Immune System and Lymphoid Tissue Study Notes

Objectives

  • At the end of this class, the learner will be able to describe and identify the histological features related to:

    • Innate and adaptive immune system and innate lymphoid cells

    • Lymphoid organs and their clinical applications

Overview of the Immune System

  • Immune System Definition: A coordinated network of cells, tissues, organs, and soluble molecules that protects the body against foreign antigens, infectious organisms, and abnormal cells.

  • Immunity Types:

    1. Innate Immunity:

    • Non-specific, immediate defense against pathogens.

    • Present at birth and does not generate immunologic memory.

    • Responses occur similarly upon repeated exposure.

    1. Adaptive Immunity:

    • Acquired, specific defense mechanism acting as the body's second line of defense.

    • Involves B and T lymphocytes recognizing specific pathogens (antigens) and establishing long-lasting immunological memory.

    • Develops following exposure to antigens and characterized by specificity, diversity, memory, and self versus non-self discrimination.

  • Immune Responses: Occur in connective tissues, blood, lymph, and specialized lymphoid organs, facilitated by soluble mediators such as antibodies, complement proteins, and cytokines that allow communication between immune cells.

Antigens

  • Definition: Any substance capable of triggering an immune response.

  • Types of Antigens:

    1. Exogenous Antigens: Originate outside the body (e.g., bacteria, viruses, toxins, allergens).

    2. Endogenous Antigens: Arise within cells (e.g., viral proteins, tumor antigens, altered self-proteins).

  • Recognition: Antigens are recognized by antigen receptors on lymphocytes or processed and presented by Antigen-Presenting Cells (APCs) like dendritic cells, macrophages, and B-cells.

  • Epitope vs. Paratope:

    • Epitope: Specific part of an antigen recognized by the immune system.

    • Paratope: Matching antigen-binding site on an antibody.

Components of Innate Immunity

  • Definition: The first-line defense and the system that bridges to adaptive immunity.

  • Characteristics:

    • Non-specific

    • Fast response (0 to 4 hours).

    • Does not generate immunologic memory; responds similarly on re-exposure.

  • Key Components:

    1. Physical Barriers: Skin and mucous membranes.

    2. Chemical Defenses: Low pH, lysozyme, interferons, and antimicrobial peptides.

    3. Cellular Components:

    • Neutrophils

    • Macrophages

    • Monocytes

    • Dendritic cells

    • Mast cells

    • Eosinophils

    • Natural Killer (NK) cells

    1. Pattern Recognition Mechanisms:

    • Pattern Recognition Receptors (PRRs):

      • Toll-like receptors (TLRs) on cell membranes.

      • NOD-like receptors (NLRs) within the cytoplasm.

      • RIG-I-like receptors that detect viral RNA.

    1. Circulating Proteins: Antimicrobial proteins like defensins and acute phase proteins (C-reactive protein, serum amyloid A, and collectins).

  • Cytokines: