Chapter 21: The Lymphoid and Immune Systems

Comprehensive vocabulary flashcards covering anatomical, physiological, and clinical terminology of the lymphoid system and innate and adaptive immunity from Chapter 21.

Microbiome

The microorganisms that reside on and within the human body, many of which are beneficial while others possess the potential to cause disease.

Immune System

A widely distributed cell population that inhabits all organs and defends the body from agents of disease rather than forming a discrete, isolated organ system.

Lymphoid System

A true organ system consisting of lymphatic vessels, tissues, and organs that recovers fluid filtered from capillaries, inspects it for disease agents, activates immune responses, and returns it to the bloodstream.

Lymphedema

Tissue swelling that occurs when there is an obstruction or interference with lymphatic drainage, preventing the return of the roughly $15\%$ of interstitial fluid not reabsorbed by blood capillaries.

Lacteals

Specialized lymphatic vessels located in the small intestine that absorb dietary lipids that cannot be absorbed directly into blood capillaries.

Lymph

A clear, colorless, low-protein fluid originating as interstitial tissue fluid taken up by microscopic lymphatic vessels.

Lymphatic Capillaries

Microscopic, closed-ended terminal vessels penetrating nearly every tissue (absent from cartilage, cornea, bone, and bone marrow) made of overlapping endothelial cells that act as one-way valves.

Anchoring Filaments

Protein filaments that tether lymphatic endothelial cells to surrounding tissues, preventing the collapse of the lymphatic capillaries under high interstitial fluid pressure.

Right Lymphatic Duct

A collecting duct formed by the convergence of lymphatic trunks that drains lymph from the right upper limb and the right side of the head and thorax, emptying into the right subclavian vein.

Thoracic Duct

The larger and longer lymphatic collecting duct beginning at the cisterna chyli in the abdomen that drains lymph from all parts of the body below the diaphragm and the left side of the upper body into the left subclavian vein.

Cisterna Chyli

A prominent sac located in the abdomen that marks the beginning of the thoracic duct and receives lymph from the lumbar and intestinal trunks.

Natural Killer (NK) Cells

Large lymphocytes of the innate immune system that patrol the body and destroy bacteria, transplanted foreign cells, virus-infected host cells, and cancerous cells.

T Lymphocytes (T Cells)

Lymphocytes that originate in the red bone marrow and mature within the thymus to carry out cellular adaptive immune responses.

B Lymphocytes (B Cells)

Lymphocytes that differentiate into antibody-producing plasma cells to mediate humoral adaptive immunity.

Macrophages

Large phagocytic cells derived from blood monocytes that engulf tissue debris and foreign matter and display foreign epitopes on their surface as antigen-presenting cells (APCs).

Dendritic Cells

Specialized antigen-presenting cells (APCs) located within the epidermis, mucous membranes, and lymphoid organs that alert the immune system to foreign pathogens.

Mucosa-Associated Lymphoid Tissue (MALT)

A collection of diffuse lymphoid tissue scattered through the connective tissues of mucous membranes in passages open to the exterior, such as the respiratory and digestive tracts.

Aggregated Lymphoid Nodules

Dense, permanent clusters of lymphoid nodules (formerly termed Peyer patches) situated within the mucous membrane of the small intestine.

Primary Lymphoid Organs

The red bone marrow and thymus, where B and T lymphocytes develop and become immunocompetent.

Secondary Lymphoid Organs

Lymph nodes, tonsils, and the spleen, which provide sites where immunocompetent lymphocytes migrate, encounter foreign antigens, and launch defensive responses.

Immunocompetence

The ability of a developing lymphocyte to successfully recognize and respond to a specific foreign antigen.

Involution

The shrinkage and degeneration of an organ with advancing age, prominently seen in the thymus, which ceases creating new naive T cells around age $65$.

Blood-Thymus Barrier

A protective isolation barrier formed by cortical epithelial cells and pericytes that shields developing T lymphocytes in the thymic cortex from antigens circulating in the bloodstream.

Lymph Nodes

Bean-shaped secondary lymphoid organs numbering around $450$ in a young adult that cleanse passing lymph and serve as activation centers for B and T lymphocytes.

Germinal Centers

Light-staining central areas within the lymphoid nodules of the lymph node cortex where B cells rapidly multiply and differentiate into plasma cells.

Metastasis

The dissemination of cancerous cells from a primary tumor through lymphatic vessels and bloodstream to establish secondary tumors in distant tissues or organs.

Tonsils

Patches of non-encapsulated or partially encapsulated lymphoid tissue located at the pharynx entrance (including pharyngeal, palatine, and lingual tonsils) that intercept inhaled or ingested pathogens.

Tonsillar Crypts

Deep epithelial pits within the tonsils lined with lymphoid nodules that trap debris and ingested or inhaled microorganisms.

Red Pulp

Splenic tissue composed of sinuses packed with concentrated erythrocytes where aged and fragile red blood cells are broken down.

