reactions, particularly those causing allergic symptoms. Their granules contain heparin and histamine.
• Eosinophils: Polymorphic multi-functional leukocytes involved in inflammatory responses, especially helminth infections and allergic disease.
• Neutrophils: First immune cells to arrive at the site of infection. They engulf microorganisms and release enzymes that kill them.
• Dendritic cells: Found in tissues like the skin, they boost immune responses by presenting antigens to other immune cells.
• Macrophages: White blood cells that engulf and kill microorganisms, remove dead cells, and stimulate other immune cells.
• Mast cells: Granular cells found in body tissues like connective tissue. They activate inflammation by releasing substances like histamine, tumor necrosis factor, and interleukins.

Role of The Immune System

• Protects from viruses, parasites, fungi, and bacteria.

Main Parts of the Immune System

• White blood cells
• Antibodies
• Complement system
• Lymphatic system (Lymph nodes)
• Spleen
• Bone marrow
• Thymus
• T cells
• B cells

Immune Cells and Organs

• Lymphoid tissue:

  • Primary lymphoid tissue: Lymphocytes differentiate, proliferate, and mature (e.g., bone marrow, thymus, fetal liver).
  • Secondary lymphoid tissues: Lymphocytes exert their effector function (e.g., spleen, lymph nodes, mucosal-associated lymphoid tissue (MALT)).

• T cells:

  • Pre-T lymphocytes mature in the thymus gland.
  • They leave the thymus and enter secondary lymphoid tissues.
  • In the spleen, they stay around central arterioles in the white pulp.
  • In lymph nodes, they are found in the paracortex.

• Types of T cells:

  • T helper (TH1-2) Cells (CD4+): Stimulate B cells to produce antibodies.
  • Cytotoxic T (Tc) Cells (CD8+): Destroy target cells.

• B cells:

  • Pre-B lymphocytes mature in the bone marrow.
  • They exit the bone marrow and enter secondary lymphoid tissues.
  • In the spleen, they reside in the white pulp and form primary and secondary follicles.
  • In lymph nodes, they are found in the cortex.
  • They differentiate into plasma cells and memory cells.

• Leukocytes:

  • Granulocytes: Short-lived (2-3 days) with granules in their cytoplasm.
    • Neutrophils
    • Basophils
    • Eosinophils
  • Agranulocytes: Long-lived (months) without cytoplasmic granules.
    • Lymphocytes
    • Monocytes

• NK cells:

  • Large granular lymphocytes.
  • Innate cytotoxic lymphocytes.
  • Function:
    • Kill tumor cells (immune surveillance).
    • Kill virally infected cells.
    • Contribute to bacterial, fungal, and parasitic infection control.
    • Release perforin, which damages cell membranes and causes lysis.
    • Produce interferons (IFNs) that protect against viral infection.

• APC (Antigen Presenting cells):

  • Heterogeneous population of leukocytes.
  • Found primarily in the skin, lymph nodes, thymus, and mucosal epithelia.
  • Types:
    • Macrophages
    • Dendritic cells:
      • Interdigitating dendritic cells (ID): Present antigens to T cells.
      • Follicular dendritic cells (FD): Present antigens to B cells.

The Immune System

• The immune system defends the body against foreign organisms and substances.
• These invaders include:
  • Living organisms (microbes)
  • Microbial toxins and byproducts
  • Foreign substances like pollen, pet dander, and chemicals
• Immunity is defined as resistance to disease, specifically infectious disease.
• There are two main types of immunity:
  • Innate (natural) immunity: This is the body's first line of defense, providing non-specific protection against invaders.
  • Adaptive (acquired) immunity: This is a specific response that targets specific pathogens or antigens, creating immunological memory for future encounters.
• The most important physiological function of the immune system is to prevent or eradicate infections.

