Host Defenses I: Overview and Innate Immunity

Seventy-five vocabulary flashcards covering key terms and definitions from the lecture on Host Defenses I and Innate Immunity.

Host defenses

Collective physical, chemical, cellular, and molecular strategies that protect the body from infection.

First line of defense

Surface barriers that block invasion at portals of entry; nonspecific physical, chemical, and microbiota barriers.

Second line of defense

Internal, nonspecific system of protective cells and fluids including inflammation, phagocytosis, fever, and antimicrobial products.

Third line of defense

Specific, acquired immunity produced by B and T lymphocytes that develops uniquely for each microbe and provides long-term protection.

Innate immunity

Inborn, nonspecific host defenses present at birth that act rapidly without prior exposure.

Acquired immunity

Specific immune responses that develop after exposure to a particular antigen and create immunological memory.

Physical barriers

Anatomical structures such as intact skin, mucous membranes, cilia, and nasal hair that prevent microbial entry.

Chemical barriers

Antimicrobial substances (e.g., lysozyme, stomach acid, sweat acids) that inhibit or destroy pathogens.

Microbiota barrier

Resident commensal microbes that block pathogens by competition, pH alteration, and immune training.

Phagocytosis

Engulfment and destruction of microbes or debris by neutrophils, monocytes, and macrophages.

Inflammation

Localized tissue response to injury marked by rubor, calor, tumor, dolor, and loss of function.

Fever

Abnormally elevated body temperature triggered by pyrogens, aiding host defense.

Antimicrobial products

Innate molecules such as interferons, complement, and antimicrobial peptides that target microbes.

Immunology

Study of the immune system’s surveillance, recognition of foreign material, and destruction of threats.

Antigen (marker)

Protein or sugar molecule on cell surfaces recognized by immune cells as self or non-self.

Pathogen-associated molecular pattern (PAMP)

Shared microbial marker (e.g., LPS, peptidoglycan, dsRNA) recognized by innate receptors.

Pattern recognition receptor (PRR)

Host sensor on phagocytes and other cells that binds PAMPs to trigger innate responses.

Mononuclear phagocyte system (MPS)

Network of connective tissue fibers and phagocytic cells that interconnects organs and provides passageways for immune cells.

Extracellular fluid (ECF)

Fluid in interstitial spaces surrounding tissue cells where immune exchanges occur.

Interstitium

Collective term for extracellular spaces and their lymph fluid between cells.

Lymphatic system

Branching network of lymph vessels, nodes, and organs returning fluid to circulation and enabling immune surveillance.

Lymph

Plasmalike fluid in lymphatic vessels carrying WBCs, fats, debris, and microbes.

Lymphatic vessels

Thin-walled tubes that transport lymph one way—from extremities toward the heart—by skeletal muscle contraction.

Red bone marrow

Primary lymphoid organ in long bones where all blood cells originate and B cells mature.

Thymus

Primary lymphoid organ in upper thorax where naïve T lymphocytes mature and gain specificity.

Lymph nodes

Encapsulated, bean-shaped secondary lymphoid organs that filter lymph and house immune cells.

Spleen

Secondary lymphoid organ filtering blood, removing old erythrocytes, and trapping blood-borne pathogens.

Skin-associated lymphoid tissue (SALT)

Aggregations of lymphocytes located beneath the skin surface for localized immunity.

Mucosa-associated lymphoid tissue (MALT)

Lymphoid clusters under mucous membranes, including tonsils and breast tissue, providing mucosal immunity.

Gut-associated lymphoid tissue (GALT)

Intestinal lymphoid structures such as Peyer’s patches, appendix, and lacteals involved in gut immunity.

Hematopoiesis

Process of blood cell production from hematopoietic stem cells in bone marrow.

Stem cell (hematopoietic)

Pluripotent precursor in marrow that differentiates into all blood cell lineages.

Granulocyte

Leukocyte with cytoplasmic granules (e.g., neutrophils, eosinophils, basophils) releasing antimicrobial chemicals.

Agranulocyte

Leukocyte lacking visible granules, possessing a large nucleus (e.g., lymphocytes, monocytes).

Neutrophil

Short-lived granulocyte, primary phagocyte in early inflammation; major component of pus.