White Pulp

Splenic tissue consisting of aggregates of lymphocytes and macrophages surrounding branches of the splenic artery to monitor circulating blood for foreign antigens.

Acid Mantle

A thin, protective film of lactic and fatty acids on the epidermal surface originating from sweat and sebum that inhibits cutaneous microbial growth.

Lysozyme

An antibacterial enzyme present in mucus, tears, and saliva that dissolves bacterial cell walls by breaking peptidoglycan bonds.

Hyaluronidase

An enzyme produced by pathogenic microbes to break down hyaluronic acid in subepithelial ground substance, reducing its viscosity and enabling tissue invasion.

Neutrophil Extracellular Trap (NET)

A web of nuclear chromatin and antimicrobials extruded by dying neutrophils to ensnare and kill nearby bacteria extracellularly.

Histamine

A vasoactive chemical secreted by basophils and mast cells that causes local vasodilation and increases capillary permeability during inflammation.

Heparin

An anticoagulant secreted by basophils and mast cells that prevents local blood clotting to preserve leukocyte mobility within an inflamed site.

Interferons

Proteins secreted by virus-infected cells that alert neighboring cells to synthesize antiviral proteins and activate NK cells and macrophages.

Complement System

A family of roughly $30$ inactive globular plasma proteins synthesized primarily by the liver that activate in cascades to facilitate inflammation, immune clearance, phagocytosis, and cytolysis.

Classical Pathway

An antibody-dependent complement activation mechanism triggered when complement protein $C1$ binds to antigen-antibody complexes on a pathogen surface.

Alternative Pathway

An antibody-independent complement activation cascade initiated when active fragment $C3b$ binds directly to targets on microbial surfaces.

Lectin Pathway

An antibody-independent complement cascade initiated when plasma lectins bind to specific carbohydrate moieties on pathogen surfaces.

Immune Clearance

The process by which complement fragment $C3b$ binds antigen-antibody complexes to red blood cells, which transport them to the liver and spleen where resident macrophages remove them.

Opsonization

The deposition of complement fragment $C3b$ or antibodies onto microbial surfaces to provide binding handles that accelerate phagocytosis.

Membrane Attack Complex (MAC)

A ring-like cytolytic pore assembled in a target membrane by complement proteins $C5b$, $C6$, $C7$, $C8$, and multiple $C9$ molecules that induces osmotic cytolysis.

Perforins

Proteins released by natural killer cells and cytotoxic T cells that polymerize into a ring to create open holes in target cell membranes.

Granzymes

Protein-degrading enzymes secreted by natural killer cells and cytotoxic T cells that enter target cells through perforin pores to trigger apoptosis.

Pyrexia

An abnormal elevation of body core temperature (fever) driven by pyrogens resetting the hypothalamic set point to accelerate metabolic repair and inhibit pathogen proliferation.

Exogenous Pyrogens

Fever-producing substances of external origin, such as bacterial and viral surface glycolipids and endotoxins.

Endogenous Pyrogens

Internal fever-inducing polypeptides, including interleukins and other cytokines secreted by activated neutrophils and macrophages, that stimulate the hypothalamus to elevate the body's set point.

Stadium

The maintenance phase of a fever during which core body temperature oscillates around the elevated hypothalamic set point.

Defervescence

The final stage of a fever in which the hypothalamic thermostat resets to normal, causing sweating and cutaneous vasodilation to return temperature to $37^{\circ}\text{C}$.

Reye Syndrome

A severe pediatric complication following viral illness (such as chickenpox or flu) linked to aspirin administration, characterized by cerebral edema, fatty liver infiltration, seizures, and a $30\%$ mortality rate.

Hyperemia

An increased volume of local blood flow driven by vasoactive mediators like histamine, which accounts for the redness (rubor) and heat (calor) of inflammation.

Margination

The adhesion of circulating leukocytes to endothelial cell-adhesion molecules on vessel walls during the initial vascular phase of inflammation.

Diapedesis

The active crawling of leukocytes through intercellular gaps between contracted endothelial cells to exit the bloodstream and reach inflamed interstitial tissue.

Chemotaxis

The directional migration of immune cells toward higher concentrations of specific chemical mediators, such as leukotrienes or bradykinin.

Pus

A viscous accumulation of dead and dying neutrophils, digested tissue debris, liquefied bacteria, and interstitial exudate at a site of infection.

Cellular Immunity

A branch of adaptive immunity in which sensitized T lymphocytes directly attack foreign cells or host cells infected with intracellular pathogens.

Humoral Immunity

A branch of adaptive immunity mediated by circulating antibodies produced by plasma cells that mark extracellular pathogens and toxins for destruction.

Natural Active Immunity

Long-term immunity acquired through natural exposure to a virulent pathogen and the body's subsequent production of its own memory cells and antibodies.

Artificial Active Immunity

Acquired immunity induced by vaccination with dead, attenuated, or fragmented antigens, stimulating protective antibodies and memory cells without causing full disease.