Immune System Components

• Lymphocytes: These mononuclear cells are part of the leukocyte lineage and are subdivided into:
  • B lymphocytes (B cells): Derived from bone marrow; express antigen receptors for recognizing specific antigens.
  • T lymphocytes (T cells): Derived from the thymus; express antigen receptors for specific antigens, playing diverse roles in immune responses.
• Basophils: These granulocytes are involved in inflammatory reactions, especially those linked to allergic symptoms. They contain heparin and histamine within their granules.
• Eosinophils: These multi-functional leukocytes are associated with inflammatory responses, particularly in response to helminth infections and allergic diseases.
• Neutrophils: These are among the first immune cells to arrive at infection sites. They engulf microorganisms and release enzymes to kill them.
• Dendritic cells: These specialized immune cells are found in tissues (like skin) and stimulate immune responses by presenting antigens to other immune cells. They act as both phagocytes and antigen-presenting cells (APCs).
• Macrophages: These white blood cells engulf and kill microorganisms, remove dead cells, and activate other immune system cells.
• Mast cells: These granular cells in tissues (especially connective tissue) activate inflammation by releasing substances like histamine, tumor necrosis factor, and interleukins.

Role of the Immune System

• The immune system protects the body from:
  • Viruses
  • Parasites
  • Fungi
  • Bacteria

Main Parts of the Immune System

• White blood cells (leukocytes)
• Antibodies
• Complement system
• Lymphatic system (including lymph nodes)
• Spleen
• Bone marrow
• Thymus
• T cells
• B cells

Immune Cells and Organs

• Lymphoid Tissue:
  • Primary Lymphoid Tissue: Where lymphocytes differentiate, proliferate, and mature. This includes fetal liver, bone marrow, and the thymus.
  • Secondary Lymphoid Tissues: Where lymphocytes exert their effector functions. This includes the spleen, lymph nodes, and mucosal-associated lymphoid tissue (MALT).

T Cells

• Pre-T lymphocytes mature in the thymus gland.
• They leave the thymus to secondary lymphoid tissues (lymphatic tissues).
• In the spleen, they reside around central arterioles in white pulp.
• In lymph nodes, they are found in the paracortex.

Types of T Cells

• T Helper (TH1-2) Cells (CD4+): Stimulate B cells to produce antibodies.
• Cytotoxic T (Tc) Cells (CD8+): Destroy target cells.

B Cells

• Pre-B lymphocytes mature in the bone marrow.
• They leave the bone marrow and migrate to secondary lymphoid tissues.
• In the spleen, they reside in white pulp and form primary and secondary follicles.
• In lymph nodes, they are found in the cortex.
• They differentiate into plasma cells and memory cells.

Leukocytes: Granulocytes and Agranulocytes

• Granulocytes: Characterized by granules in their cytoplasm; short-lived (approximately 2-3 days).
  • Neutrophils
  • Basophils
  • Eosinophils
• Agranulocytes: Lack granules; long-lived (for months).
  • Lymphocytes
  • Monocytes

NK Cells

• Natural killer (NK) cells are large granular lymphocytes.
• They are innate cytotoxic lymphocytes, meaning they can kill cells without prior sensitization.
• They play a role in immune surveillance, recognizing and killing:
  • Tumor cells
  • Virus-infected cells
  • Bacteria, fungi, parasites
• They release perforin, which perforates cell membranes, leading to cell lysis.
• They also produce interferon (IF-), which protects against viral infections.

Antigen Presenting Cells (APCs)

• APCs are a diverse population of leukocytes that present antigens to T cells.
• They are found in various tissues such as the skin, lymph nodes, thymus, and mucosal epithelia.
• Types of APCs include:
  • Macrophages:
  • Dendritic cells:
    • Interdigitating dendritic cells (ID): Present antigens to T cells.
    • Follicular dendritic cells (FD): Present antigens to B cells.

The Immune System

• Protects the body from foreign invaders such as bacteria, viruses, fungi, parasites, toxins, and other substances like pollens, pet danders, and chemicals.
• Two main types of immunity: innate (natural) and adaptive (acquired).
• The most important function of the immune system is to prevent or eradicate infections.

