Immunology: T-Cell Development, Antigen Recognition, and Adaptive Immunity

Comprehensive vocabulary flashcards covering adaptive immunity, somatic V(D)J recombination, T-cell development and selection in the thymus, MHC structure and antigen presentation pathways, and effector T-cell activation and subsets.

Adaptive Immunity

A pathogen-recognition system characterized by variable antigen receptors on B and T lymphocytes that provides highly specific recognition of novel pathogens and generates long-lasting immunological memory.

Recombination Signal Sequences (RSSs)

Conserved DNA sequences flanking V, D, and J gene segments that consist of a conserved heptamer, a conserved nonamer, and an intervening spacer of either $12\,\text{bp}$ or $23\,\text{bp}$, serving as binding and cleavage sites for the RAG recombinase.

12/23 Rule

The requirement in somatic V(D)J recombination that efficient joining occurs only between an RSS containing a $12\,\text{bp}$ spacer and an RSS containing a $23\,\text{bp}$ spacer, corresponding to one and two full turns of the DNA helix.

RAG1 and RAG2

Recombination activating genes that encode lymphocyte-specific endonuclease complexes which recognize RSSs and introduce double-strand DNA breaks at the junctions between coding sequences and flanking signal sequences.

P-nucleotides

Short palindromic nucleotide sequences generated during somatic recombination when the hairpin coding ends created by RAG cleavage are nicked asymmetrically by Artemis.

Terminal Deoxynucleotidyl Transferase (TdT)

A template-independent DNA polymerase that adds non-templated nucleotides (N-nucleotides) randomly to single-stranded coding ends during V(D)J recombination to generate junctional diversity.

Somatic Hypermutation

An activation-induced cytidine deaminase (AID)-dependent process occurring in activated B cells within germinal centers that introduces point mutations into rearranged immunoglobulin variable regions, driving affinity maturation.

Isotype Switching

A biological mechanism that replaces the heavy chain constant region ($C_H$) gene of an antibody with a downstream $C_H$ gene segment, changing the antibody's effector class without altering its antigen specificity.

Thymus

A bilobed primary lymphoid organ encapsulated in connective tissue, featuring an outer cortex and inner medulla, responsible for supporting T-cell receptor gene rearrangement, thymocyte proliferation, and positive and negative selection.

Hassall's Corpuscles

Epithelial structures located in the thymic medulla that produce thymic stromal lymphopoietin (TSLP) and assist in the development of regulatory T cells.

Notch1

A cell-surface transmembrane receptor on lymphoid progenitor cells that undergoes proteolytic cleavage upon engaging ligands like Jagged or Delta-like (DLL) on thymic epithelial cells, releasing an intracellular domain that translocates to the nucleus to commit progenitors irreversibly to the T-cell lineage.

Double-Negative (DN) Thymocytes

Early immature thymocytes that lack cell-surface expression of both CD4 and CD8 coreceptors and progress sequentially through stages DN1 ($\text{CD44}^+\text{CD25}^-$), DN2 ($\text{CD44}^+\text{CD25}^+$), DN3 ($\text{CD44}^-\text{CD25}^+$), and DN4 ($\text{CD44}^-\text{CD25}^-$).

Pre-T-Cell Receptor (pre-TCR)

A transient receptor complex formed by a successfully rearranged TCR $\beta$ chain paired with the invariant pre-T $\alpha$ ($\text{pT}\alpha$) surrogate chain and the CD3 signaling complex, mediating $\beta$-selection, allelic exclusion, and proliferation.

Positive Selection

The thymic process in the cortex where double-positive thymocytes that bind self-peptide:MHC complexes on cortical thymic epithelial cells (cTECs) with low-to-intermediate affinity receive survival signals and differentiate into single-positive cells.

Death by Neglect

The apoptotic demise of double-positive thymocytes whose rearranged TCRs exhibit no affinity for self-peptide:MHC complexes presented by cortical thymic epithelial cells.

Instructive Model

A lineage commitment model proposing that coreceptor downregulation is driven directly by whether the TCR binds MHC class I (instructing down-regulation of CD4 and retention of CD8) or MHC class II (instructing down-regulation of CD8 and retention of CD4).

