Innate and Adaptive Immunity: Principles and Mechanisms

A set of vocabulary flashcards reviewing the temporal phases, cell types, molecular structures, antigen recognition mechanisms, and lymphoid architecture of the innate and adaptive immune systems.

Immediate innate immune response

The immune response phase occurring between $0$ and $4$ hours after pathogen invasion, in which pathogens are recognized and eliminated by preformed effectors and tissue-resident effector cells, leading to minor tissue damage that is repaired.

Induced innate immune response

The immune response occurring from $4$ hours to $4$ days post-infection, characterized by activation of tissue-resident cells, recruitment of soluble and cellular effectors to infected tissues, inflammation, fever, and an acute-phase response.

Adaptive immune response

An instructed immune response initiating after approximately $4$ days and lasting until defeat of the pathogen, host defeat, or chronic disease compromise, mediated by the expansion of pathogen-specific B and T cells.

Antigen

Antibodies are generated due to their presence. They are bound specifically by adaptive immune receptors; it can virtually be any structure including proteins, carbohydrates, nucleic acids, or small molecules.

B cell receptor (BCR)

A surface-bound immunoglobulin that recognizes and binds intact, whole antigens in their native natural conformation.

Antibody structure

A protein molecule composed of constant regions, variable regions that form the antigen-binding sites, and a flexible hinge region.

Epitope

The specific small portion of an antigen that is directly bound by a B-cell receptor or T-cell receptor.

Linear epitope

An epitope consisting of sequential, contiguous amino acids in the primary structure of a protein antigen.

Discontinuous epitope

A conformational epitope composed of amino acid residues that are separated in the primary sequence but brought into proximity when the protein folds into its secondary, tertiary, or quaternary structure.

T cell receptor (TCR)

An adaptive immune receptor that does not bind intact whole antigens, but instead recognizes processed peptide fragments presented on MHC molecules by antigen-presenting cells.

CD4

A cell-surface glycoprotein co-receptor expressed on helper T cells that interacts with MHC class II molecules.

CD8

A cell-surface glycoprotein co-receptor expressed on cytotoxic T cells that interacts with MHC class I molecules.

Cytotoxic T cells ($\text{CD8}^+$ T cells)

Effector T cells that detect viral or abnormal antigens presented on MHC class I and directly kill infected or mutated host cells by inducing programmed cell death (apoptosis).

Helper T cells ($\text{CD4}^+$ T cells)

Effector T cells that detect processed antigens presented on MHC class II and provide essential signals to assist and direct the immune functions of other cells.

MHC class I

Cell surface molecules expressed on all nucleated cells that present intracellular peptide antigens to $\text{CD8}^+$ T cells for immune surveillance.

MHC class II

Cell surface molecules expressed selectively by professional antigen-presenting cells (APCs) to present processed exogenous peptide fragments to $\text{CD4}^+$ helper T cells.

Antigen-presenting cell (APC)

A specialized cell expressing MHC class II—primarily dendritic cells ('professional APCs'), macrophages, and B cells—that displays peptide antigens to activate $\text{CD4}^+$ T cells.

$\text{T}_\text{H}1$ cell

A specialized subset of effector $\text{CD4}^+$ helper T cells whose primary function is to activate macrophages.

$\text{T}_\text{H}2$ cell

A specialized subset of effector $\text{CD4}^+$ helper T cells that activates cellular and antibody-mediated responses against parasites.

$\text{T}_\text{H}17$ cell

A specialized subset of effector $\text{CD4}^+$ helper T cells that enhances the recruitment and activation of neutrophils.

$\text{T}_\text{FH}$ cell

Follicular helper T cells that activate B cells within lymphoid tissues to refine and mature the antibody response.

Plasma cell

A fully differentiated, antibody-secreting effector B cell that synthesizes large amounts of antibodies within its endoplasmic reticulum.

Lymph node

A secondary lymphoid organ featuring specialized compartments—such as lymphoid follicles (mostly B cells), germinal centers, and T-cell areas—where naïve lymphocytes encounter antigens.

Draining lymph node activation

The process whereby tissue dendritic cells take up pathogen components at an infection site and travel via afferent lymphatic vessels into the regional lymph node to activate antigen-specific T cells and B cells.

Immediate innate immune response

The immune response phase occurring between $0$ and $4$ hours post-infection, wherein pathogens are recognized and cleared by preformed soluble effectors and tissue-resident cells without significant delay.

Induced innate immune response

The phase occurring between $4$ hours and $4$ days post-infection, characterized by tissue-resident cell activation, recruitment of effector leukocytes, localized inflammation, fever, and the acute-phase response.

Adaptive immune response

An instructed immune defense that initiates around $4$ days after infection and continues until pathogen clearance, host defeat, or chronic pathology, driven by pathogen-specific lymphocyte clonal expansion.

Antigen

Any antibody-generating structure specifically bound by adaptive immune receptors, encompassing proteins, carbohydrates, lipids, nucleic acids, and synthetic haptens.

B cell receptor (BCR)

A membrane-bound immunoglobulin that recognizes and binds intact, native macromolecular antigens in their physiological three-dimensional confirmation.

Antibody structure

A Y-shaped glycoprotein consisting of two identical heavy chains, two identical light chains, antigen-binding variable regions, a constant region, and a flexible hinge region.

Epitope

The specific localized structural motif on an antigen that directly interfaces with the binding site of a B-cell receptor or T-cell receptor.

