Innate Immune System and Host Defenses

Vocabulary flashcards covering the key terms, cell types, chemical mediators, and processes of the innate immune system from Bio-240 Chapter 15.

Primary Lymphoid Organs

Structures such as the red bone marrow and thymus where lymphocytes arise and undergo maturation.

Secondary Lymphoid Organs

Tissues such as lymph nodes and the spleen where mature lymphocytes reside and encounter foreign microbes and antigens.

Thymus

A primary lymphoid organ to which immature T cells migrate from the bone marrow to complete their maturation.

Lymph

Fluid formed when blood plasma leaks out into tissue spaces as interstitial fluid and is absorbed by lymphatic capillaries.

First Line of Defense

Physical and chemical barriers—including intact skin, mucous membranes, secretions, and mechanical removal—that prevent microbes from entering body tissues.

Second Line of Defense

Rapid, non-specific innate internal defenses—including immune cells, inflammation, complement, antimicrobial molecules, and fever—activated after microbes cross surface barriers.

Third Line of Defense

Adaptive immunity consisting of highly specific responses and immune memory mediated by B cells, T cells, and antibodies.

Mucociliary Escalator

A mechanical defense mechanism in which respiratory cilia transport mucus-trapped microbes upward and away from the lungs to be swallowed, coughed, or sneezed out.

Lysozyme

An antimicrobial enzyme present in tears, saliva, mucus, and breast milk that damages bacterial cell walls by cleaving the bonds between NAG and NAM in peptidoglycan.

Antimicrobial Peptides (AMPs)

Small peptide molecules produced by host epithelial cells, immune cells, or bacteria that rapidly disrupt microbial membrane integrity.

Defensins

Positively charged antimicrobial peptides produced by epithelial cells, macrophages, and neutrophils that target and disrupt negatively charged microbial membranes.

Bacteriocins

Antimicrobial peptides produced by resident gastrointestinal microbiota that bind and disrupt the membranes of competing susceptible bacteria.

Pathogen-Associated Molecular Patterns (PAMPs)

Conserved molecular structures shared across broad groups of microorganisms, such as LPS, flagellin, and viral nucleic acids, recognized by innate receptors.

Damage-Associated Molecular Patterns (DAMPs)

Molecules released or exposed by host cells that have been injured, stressed, or damaged.

Pattern-Recognition Receptors (PRRs)

Innate host receptors located on the cell surface or within internal compartments that detect conserved PAMPs or DAMPs to trigger immune cell activation.

Toll-Like Receptors (TLRs)

A major class of pattern-recognition receptors, such as TLR4, that bind specific microbial ligands and initiate intracellular signaling to alter nuclear gene expression.

Lipopolysaccharide (LPS)

A major component of the outer membrane of Gram-negative bacteria whose lipid A portion binds TLR4 to stimulate inflammatory cytokine production.

Cytokines

Small protein chemical messengers—such as interleukins, chemokines, and interferons—that allow immune cells to communicate and coordinate an immune response.

Interleukins

A major class of cytokines that regulate and coordinate the activities and development of immune cells.

Chemokines

Chemotactic cytokines that attract specific leukocytes toward sites of infection, tissue injury, and inflammation.

Autocrine Signaling

A pathway in which a cell secretes a chemical signal that binds to receptors on its own cell surface.

Paracrine Signaling

A pathway in which a cell secretes chemical signals that diffuse to act on nearby target cells.

Endocrine Signaling

A signaling mechanism in which chemical mediators travel through the bloodstream to act on distant target cells.

Histamine

An inflammatory mediator released by mast cells and basophils that induces vasodilation and increases vascular permeability, causing local redness and swelling.

Prostaglandins

Lipid mediators generated from membrane arachidonic acid during inflammation that sensitize local pain receptors and help raise the hypothalamic temperature set point.

Erythema

Local redness of the skin or tissue caused by increased blood flow through dilated blood vessels during inflammation.

Edema

Tissue swelling resulting from increased vascular permeability and fluid accumulation in interstitial spaces.

Hematopoietic Stem Cells (HSCs)

Bone marrow stem cells that differentiate into both myeloid and lymphoid lineages, generating all blood and immune cell types.

Granulocytes

A category of leukocytes containing prominent cytoplasmic granules filled with chemical mediators; includes neutrophils, eosinophils, and basophils.

Agranulocytes

Leukocytes that lack prominent visible cytoplasmic granules; includes monocytes and lymphocytes.

Neutrophils

The most abundant circulating leukocytes that rapidly migrate to infection sites to kill extracellular bacteria via phagocytosis, degranulation, and NET formation.

Neutrophil Extravasation Traps (NETs)

Extracellular webs of DNA and antimicrobial proteins extruded by neutrophils to trap and kill invading microbes.

Phagocytosis

A cellular process involving chemotaxis, attachment, engulfment, phagosome formation, phagolysosome fusion, killing/digestion, and exocytosis to destroy microbes.

Phagolysosome

A vesicle created by the fusion of a phagosome with a lysosome, providing low pH, digestive enzymes, and reactive oxygen species to kill internalized pathogens.

Opsonization

The coating of a pathogen surface with molecules like C3b or antibodies (opsonins) to facilitate recognition and attachment by phagocytes.

Complement System

A cascade of approximately 30 blood proteins that amplify innate immunity through opsonization (C3b), inflammation/chemotaxis (C3a, C5a), and membrane attack complex-mediated cytolysis.

Membrane Attack Complex (MAC)

A pore-forming structure assembled on microbial membranes by complement proteins C5b, C6, C7, C8, and C9 that causes osmotic cell lysis.

Monocytes

Large circulating phagocytic agranulocytes that exit the bloodstream into tissue spaces to differentiate into macrophages or dendritic cells.

Macrophages

Large, highly phagocytic tissue-resident cells that engulf pathogens and debris, produce cytokines, and display antigen fragments on MHC class II molecules.

Antigen-Presenting Cells (APCs)

Immune cells—specifically dendritic cells, macrophages, and B cells—that process pathogen proteins and display antigen fragments on surface MHC II molecules to activate T cells.

Dendritic Cells

Specialized phagocytic cells in barrier tissues that capture antigens, process them, and migrate to secondary lymphoid organs to activate naive T cells.

Eosinophils

Granulocytes containing major basic protein (MBP) and degradative enzymes that attack large extracellular parasites like helminths by degranulation.

Mast Cells

Tissue-resident cells situated near blood vessels and mucosal surfaces that release histamine to stimulate local inflammation upon activation.

Natural Killer (NK) Cells

Cytotoxic lymphocytes that identify virus-infected or abnormal host cells with altered/reduced MHC I expression and induce target cell apoptosis using perforin and granzymes.

Type I Interferons

Cytokines (IFN-α and IFN-β) secreted by virus-infected cells that induce an antiviral state in nearby uninfected cells by suppressing protein synthesis and degrading viral RNA.

Type II Interferon (IFN-γ)

A cytokine produced primarily by NK cells and T lymphocytes that functions mainly to activate macrophages and enhance cell-mediated immune responses.

Pyrogens

Fever-inducing substances, including exogenous microbial molecules like LPS and endogenous cytokines (IL-1, IL-6, TNF, IFN-γ), that stimulate hypothalamic PGE₂ production.