Natural Passive Immunity

Temporary immune protection conferred when a fetus or infant receives maternal antibodies across the placenta or through breast milk.

Artificial Passive Immunity

Temporary protection achieved by directly injecting exogenous antibodies harvested from another human or animal, used for acute emergency treatments like antivenoms.

Epitope

A specific localized region or antigenic determinant on a large antigen molecule that binds an antibody or T cell receptor to elicit an immune response.

Haptens

Incomplete, small chemical molecules that are not antigenic by themselves but stimulate an immune attack when bound to host macromolecular carrier proteins.

Antibody Monomer Structure

A four-polypeptide Y-shaped protein consisting of two identical heavy chains, two identical light chains, a flexible hinge region, constant regions, and terminal variable (V) regions forming antigen-binding sites.

IgA

An immunoglobulin existing as a monomer in blood plasma and as a secretory dimer in tears, saliva, milk, and mucus, preventing pathogen adherence to epithelial linings.

IgE

A monomeric immunoglobulin bound to basophils and mast cells that stimulates histamine release during allergic responses and mediates antiparasitic defenses.

IgG

The predominant circulating immunoglobulin monomer, making up $80\%$ of plasma antibodies, capable of crossing the placenta and driving the secondary immune response.

IgM

A large pentameric immunoglobulin in blood and lymph that acts as the initial antibody secreted during the primary immune response and excels at agglutination and complement fixation.

Somatic Recombination

The DNA shuffling and joining process in developing B lymphocytes that generates immense diversity in antibody variable regions from a limited gene pool.

Positive Selection

The thymic process in the cortex testing naive T cells for functional antigen receptors capable of binding MHC molecules; non-functional cells die.

Negative Selection

The elimination or inactivation in the thymic medulla of T cells that react destructively against self-antigens, establishing self-tolerance.

Anergy

A state of non-responsiveness induced in self-reactive lymphocytes, allowing them to survive while permanently silencing their ability to attack self-tissues.

Major Histocompatibility Complex (MHC) Proteins

Cell surface glycoproteins that display peptide fragments to T cells; MHC-I presents endogenous peptides on all nucleated cells, while MHC-II displays exogenous antigens on professional APCs.

MHC Restriction

The regulatory rule stating that cytotoxic T ($T_C$) cells bind and respond only to antigens presented on MHC-I proteins, whereas helper T ($T_H$) cells respond strictly to antigens on MHC-II proteins.

Costimulation

The required secondary molecular checkpoint during T cell activation where a T cell binds a second APC surface protein, preventing accidental auto-reactivity.

Clonal Selection

The mitotic multiplication of an activated lymphocyte upon antigen binding, producing a large army of identical effector and memory cells specific to that single epitope.

Cytotoxic T (Tc) Cells

Effector T lymphocytes bearing CD8 markers that directly dock onto and destroy diseased or foreign target cells by deploying perforins, granzymes, and tumor necrosis factor.

Helper T (TH) Cells

Central regulatory lymphocytes bearing CD4 markers that recognize MHC-II complexes and secrete interleukins to coordinate innate, cellular, and humoral responses.

Regulatory T (Treg) Cells

Inhibitory T cells that modulate the immune response by suppressing excessive lymphocyte activity and cytokine production, limiting potential autoimmune damage.

Plasma Cells

Differentiated, organelle-rich B cells containing abundant rough endoplasmic reticulum that produce and secrete up to $2{,}000$ specific antibody molecules per second over a lifespan of $4$ to $5\text{ days}$.

Agglutination

The antibody-mediated cross-linking and clumping of intact enemy cells (such as bacteria or mismatched erythrocytes) into large, immobile clusters for clearance by phagocytes.

Precipitation

The clumping of soluble antigen molecules (rather than whole cells) by antibodies, creating insoluble complexes that are cleared by phagocytes.

Secondary (Anamnestic) Response

The rapid and robust humoral reaction occurring upon re-exposure to a known antigen, wherein memory B cells yield high titers of IgG antibodies within hours to prevent clinical illness.

Severe Combined Immunodeficiency Disease (SCID)

A rare hereditary disorder caused by a lack of both functional T and B lymphocytes, rendering patients defenseless against opportunistic infections and requiring life in sterile, germ-free isolators.

Human Immunodeficiency Virus (HIV)

An enveloped retrovirus containing reverse transcriptase that enters and destroys helper T cells, macrophages, and dendritic cells via receptor-mediated endocytosis, ultimately inducing AIDS.

Acquired Immunodeficiency Syndrome (AIDS)

A nonhereditary terminal immunodeficiency contracted from HIV infection, clinically defined when the helper T cell count drops below $200\text{ cells/}\mu\text{L}$ of blood.

Kaposi Sarcoma

An opportunistic endothelial cancer prevalent in immunocompromised AIDS patients that manifests as dark, elevated purple skin lesions.