Immune Recognition – BCR, TCR & MHC Vocabulary

Vocabulary flashcards summarizing core terms and definitions related to B-cell receptors, T-cell receptors, MHC molecules, and antigen recognition mechanisms.

Humoral Immunity

Branch of adaptive immunity mediated by B lymphocytes and antibodies that targets extracellular microbes.

Cell-Mediated Immunity

Branch of adaptive immunity mediated by T lymphocytes that targets intracellular microbes and infected cells.

B Cell Receptor (BCR)

Membrane-bound immunoglobulin composed of two identical heavy chains and two identical light chains that directly binds native antigen on B cells.

T Cell Receptor (TCR)

Heterodimer of one α chain and one β chain (or γ and δ in some cells) that recognizes processed peptide antigen presented by MHC molecules.

Variable Region

Portion of Ig or TCR chains with highly diverse amino acid sequences that forms the antigen-binding site.

Constant Region

Relatively invariant portion of Ig or TCR chains responsible for effector functions or structural support.

Complementarity Determining Regions (CDRs)

Three hypervariable loops within each variable domain of Ig or TCR that make direct contact with antigen.

Paratope

Antigen-binding pocket on an antibody or BCR that fits the corresponding epitope like a lock-and-key.

Epitope

Specific molecular site on an antigen recognized by a paratope of an antibody or TCR.

Disulfide Bridge

Covalent bond linking Ig heavy and light chains or TCR α and β chains, stabilizing receptor structure.

Ig-α/Ig-β (CD79a/CD79b)

Signal-transducing heterodimer associated with the BCR that contains ITAM motifs.

Immunoreceptor Tyrosine-Based Activation Motif (ITAM)

Conserved YxxL/I sequence in signaling chains (Ig-α/β, CD3) that becomes phosphorylated to initiate lymphocyte activation.

BCR Clustering

Aggregation of two or more BCR complexes upon antigen binding that amplifies intracellular signaling.

BCR Co-Receptor Complex

Surface complex of CD19, CD21 (CR2), CD81, and CD225 that delivers Signal 2 to enhance B-cell activation.

CD21 (CR2)

Complement receptor for C3d that links complement-tagged antigen to the BCR co-receptor complex.

Plasma Cell

Terminally differentiated B cell that secretes large amounts of antibody.

αβ T Cell

Major T-cell subset expressing αβ TCRs, includes helper (CD4+) and cytotoxic (CD8+) cells.

γδ T Cell

Minor T-cell subset expressing γδ TCRs, often found in epithelial tissues with innate-like functions.

CD4 Co-Receptor

Glycoprotein on helper T cells that binds the β2 domain of MHC class II to stabilize TCR–pMHC interaction.

CD8 Co-Receptor

Heterodimer on cytotoxic T cells that binds the α3 domain of MHC class I, aiding TCR recognition.

CD3 Complex

Invariant signaling complex (γε, δε, ζζ chains) associated with the TCR that contains ITAMs.

Lck Kinase

Src-family tyrosine kinase constitutively bound to CD4 or CD8 that phosphorylates CD3 ITAMs upon TCR engagement.

Major Histocompatibility Complex (MHC)

Family of cell-surface glycoproteins that present antigenic peptides to T cells; called HLA in humans.

MHC Class I

MHC isotype expressed on nearly all nucleated cells; presents endogenous peptides to CD8+ T cells.

MHC Class II

MHC isotype expressed mainly on professional APCs; presents exogenous peptides to CD4+ T cells.

HLA-A, HLA-B, HLA-C

Highly polymorphic human MHC class I isotypes responsible for presenting peptides to cytotoxic T cells.

HLA-DP, HLA-DQ, HLA-DR

Polymorphic human MHC class II isotypes involved in presenting peptides to helper T cells.

Antigen-Presenting Cell (APC)

Cell such as dendritic cell, macrophage, or B cell that processes antigen and displays peptide–MHC complexes.

Cytosolic Pathogen

Intracellular microbe residing in the cytoplasm; its peptides are presented by MHC class I to CD8+ T cells.

Intravesicular Pathogen

Microbe living within endosomes/lysosomes; its peptides are presented by MHC class II to CD4+ Th1 cells.

Extracellular Pathogen

Microbe or toxin located outside host cells; induces antibody production via Th2-mediated B-cell help.

Perforin

Cytolytic protein released by CD8+ T cells that forms pores in infected target cell membranes leading to lysis.

Signal 1 (B Cells)

Initial activation signal delivered when antigen cross-links the BCR and its Ig-α/β ITAMs are phosphorylated.

Signal 2 (B Cells)

Co-stimulatory signal provided by the BCR co-receptor complex (CD19/CD21/CD81) enhancing activation.

Endogenous Antigen Processing

Proteasomal degradation of cytosolic proteins followed by peptide loading onto MHC class I in the ER.

Exogenous Antigen Processing

Endocytosis of extracellular proteins, lysosomal degradation, and peptide loading onto MHC class II.

B29 and mb-1 Genes

Genes encoding the Ig-β and Ig-α components of the BCR signaling heterodimer.

Complement (C3d)

Complement cleavage fragment that opsonizes antigen and binds CD21 to aid B-cell activation.

Hypervariability

High degree of sequence diversity in Ig/TCR variable regions enabling recognition of diverse antigens.