Kinetic Signaling Model

A lineage commitment model proposing that continuous TCR/coreceptor engagement signals thymocytes to mature into $\text{CD4}^+$ cells, whereas a disruption or reduction in the TCR signaling cascade directs differentiation into $\text{CD8}^+$ cells.

Central Tolerance

Negative selection in primary lymphoid tissues that deletes developing thymocytes whose TCRs bind self-peptide:MHC complexes with high affinity, preventing autoreactive T cells from entering circulation.

AIRE (Autoimmune Regulator)

A transcriptional activator expressed in medullary thymic epithelial cells (mTECs) containing CARD, SAND, and PHD domains that drives promiscuous expression of tissue-restricted peripheral antigens for negative selection.

Major Histocompatibility Complex (MHC) Class I

A cell-surface heterodimer composed of a polymorphic transmembrane $\alpha$ chain (with $\alpha_1$, $\alpha_2$, and $\alpha_3$ domains) noncovalently associated with invariant $\beta_2$-microglobulin, presenting peptides of $8\text{ to }10$ amino acids to CD8 T cells.

Major Histocompatibility Complex (MHC) Class II

A cell-surface heterodimer composed of two polymorphic transmembrane polypeptide chains ($\alpha$ and $\beta$) whose interface forms an open-ended peptide-binding groove accommodating peptides of $13\text{ to }25$ amino acids for presentation to CD4 T cells.

Anchor Residues

Specific amino acid residues located within an antigenic peptide that fit precisely into matching pockets of a particular MHC peptide-binding groove, allowing promiscuous binding of diverse peptides that share the same binding motif.

Tapasin

A dedicated chaperone within the ER peptide-loading complex that bridges the MHC class I heterodimer to TAP and optimizes peptide loading by facilitating the exchange of low-affinity peptides for high-affinity peptides.

Transporter Associated with Antigen Processing (TAP)

An ATP-binding cassette transporter formed by a TAP1:TAP2 heterodimer in the ER membrane that preferentially pumps proteasome-derived cytosolic peptides of $8\text{ to }15$ amino acids into the lumen of the ER.

Endoplasmic Reticulum Aminopeptidase (ERAP)

An enzyme in the ER lumen that trims amino-terminal residues from peptides that are too long to fit into the MHC class I peptide-binding groove, bringing them to an optimal length of $8\text{ to }10$ amino acids.

Invariant Chain (Ii)

A protein that trimerizes with newly synthesized MHC class II heterodimers in the endoplasmic reticulum, physically blocking the peptide-binding groove from binding premature cytosolic peptides and directing trafficking to endosomal compartments.

Class II-Associated Invariant Chain Peptide (CLIP)

A residual peptide fragment of the invariant chain that remains bound in the MHC class II peptide-binding groove following sequential cleavage by endosomal proteases such as cathepsin S.

HLA-DM

An intracellular, non-classical MHC class II-like molecule that catalyzes the removal of CLIP from the peptide-binding groove of MHC class II molecules, permitting exogenous antigenic peptides to load.

Cross-Presentation

The pathway in specialized professional APCs (notably dendritic cells) where endocytosed or phagocytosed extracellular antigens are diverted to the cytosol and presented on MHC class I molecules to activate naïve CD8 T cells.

Balancing Selection

An evolutionary selective pressure that maintains multiple diverse alleles of MHC genes within a population because heterozygous individuals present a broader repertoire of pathogen-derived peptides.

Directional Selection

An evolutionary process in which an infectious outbreak selectively favors specific MHC alleles capable of presenting dominant pathogen epitopes, shifting allele frequencies at the expense of general heterozygosity.

T-Cell Priming

The initial antigen-dependent activation and clonal expansion of naïve T cells in secondary lymphoid organs upon encountering professional antigen-presenting cells expressing cognate MHC-peptide complexes and costimulatory ligands.

CCR7

A chemokine receptor expressed on mature dendritic cells and naïve T cells that binds the chemokines CCL19 and CCL21, mediating homing from circulation and peripheral tissues into secondary lymphoid tissues.