Eosinophil

Granulocyte active against parasitic worms and in antigen-antibody reactions; minor phagocyte.

Basophil

Granulocyte that releases histamine and other mediators during inflammation and allergy.

Monocyte

Large agranulocyte that exits blood, differentiates into macrophages or dendritic cells in tissues.

Macrophage

Large phagocytic cell derived from monocytes; scavenges, presents antigen, and orchestrates immune responses.

Dendritic cell

Antigen-presenting cell residing in tissues; processes foreign matter and activates lymphocytes.

Natural killer (NK) cell

Innate lymphocyte that kills virus-infected and tumor cells without prior sensitization.

Gamma-delta T cell

T cell subset displaying innate and adaptive traits; responds to antigens and PAMPs quickly.

Mast cell

Tissue cell resembling basophils; triggers local inflammation and allergic symptoms via histamine release.

Cytokine

Small signaling protein released by immune and other cells to regulate immunity, inflammation, and hematopoiesis.

Pro-inflammatory cytokine

Cytokine (e.g., IL-1, TNF-β) that stimulates innate and adaptive immune responses.

Anti-inflammatory cytokine

Cytokine (e.g., IL-10) that suppresses or dampens immune reactions.

Vasodilator/Vasoconstrictor cytokine

Cytokine (e.g., histamine, serotonin) that alters blood vessel diameter and permeability during inflammation.

Growth factor cytokine

Cytokine (e.g., IL-7, erythropoietin) that regulates lymphocyte or blood cell growth and activation.

Sebaceous gland secretions

Oily skin exudates with antimicrobial properties forming part of chemical defenses.

Lysozyme

Enzyme in tears and saliva that hydrolyzes bacterial peptidoglycan cell walls.

Stratum corneum

Keratinized, waterproof outer skin layer forming a tough physical barrier to pathogens.

Mucous membrane

Moist, permeable epithelial lining producing mucus that blocks microbial attachment in body tracts and eye.

Respiratory ciliary escalator

Ciliated epithelium moving mucus-trapped particles toward pharynx for removal from respiratory tract.

Genitourinary flushing

Protective cleansing via continuous urine flow and vaginal secretions that remove microbes.

Resident microbiota

Normal flora that prevent pathogen colonization by competition, pH changes, and immune modulation.

Myeloperoxidase

Enzyme in neutrophils that generates halogen ions for oxygen-dependent microbial killing.

Rubor

Redness, first classic sign of inflammation due to increased blood flow.

Calor

Heat at an inflamed site resulting from augmented circulation and metabolism.

Tumor (inflammation)

Swelling caused by fluid exudate accumulation in tissues.

Dolor

Pain produced by stimulation of nerve endings during inflammation.

Diapedesis

Process by which WBCs exit bloodstream by squeezing through endothelial gaps into tissues.

Chemotaxis

Directed migration of WBCs toward chemical signals at sites of injury or infection.

Exudate

Protein-rich fluid escaping blood vessels during inflammation; may be serous, sanguineous, or purulent.

Pyrogen

Substance that resets hypothalamic thermostat, inducing fever.

Exogenous pyrogen

Fever-inducing molecule originating outside the host, e.g., bacterial endotoxin.

Endogenous pyrogen

Host-derived fever mediator such as IL-1 or TNF released by phagocytes.

Interferon (IFN)

Family of small antiviral proteins (alpha, beta, gamma) produced by WBCs and tissues; modulate immunity.

Complement system

Cascade of >50 blood proteins that, once activated, lyse microbes, enhance phagocytosis, and promote inflammation.

Classical complement pathway

Complement activation route triggered by antibodies bound to microbial surfaces.

Lectin pathway

Complement activation via host lectins binding specific microbial sugars.

Alternative complement pathway

Complement activation directly by repetitive microbial surface molecules like LPS.

Membrane attack complex (MAC)

Ring-shaped complement end product that punches holes in microbial membranes, causing lysis.

Antimicrobial peptide

Short (12–50 aa) innate peptide (e.g., defensins) that inserts into and disrupts bacterial membranes.

Host restriction factor

Host molecule (protein or nucleic acid) that blocks viral replication steps inside infected cells.

Chronic inflammation

Long-term inflammatory state linked to autoimmune disease, cardiovascular disease, and some long COVID cases.