B Cell Development and Activation

Comprehensive vocabulary flashcards covering the stages, molecular checkpoints, tolerance mechanisms, and activation pathways of B-cell biology from MICI 3115 Lecture 14.

Clonal Selection Hypothesis

A foundational hypothesis proposed by Frank Macfarlane Burnet in 1957 stating that developing lymphocytes possess predetermined immunoglobulin specificity prior to antigen encounter; upon contact with matching antigen, specific cells proliferate into clones that differentiate into antibody-secreting plasma cells and long-lived memory cells, while self-reactive clones are eliminated.

Common Lymphoid Progenitor (CLP)

A bone marrow precursor derived from pluripotent hematopoietic stem cells that possesses the developmental potential to produce pro-B cells, T cells, and innate lymphoid cells (ILCs).

PAX5

The master transcription factor for B cell development that drives committed B lineage differentiation, directs $V\text{-}DJ$ recombination at the immunoglobulin heavy-chain locus, and induces expression of $Ig\alpha / Ig\beta$ and surrogate light-chain components ($VpreB$ and $\lambda 5$).

E2A and EBF1

Early transcription factors that act in concert within developing pro-B cells to open chromatin structure across the heavy-chain locus and activate the expression of $RAG1/2$ and $PAX5$.

Surrogate Light Chain (SLC)

An invariant, non-rearranged protein complex composed of $VpreB$ and $\lambda 5$ subunits that pairs with a newly synthesized $\mu$ heavy chain in pre-B cells to form the pre-B-cell receptor.

Pre-B-Cell Receptor (Pre-BCR)

A transient cell-surface receptor complex found on pre-B cells composed of a rearranged $\mu$ heavy chain, a surrogate light chain ($VpreB$ and $\lambda 5$), and the $Ig\alpha / Ig\beta$ signaling heterodimer; clustering provides the essential survival and proliferation signal representing the first developmental checkpoint.

Allelic Exclusion

The process initiated by pre-BCR signaling that transiently downregulates $RAG1/2$ expression and induces chromatin condensation at the unused homologous heavy-chain allele, ensuring that each B cell expresses heavy chains from only one parental chromosome.

Receptor Editing

A central tolerance mechanism in autoreactive immature B cells where $RAG$ genes are reactivated to induce secondary gene rearrangement at the immunoglobulin light-chain loci ($\kappa$ or $\lambda$), altering the antigen-binding specificity of the BCR away from self.

Clonal Deletion

A central tolerance mechanism triggered when immature B cells encounter and cross-link multivalent self-antigens in the bone marrow, resulting in the apoptotic clearance of autoreactive clones.

Anergy (B Cells)

A state of non-responsiveness and shortened lifespan induced in immature B cells following exposure to soluble, monovalent self-antigens that fail to cross-link the BCR.

Transitional 1 (T1) B Cell

An early stage transitional B cell newly exported from the bone marrow to the splenic periarteriolar lymphoid sheath (PALS), characterized phenotypically as $mIgM^{hi}/mIgD^{lo}$, that undergoes negative selection upon antigen encounter.

Transitional 2 (T2) B Cell

A transitional B cell phenotype ($mIgM^{hi}/mIgD^{int}$) that enters the splenic follicle, upregulates CD21/CR2, CD23, and BAFF-R, and receives tonic BCR signals and BAFF survival factors from follicular dendritic cells to complete maturation into a follicular B cell ($mIgM^{lo}/mIgD^{hi}$).

B-Cell Co-Receptor Complex

A membrane-bound signaling triad composed of CR2 (CD21), CD19, and CD81 that binds complement component C3d deposited on antigens, augmenting BCR signaling by $1{,}000\text{--}10{,}000$-fold.

CD69

An early activation marker rapidly upregulated on stimulated B cells that prevents responsiveness to the sphingosine-1-phosphate (S1P) gradient, thereby retaining activated cells within the secondary lymphoid organ.

Primary Focus

An initial site of B cell expansion located in the medullary cords of lymph nodes where paired B cells and $T_{FH}$ cells proliferate into plasmablasts that secrete early, low-affinity IgM under the control of BLIMP-1 and IRF4.

Germinal Center Dark Zone

The histological compartment of a germinal center where densely packed centroblasts rapidly divide and express activation-induced cytidine deaminase (AID) to drive somatic hypermutation of immunoglobulin variable regions.

Germinal Center Light Zone

The germinal center compartment where centrocytes cease rapid proliferation to undergo selection based on affinity maturation through competition for antigens held on FDCs and receive cognate help from $T_{FH}$ cells directing class-switch recombination.

Instructive Fate Model

A germinal center fate-determination model stating that the level of T-cell help and BCR signaling strength directly instructs outcome: absent signal leads to apoptosis, low signal (+$) produces memory B cells, moderate signal (++$) returns cells to dark zone proliferation, and high signal ($+++$$) promotes differentiation into plasma cells.

B-1 B Cells

An innate-like B-cell subset that arises in the fetal liver during embryonic development, resides primarily in peritoneal and pleural cavities, maintains numbers through IL-10-dependent self-renewal, and secretes low-affinity, polyspecific natural IgM antibodies without requiring T-cell help.

T-Dependent (TD) Antigen

A soluble protein antigen that requires both BCR cross-linking and direct cognate assistance from helper T cells (via CD40/CD40L costimulation and secreted cytokines) to drive full B-cell activation, germinal center entry, and isotype switching.

T-Independent-1 (TI-1) Antigen

A pathogen-associated molecule (such as bacterial lipopolysaccharide) capable of directly activating B cells by binding both the BCR and a Toll-like receptor (TLR); induces antigen-specific responses at low doses and polyclonal B-cell activation at high doses.

T-Independent-2 (TI-2) Antigen

A repetitive polymeric molecule (such as a bacterial capsular polysaccharide or flagellin) that cross-links dense arrays of BCRs and activates the CD21 coreceptor through C3d complement deposition to stimulate mature B cells without cognate T-cell contact.

Follicular Dendritic Cell (FDC)

A non-hematopoietic stromal cell resident in lymphoid follicles that captures and displays intact, un-internalized antigen-antibody-complement complexes on its surface to provide survival and selection signals to B cells.