Immunology: B-Cell Biology, Effector Responses, and Barrier Immunity

Comprehensive vocabulary flashcards covering B-cell development, receptor signaling, germinal center reactions, antibody effector functions, cell-mediated cytotoxicity, and barrier immunity.

Clonal Selection Hypothesis

The model stating that each immature B-cell has a unique antigen specificity, antigen stimulation drives clonal expansion, memory B-cells persist after the response, and self-reactive B-cells are deleted during embryonic development.

Plasma Cells

Fully differentiated antibody-producing B cells that can no longer divide, bear little to no cell-surface immunoglobulin, and rapidly secrete large numbers of antibodies to combat an active infection.

Memory B Cells

Differentiated B cells that persist following an initial antigen exposure to support a faster and enhanced secondary immune response upon future re-exposure.

T-dependent (TD) Responses

B-cell responses mounted against protein antigens that require helper T-cell assistance and are predominantly mediated by B-2 cells.

T-independent (TI) Responses

B-cell responses triggered by multivalent or polymerized non-protein antigens that do not require helper T-cell assistance.

TI-1 Antigens

T-independent antigens that bind directly to B cells via pattern recognition receptors (PRRs/Toll-like receptors) and B-cell receptors (BCRs).

TI-2 Antigens

T-independent antigens that cross-link extensive numbers of surface B-cell receptors (BCRs) to trigger activation.

Subcapsular Sinus Macrophages (SCSM)

Specialized lymph node macrophages that act as antigen catchers by capturing large antigens ($>70\,\text{kDa}$) and transferring them to follicular B cells.

Follicular Dendritic Cells (FDCs)

Non-lymphoid stromal cells in B-cell follicles where antigens are concentrated on their surfaces for future B-cell selection and differentiation.

Follicular Reticular Cells (FRCs)

A supporting network of stromal cells in lymphoid tissue forming conduits that allow small soluble antigens ($<70\,\text{kDa}$) to travel directly to follicular B cells.

Membrane Spreading

A rapid morphological process occurring within minutes of antigen recognition on an APC where a B cell expands its membrane contact area across the target surface and subsequently contracts to strengthen signaling.

BCR Oligomerization

The gathering and clustering of antigen-bound B-cell receptors within the plasma membrane following antigen binding.

Lipid Rafts

Small, specialized microdomains of the cell membrane enriched in signaling molecules into which clustered BCRs migrate during activation.

ITAMs

Immunoreceptor tyrosine-based activation motifs that become phosphorylated by Src-family kinases (such as Lyn) upon BCR clustering, transmitting activation signals into the cell interior.

Germinal Center Fate

A B-cell differentiation path driven by the transcription factor network of Pax5, Bcl-6, and low IRF-4, leading to somatic hypermutation and class switch recombination.

Plasma-Cell Fate

A B-cell differentiation path directed by BLIMP-1 and high IRF-4 that promotes the generation of antibody-secreting plasma cells.

Plasmablasts

Intermediate differentiated B cells that have begun secreting antibodies and maintain cell-surface BCR expression while retaining the ability to proliferate.

Dark Zone

The microenvironmental compartment of a germinal center where activated B cells undergo clonal proliferation and somatic hypermutation (SHM).

Light Zone

The germinal center microenvironment where mutated B cells undergo affinity selection via competition for antigen on FDCs and helper T-cell signals.

Somatic Hypermutation (SHM)

A process occurring in the germinal center after VDJ recombination that introduces point mutations into the rearranged heavy- and light-chain variable regions of immunoglobulin genes.

Class Switch Recombination (CSR)

An AID-dependent DNA rearrangement process in the germinal center between donor and acceptor switch (S) regions on transcribed single-stranded DNA that alters the antibody constant region and requires CD40 signaling.

B-1 B Cells

An innate-like B-cell subset that constitutes approximately $5\%$ of B-cell production, self-renews in the periphery, produces IgM, and provides protection in body cavities.

Marginal Zone B Cells

Splenic B cells that self-renew, require continuous low-level BCR tonic signaling for survival, and specialize in capturing blood-borne antigens entering the spleen.

Neutralization

An antibody-mediated effector function where antibodies bind to viral particles, bacterial surfaces, or toxins to physically block their biological activity.

Agglutination

The cross-linking and clumping of multiple pathogens by multivalent antibodies to enhance neutralization via steric hindrance and facilitate clearance through peristalsis or mucus trapping.

Opsonization

An effector process mediated predominantly by IgG antibodies that coats pathogens to promote and enhance recognition and engulfment by phagocytic cells.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

A mechanism in which Fc receptors on cytotoxic effector cells (such as NK cells) bind antibody-coated targets, triggering target cell lysis.

PolyIgR

Polymeric immunoglobulin receptor expressed by epithelial cells that binds and transports polymeric IgA and IgM from the blood across mucosal barriers into lumen secretions such as tears and breast milk.

FcRn

The neonatal Fc receptor that binds and transports maternal IgG antibodies across the intestinal epithelial cell layer into neonatal circulation.

Dendritic Cell Licensing

The CD40-CD40L-mediated activation of a dendritic cell by a CD4+ helper T cell, empowering the dendritic cell to cross-present antigen and effectively stimulate naïve CD8+ T cells.

Perforin

A pore-forming protein discharged from cytotoxic T lymphocyte (CTL) and NK cell granules that integrates into the target cell plasma membrane to facilitate granzyme entry.

Granzymes

Serine proteases released by CTLs and NK cells that enter target cells through perforin pores to enzymatically trigger apoptosis from within.

Missing Self Model

The mechanism whereby NK cells spare normal cells expressing MHC Class I (which activates inhibitory receptors) but kill abnormal or infected cells that downregulate MHC Class I, allowing activating signals to dominate.

Natural Killer T (NKT) Cells

Innate-like T lymphocytes that express an invariant T-cell receptor (TCR) alongside NK-cell surface markers, act as helper cells, and do not establish immunological memory.

Mucosa-Associated Lymphoid Tissue (MALT)

The organized barrier lymphoid tissue system protecting mucosal surfaces of the gastrointestinal, respiratory, and urogenital tracts.

Intraepithelial Lymphocytes (IELs)

A specialized population of CD8+ T cells located directly between the epithelial cells of barrier tissues, comprising memory, conventional, and invariant T cells.