Immunology I Quiz

Immune System

• The immune system is a collection of cells, tissues, and molecules that mediate resistance to foreign elements (antigens).
• Immune response is a coordinated reaction of cells and molecules to antigens.

Antigens and Epitopes

• Antigens are molecules that induce an immune response when introduced into the body.
• Epitopes or antigenic determinants are parts of an antigen that induce the immune response and bind to lymphocyte receptors and antibodies.

Self vs Non-Self

• The immune system must be able to discriminate between foreign elements and components of its own organism to prevent autoimmunity.
• Immune responses should be generated only when components of the immune system come into contact with non-self elements.

Innate and Adaptive Immunity

• There are two types of defense: Innate Immunity (non-specific) and Adaptive Immunity (specific).
• Innate Immunity is always active, whereas Adaptive Immunity is activated only when in contact with antigens.

Cells of the Immune System

• Neutrophils:
  • Role: Antimicrobial effectors, particularly in acute bacterial infection/inflammation
  • Mechanism: Phagocytosis; degranulation; neutrophil extracellular trap formation
• Eosinophils:
  • Role: Antiparasitic effectors, particularly in helminthic infection; some antiviral action
  • Mechanism: degranulation; limited phagocytosis
• Basophils:
  • Role: Mediator of inflammation
  • Mechanism: degranulation
• Mast cells:
  • Role: Immune surveillance, mediator and amplifier of inflammation and allergy
  • Mechanism: detection of threats and release of inflammatory mediators via degranulation or synthesis of lipid mediators and cytokines
• Macrophages:
  • Role: Immune surveillance, moderate antimicrobial capacity, limited antigen presentation
  • Mechanism: detection of threats and release of inflammatory mediators; phagocytosis
• Dendritic cells:
  • Role: Immune surveillance, antigen processing, and presentation
  • Mechanism: detection of threats and release of inflammatory mediators; endocytosis and phagocytosis
• Lymphocytes:
  • NK cells:
    • Role: Destruction of virally infected or abnormal host cells (including tumor cells)
    • Mechanism: recognition of virally infected or abnormal host cells and targeted release of cytotoxic granules
  • T cells and B cells:
    • Role: adaptive immunity; recognition of specific antigens/epitopes
    • Mechanism: activation and regulation of cell-mediated and antibody responses; memory cells

Cell Communication in the Immune System

• Cell-to-cell contact:
  • Receptors: expressed on the surface of a cell or in intracellular compartments
  • Ligands: molecules that activate receptors, can be soluble or membrane-bound
• Cytokines:
  • Proteins with diverse functions, such as cell growth, activation, and communication between cells
  • Examples: interleukins, chemokines

Innate Immunity

• Recognition of Pathogens:
  • Pathogen-Associated Molecular Patterns (PAMPs): recognizable by the innate immune system
  • Pattern Recognition Receptors (PRRs): receptors of innate immunity that recognize PAMPs
• Recognition of Damage:
  • Damage-Associated Molecular Patterns (DAMPs): released when cells die or connective tissue is damaged
• Sentinel Cells:
  • Recognition of threats: PAMPs and DAMPs
  • Examples: dendritic cells, macrophages, and mast cells
• Inflammation:
  • Sentinel cells exposed to PAMPs, DAMPs, or pro-inflammatory cytokines
  • Cell activation: upregulation of cellular antimicrobial defenses and release of pro-inflammatory mediators
• Phagocytosis:
  • Active process of capturing and ingesting foreign objects/microorganisms by phagocytes
  • Destruction of microorganisms, damaged cells, and cellular debris
  • Induction of cytokine production and processing and presentation of antigens
• Complement System:
  • Collection of circulating and membrane-associated proteins
  • Activation occurs in a sequential manner through three pathways: alternative, classical, and lectin
  • Leads to the formation of the Membrane Attack Complex (MAC), which breaches the cell membrane of the microbe, leading to destruction by osmotic lysis
• NK cells:
  • Destruction of virally infected or abnormal host cells (including tumor cells)
  • Mechanism: recognition of virally infected or abnormal host cells and targeted release of cytotoxic granules

Objectives

• Understand the mechanisms of adaptive immunity

Antigen Presentation

• Antigen receptors on T cells and B cells recognize who introduced the antigen
• Antigen presentation is required for activation of T cells
• T cells recognize pathogenic epitopes

Antigen Processing & Presentation

• Major Histocompatibility Complex (MHC) molecules present antigens to T cells
• Class I MHC presents antigens to CD8+ T cells
• Class I MHC takes any protein, breaks it down, and presents it on the cell surface
• Class II MHC presents antigens to CD4+ T cells
• Professional antigen-presenting cells (APCs) present antigens on MHC II molecules

Activation of Naive T cells

• Naive T cells require a level of stimulation to be activated by dendritic cells
• Activation always done by a dendritic cell
• Proliferation and differentiation of activated T cells into effector and memory T cells

Immature Dendritic Cells

• Encounter antigens in tissues
• Migrate to lymphoid tissue and mature
• Present antigens to T cells in lymph nodes

Differentiation of Activated T cells

• T cell subsets: CD4+ and CD8+
• CD4+ T cells: Th1, Th2, Th17 differ due to cytokines
• CD4+ T cells express surface molecules and secrete cytokines that activate other cells
• CD8+ T cells: Cytotoxic T Lymphocytes (CTLs) that kill infected cells

Adaptive Immunity - Cellular Response

• CD4+ Th1 cells: secrete cytokines, activate macrophages, and activate CD8+ CTLs
• CD8+ CTLs: induce apoptosis of infected cells

Adaptive Immunity - Humoral Response

• B cells recognize antigens without MHC presentation
• B cells are activated by Th cells (T-dependent) or polysaccharides (T-independent)
• Immunoglobulins (Ig): isotypes or classes include IgG, IgM, IgA, IgE, and IgD

Immunoglobulins

• Basic structure: two heavy chains and two light chains
• Isotypes differ in heavy chain structure, monomeric vs polymeric, subclasses, serum concentration, half-life, and functions

Distribution of Ig Isotypes

• IgM: found in blood
• IgG: found in blood and tissues
• IgA: found in mucous membranes
• IgE: found in epithelial tissue

Primary and Secondary Responses

• IgM: major Ab of primary responses
• IgG: major Ab of secondary responses