Immunology Terms

• Immunity: Resistance to disease, especially infectious disease.
• Immune system: The collection of cells, tissues, and molecules that protect the body from infection.
• Immune response: The coordinated reaction of immune cells and molecules to infectious agents.
• Lymphocytes: Mononuclear white blood cells that are part of the leukocyte lineage and are subdivided into B and T lymphocytes. They express antigen receptors for recognizing specific antigens.
• Basophils: Granulocytes involved in inflammatory reactions, particularly those causing allergic symptoms. They contain granules with heparin and histamine.
• Eosinophils: Polymorphic leukocytes involved in inflammatory responses, especially against helminths and allergies.
• Neutrophils: Immune cells that are among the first responders to infection. They ingest microorganisms and release enzymes to kill them.
• Dendritic cells: Immune cells found in various tissues which boost immune responses by presenting antigens to other immune cells. They are phagocytes and antigen-presenting cells (APCs).
• Macrophages: White blood cells that engulf and kill microorganisms, remove dead cells, and stimulate other immune cells.
• Mast cells: Granular cells located in tissues, especially connective tissue, that activate inflammation by releasing histamine, tumor necrosis factor, and interleukins.

Parts of the Immune System

• White blood cells: Leukocytes involved in immune responses.
• Antibodies: Proteins produced by B lymphocytes that recognize and neutralize specific antigens.
• Complement system: A group of proteins that work together to destroy pathogens.
• Lymphatic system: A network of vessels and lymph nodes that transports lymph fluid and contains immune cells.
• Spleen: An organ that filters blood and removes old red blood cells, and hosts immune cells.
• Bone marrow: Site of blood cell production, including immune cells like B lymphocytes.
• Thymus: Gland responsible for the maturation of T lymphocytes.
• T cells: Lymphocytes involved in cell-mediated immunity.
• B cells: Lymphocytes involved in humoral immunity (antibody production).

Immune Cells and Organs

• Lymphoid tissue: Classified into primary and secondary lymphoid tissues.
  • Primary lymphoid tissue: Where lymphocytes differentiate, proliferate, and mature (e.g., bone marrow, thymus).
  • Secondary lymphoid tissues: Where lymphocytes exert their effector functions (e.g., spleen, lymph nodes, mucosal-associated lymphoid tissue (MALT)).

T Cells

• Pre-T lymphocytes mature in the thymus gland.
• Mature T cells migrate to secondary lymphoid tissues.
• In the spleen, they reside near central arterioles in the white pulp.
• In lymph nodes, they are found in the paracortex.

Types of T Cells

• T Helper (TH1-2) Cells (CD4+): Stimulate B cells to produce antibodies.
• Cytotoxic T (Tc) Cells (CD8+): Destroy target cells.

B Cells

• Pre-B lymphocytes mature in the bone marrow.
• Mature B cells migrate to secondary lymphoid tissues.
• In the spleen, they reside in the white pulp and form primary and secondary follicles.
• In lymph nodes, they are found in the cortex.
• They differentiate into plasma cells (antibody producers) and memory cells.

Leukocytes: Granulocytes & Agranulocytes

• Granulocytes: White blood cells containing granules in their cytoplasm. They are short-lived (2-3 days). Three types:
  • Neutrophils: Phagocytize and kill microorganisms.
  • Basophils: Release histamine and heparin during inflammation.
  • Eosinophils: Involved in inflammation, particularly against parasitic infections and allergies.
• Agranulocytes: White blood cells lacking granules in their cytoplasm. They are long-lived (months). Two types:
  • Lymphocytes: T cells and B cells involved in specific immunity.
  • Monocytes: Precursors to macrophages, which are phagocytes and APCs.

NK Cells

• Natural killer (NK) cells are large granular lymphocytes.
• They are cytotoxic, killing tumor cells and viral infected cells.
• They release perforin, which perforates cell membranes, and interferon (IFN) which combats viral infections.

Antigen Presenting Cells (APCs)

• Heterogeneous population of leukocytes that present antigens to other immune cells.
• Found in the skin, lymph nodes, thymus, and mucosal epithelia.
• Types:
  • Macrophages: Phagocytize and present antigens.
  • Dendritic cells:
    • Interdigitating dendritic cells (ID): Present antigens to T cells.
    • Follicular dendritic cells (FD): Present antigens to B cells.