Linear epitope

An antigenic determinant formed by a contiguous sequence of amino acids in a polypeptide chain that remains recognizable even after denaturation.

Discontinuous epitope

A conformational antigenic site formed by amino acid residues that are distant in primary sequence but brought into spatial proximity by protein folding.

T cell receptor (TCR)

An adaptive receptor that cannot bind native whole antigens, recognizing only short, processed peptide fragments displayed within MHC molecules.

CD4

A membrane co-receptor expressed on helper T cells that selectively binds non-polymorphic invariant domains of MHC class II molecules.

CD8

A membrane co-receptor expressed on cytotoxic T cells that selectively interacts with the $\\alpha_3$ invariant domain of MHC class I molecules.

Cytotoxic T cells ($\\text{CD8}^+$ T cells)

Effector lymphocytes that identify viral or endogenous mutant peptides displayed on MHC class I and kill target host cells via targeted apoptosis.

Helper T cells ($\\text{CD4}^+$ T cells)

Effector lymphocytes that recognize exogenous peptides displayed on MHC class II and provide regulatory cytokines and co-stimulatory signals to other immune cells.

MHC class I

Ubiquitous surface glycoproteins present on virtually all nucleated cells that present intracellularly derived peptide antigens to $\\text{CD8}^+$ cytotoxic T cells.

MHC class II

Surface heterodimers expressed almost exclusively on professional antigen-presenting cells that display endocytosed, processed peptides to $\\text{CD4}^+$ helper T cells.

Antigen-presenting cell (APC)

A specialized cell type—principally dendritic cells, macrophages, and B cells—that internalizes antigens, processes them, and expresses them on MHC class II molecules.

\\text{T}_\\text{H}1 cell

A specialized lineage of effector $\text{CD4}^+$ helper T cells responsible for activating macrophages to eradicate intracellular microbial infections.

\\text{T}_\\text{H}2 cell

A specialized lineage of effector $\text{CD4}^+$ helper T cells that coordinates cellular defenses, eosinophil activation, and antibody responses against helminths and parasites.

\\text{T}_\\text{H}17 cell

A lineage of effector $\text{CD4}^+$ helper T cells that secretes cytokines to stimulate the recruitment and functional activation of neutrophils against extracellular bacteria and fungi.

\\text{T}_\\text{FH} cell

Follicular helper T cells that migrate into lymphoid follicles to deliver critical activation and survival signals to B cells undergoing affinity maturation.

Plasma cell

A terminally differentiated effector B cell equipped with an extensive rough endoplasmic reticulum dedicated to the mass synthesis and secretion of soluble antibodies.

Lymph node

An encapsulated secondary lymphoid organ structured into follicles, germinal centers, and paracortical T-cell zones where naïve lymphocytes survey lymph-borne antigens.

Draining lymph node activation

The migration of antigen-bearing tissue dendritic cells via afferent lymphatics into the regional node to initiate the clonal priming of naïve T and B lymphocytes.

Instructed immunity

The defining attribute of the adaptive immune system wherein lymphocyte effector profiles are dynamically shaped and directed by initial antigen presentation rather than being fixed at baseline.

Regulatory T cell (\\text{T}_\\text{reg})

An adaptive T lymphocyte population that actively suppresses autoreactive effector cells and dampens ongoing immune responses to prevent autoimmunity and tissue damage.

Primary site of lymphocyte origin

The bone marrow, which houses hematopoietic stem cells responsible for generating the precursors of all myeloid and lymphoid lineages.

Cellular effectors of innate immunity

Preformed, germline-encoded leukocytes that include macrophages, monocytes, neutrophils, eosinophils, basophils, and natural killer (NK) cells.

Antibody neutralization

The protective mechanism wherein antibodies bind directly to pathogen surface proteins or toxins, physically preventing their attachment to host cell receptors.

Antibody opsonization

The coating of microbial surfaces by specific antibodies to facilitate direct recognition, engulfment, and destruction by phagocytic innate cells possessing Fc receptors.

Complement activation by antibodies

The binding of immune complexes by classical complement proteins, triggering a proteolytic cascade that forms membrane-attack complexes to lyse pathogens.

Afferent lymphatic vessel

The peripheral vessel through which interstitial lymph, particulate antigens, and mature dendritic cells enter the subcapsular sinus of a regional lymph node.

Efferent lymphatic vessel

The conduit at the lymph node hilum through which cleared lymph, activated effector lymphocytes, and secreted antibodies exit toward the bloodstream.

Lymphoid follicle

The defined anatomical microdomain within the cortex of a lymph node populated primarily by resting and naïve B cells.

Germinal center

A temporary, highly organized microdomain formed inside lymphoid follicles where antigen-stimulated B cells undergo clonal proliferation, somatic hypermutation, and class switching.

T-cell area (paracortex)

The deep cortical region of a lymph node packed with naïve T lymphocytes that continuously screen migratory dendritic cells presenting peptide–MHC complexes.

Apoptosis induction by $\text{CD8}^+$ T cells

The targeted release of cytotoxic granules (perforin and granzymes) that activates host-cell caspase cascades to destroy infected cells without dispersing intact viral progeny.

Denaturation impact on epitopes

A process that abolishes discontinuous conformational epitopes by unraveling folded higher-order protein structures while leaving contiguous linear epitopes intact.