Innate and Adaptive Immunity: Key Concepts from Chapters 16 & 17

Innate Immunity: Nonspecific Defenses of the Host

Phagocytosis

Phagocytosis is a critical process of the innate immune system, involving the ingestion and destruction of pathogens by specialized cells.

• Steps of Phagocytosis:

  1. Chemotaxis: Phagocytes are attracted to the site of infection by chemical signals (e.g., chemokines).

  2. Adherence: Phagocyte membrane receptors (such as PRRs) bind to pathogen-associated molecular patterns (PAMPs) on microbes.

  3. Ingestion: The phagocyte engulfs the microbe, forming a phagosome.

  4. Phagolysosome Formation: The phagosome fuses with a lysosome, forming a phagolysosome.

  5. Digestion: Enzymes and toxic substances destroy the microbe.

  6. Exocytosis: Indigestible material is expelled from the cell.

• Receptors Involved: Pattern Recognition Receptors (PRRs), such as Toll-Like Receptors (TLRs), recognize PAMPs.

• End Result: Destruction of the pathogen and presentation of antigens to adaptive immune cells.

Immune Cell Types

The immune system consists of various cell lineages, each with specialized functions.

• Myeloid Lineage: Neutrophils, eosinophils, basophils, monocytes/macrophages, dendritic cells.

• Lymphoid Lineage: T cells, B cells, natural killer (NK) cells.

• Function: Myeloid cells are primarily involved in innate immunity; lymphoid cells are central to adaptive immunity.

PRR and PAMP

Pattern Recognition Receptors (PRRs) detect conserved microbial structures called Pathogen-Associated Molecular Patterns (PAMPs).

• PRRs: Receptors on immune cells that recognize PAMPs (e.g., TLRs, NOD-like receptors).

• PAMPs: Molecules commonly found on pathogens (e.g., lipopolysaccharide, flagellin, peptidoglycan).

• Importance: Initiate innate immune responses and inflammation.

• Example: TLR4 recognizes bacterial lipopolysaccharide (LPS).

Inflammation

Inflammation is a protective response to infection or injury, aiming to eliminate the cause and repair tissue.

• Signs and Symptoms:

  • Redness (rubor)

  • Heat (calor)

  • Swelling (tumor)

  • Pain (dolor)

  • Loss of function (functio laesa)

Extravasation

Extravasation is the process by which immune cells exit the bloodstream and migrate to sites of infection.

• Steps:

  1. Rolling: Leukocytes loosely bind to endothelial cells via selectins.

  2. Margination: Leukocytes move to the periphery of blood vessels.

  3. Diapedesis (Transmigration): Leukocytes squeeze between endothelial cells.

  4. Emigration: Leukocytes migrate into the tissue toward the infection site.

• Proteins Involved: Selectins, integrins, chemokines.

Cytokine vs. Chemokine

Cytokines and chemokines are signaling proteins that regulate immune responses.

• Cytokines: Broad category of proteins (e.g., interleukins, interferons) that modulate immune cell activity.

• Chemokines: Subset of cytokines that specifically induce chemotaxis (cell movement) toward sites of inflammation.

• Examples: IL-2 (cytokine), CXCL8/IL-8 (chemokine).

Antimicrobial Proteins: Complement System

The complement system consists of plasma proteins that enhance immune responses against pathogens.

• Three Pathways:

  • Classical Pathway: Triggered by antigen-antibody complexes.

  • Lectin Pathway: Initiated by mannose-binding lectin binding to pathogen surfaces.

  • Alternative Pathway: Activated directly by pathogen surfaces.

• Activation: Each pathway leads to the cleavage of C3, a central complement protein.

• Main Effector Mechanisms:

  • Opsonization (coating pathogens for phagocytosis)

  • Inflammation (recruitment of immune cells)

  • Cell lysis (formation of the membrane attack complex, MAC)

Four Major Principles of Immunology

• Specificity: Immune responses are tailored to specific antigens.