Sphingosine-1-Phosphate (S1P)

A signaling lipid with a concentration gradient between lymphoid tissue and blood/lymph that directs unactivated T cells out of secondary lymphoid tissues into efferent lymphatics.

B7 (CD80/CD86)

Costimulatory surface glycoproteins induced on professional APCs by PAMP recognition via pattern recognition receptors that interact with CD28 to provide Signal 2 for naïve T-cell activation.

CTLA4

An inhibitory receptor expressed on activated T cells and regulatory T cells that binds B7 molecules with an affinity approximately $20\times$ stronger than CD28, dampening T-cell proliferation and signaling.

Immunological Synapse

A structured junction formed between a T cell and an antigen-presenting cell containing clustered TCRs, coreceptors (CD4 or CD8), CD3 signaling complexes, costimulatory molecules, and adhesion proteins (e.g., LFA-1/ICAM-1).

Immunoreceptor Tyrosine-Based Activation Motifs (ITAMs)

Conserved cytoplasmic sequence motifs found in the signaling chains ($\delta$, $\epsilon$, $\gamma$, and $\zeta$) of the CD3 complex that undergo tyrosine phosphorylation by Lck upon TCR engagement, creating docking platforms for ZAP-70.

Calcineurin

A calcium-activated protein phosphatase that dephosphorylates the cytoplasmic transcription factor NFAT following $IP_3$-mediated calcium release from the ER, allowing NFAT to enter the nucleus.

High-Affinity IL-2 Receptor

A heterotrimeric receptor complex composed of $\alpha$ (CD25), $\beta$, and $\gamma$ chains synthesized upon T-cell activation that binds IL-2 to drive robust clonal expansion.

Anergy

A long-lasting state of antigen unresponsiveness induced when a naïve T cell recognizes an MHC-peptide complex (Signal 1) in the absence of a costimulatory signal (Signal 2), serving as a mechanism of peripheral tolerance.

$$\text{T}_\text{H}1\text{ Helper T Cells}$$

A CD4 T-cell subset differentiated under the control of IL-12 and IFN-$\gamma$ that expresses the master transcription factor T-bet, secretes IFN-$\gamma$, and activates macrophages to clear intracellular pathogens.

$$\text{T}_\text{H}2\text{ Helper T Cells}$$

A CD4 T-cell subset differentiated under the influence of IL-4 that expresses GATA3, secretes IL-4 and IL-5, activates granulocytes, and promotes B-cell isotype switching to IgE to defend against multicellular parasites.

$$\text{T}_\text{H}17\text{ Helper T Cells}$$

A CD4 T-cell subset induced by TGF-$\beta$ and IL-6 that expresses the transcription factor $\text{ROR}\gamma\text{t}$ and secretes IL-17 to recruit and activate neutrophils against extracellular bacteria and fungi.

$$\text{T}_{\text{FH}}\text{ (Follicular Helper T) Cells}$$

A CD4 T-cell subset driven by IL-6 and IL-21 that expresses Bcl6, localizes to B-cell follicles, and delivers CD40L, IL-4, and IL-21 signals to induce B-cell proliferation, affinity maturation, and plasma cell differentiation.

Perforin and Granzymes

Cytotoxins stored in lytic granules of cytotoxic CD8 T cells; perforin polymerizes to generate pores in the target cell plasma membrane, facilitating entry of granzyme serine proteases that cleave procaspases and trigger apoptosis.

Fas/FasL Pathway

A contact-dependent apoptosis pathway in which Fas ligand (FasL) on cytotoxic T cells binds to Fas on target cells, recruiting FADD and triggering caspase-8 activation to destroy the infected target cell.

Regulatory T Cells (Tregs)

A specialized subset of CD4 T cells expressing CD25 and the lineage-defining transcription factor Foxp3 that suppresses the activation and proliferation of autoreactive or effector T cells through contact inhibition and secretion of inhibitory cytokines (such as IL-10 and TGF-$\beta$).

Natural Killer T (NKT) Cells

Thymus-derived lymphocytes expressing invariant $\alpha:\beta$ T-cell receptors that recognize glycolipid antigens presented by non-polymorphic CD1d molecules, exerting both helper and cytotoxic effector functions.