• Diversity: The immune system can recognize a vast array of antigens.

• Memory: The immune system responds more rapidly and effectively to previously encountered antigens.

• Self/Non-self Recognition: The immune system distinguishes between the body's own cells and foreign invaders.

Adaptive Immunity: Specific Defenses of the Host

Humoral vs. Cell-Mediated Immunity

Adaptive immunity is divided into humoral and cell-mediated branches, each with distinct roles.

• Humoral Immunity: Mediated by B cells and antibodies; effective against extracellular pathogens.

• Cell-Mediated Immunity: Mediated by T cells; effective against intracellular pathogens (e.g., viruses, some bacteria).

• Significance: Both branches are essential for comprehensive immune protection.

Immunocompetent vs. Self-Tolerant

These terms describe key properties of functional immune cells.

• Immunocompetent: Cells capable of mounting an immune response to foreign antigens.

• Self-Tolerant: Cells that do not react against the body's own tissues.

• Role: Ensures effective defense while preventing autoimmunity.

Epitope/Antigenic Determinant

An epitope is the specific part of an antigen recognized by immune receptors.

• Importance: Determines the specificity of antibody and T-cell responses.

• Example: A single protein antigen may have multiple epitopes, each recognized by different antibodies.

Haptens and Carrier Proteins

Haptens are small molecules that are not immunogenic by themselves but can elicit an immune response when attached to larger carrier proteins.

• Significance: Explains allergic reactions to small chemicals and is used in vaccine design.

• Example: Penicillin acts as a hapten when bound to serum proteins, potentially triggering an allergic response.

Antibody Families (Isotypes)

Antibodies (immunoglobulins) are classified into five main families, each with unique functions.

Isotype	Main Function
IgG	Most abundant; crosses placenta; opsonization, neutralization
IgM	First antibody produced; effective in agglutination and complement activation
IgA	Found in mucosal areas and secretions; protects body surfaces
IgE	Involved in allergic responses and defense against parasites
IgD	Functions mainly as a B cell receptor

Antibody Structure vs. T-Cell Receptor Structure

Both antibodies and T-cell receptors (TCRs) are antigen-specific receptors, but they differ in structure and function.

• Antibody Structure: Y-shaped molecule with two heavy and two light chains; can be secreted or membrane-bound.

• TCR Structure: Composed of alpha and beta chains; always membrane-bound; recognizes antigen only when presented by MHC molecules.

• Importance: Determines how each receptor recognizes and responds to antigens.

MHC Class I and II

Major Histocompatibility Complex (MHC) molecules present antigens to T cells, guiding immune responses.

MHC Class	Expressed On	Presents To	Type of Antigen
I	All nucleated cells	CD8+ cytotoxic T cells	Endogenous (intracellular)
II	Antigen-presenting cells (APCs)	CD4+ helper T cells	Exogenous (extracellular)

Main Functions of T-Cells

T cells differentiate into various subsets, each with specialized roles in immunity.

• TH1: Activate macrophages and cytotoxic T cells; important for intracellular pathogens.

• TH2: Stimulate B cells to produce antibodies; important for extracellular pathogens and allergies.

• TH17: Recruit neutrophils; important in defense against fungi and extracellular bacteria.

• TReg (Regulatory T cells): Suppress immune responses to maintain tolerance and prevent autoimmunity.

• T memory: Provide long-term immunity by responding rapidly to previously encountered antigens.

• TFH (Follicular Helper T cells): Help B cells in germinal centers produce high-affinity antibodies.

Immune System Communication

The innate and adaptive branches of the immune system interact through cell signaling and antigen presentation.

• Antigen Presentation: Dendritic cells and macrophages present antigens to T cells, linking innate and adaptive responses.

• Cytokine Networks: Cytokines coordinate the activities of various immune cells.

• Effector Functions: Antibodies produced by B cells (adaptive) enhance phagocytosis (innate).

Additional info: Academic context and examples have been added to ensure completeness and clarity for exam preparation.