Lymphocyte Development and Selection

Vocabulary flashcards covering lymphocyte development, receptor gene rearrangements, signal transduction complexes, MHC molecules, and selection mechanisms in B and T cell maturation.

Naïve Mature B Lymphocyte Antigen Receptors

Surface-bound antigen receptors composed of two immunoglobulin isotypes, specifically $IgM$ and $IgD$, co-expressed on the cell membrane with a single antigen specificity (idiotype).

Idiotype

The unique three-dimensional antigen-binding structure located on the N-terminal end of an immunoglobulin or T-cell receptor that determines its specific antigen-binding capacity.

Isotype

The class or category of an immunoglobulin or T-cell receptor heavy/light or $\text{alpha/beta}$ chain determined by its constant domain amino acid sequences.

B-Cell Signal Transduction Complex

A cell-surface protein structure comprising two invariant chains, Ig\text{-}\frac{\text{alpha}}{\text{}}} \text{ and } Ig\text{-}\beta (Ig\text{-}\frac{\text{alpha}}{\text{}}} and $Ig\text{-}\beta$), co-expressed with a B-cell co-receptor consisting of $CD19$, $CD21$, and $CD81$.

CD3 Complex

An invariant multichain structure associated with the T-cell receptor that mediates intracellular signal transduction leading to lymphocyte activation upon antigen recognition.

VDJ Recombination

The random somatic gene rearrangement of variable ($V$), diversity ($D$), and joining ($J$) gene segments that constructs unique variable domains for immunoglobulin heavy chains and T-cell receptor $\beta$ chains.

RAG1 and RAG2

Genes that encode essential protein components of the recombinase complex, which direct V(D)J gene rearrangements during early B-cell and T-cell maturation.

Terminal Deoxyribonucleotidyl Transferase (Tdt)

An enzyme expressed during lymphocyte development that inserts non-templated nucleotides (N-nucleotides) at the junctions of $V$, $D$, and $J$ segments, generating junctional diversity.

Allelic Exclusion

The process by which the productive functional rearrangement of an antigen-receptor gene allele on one chromosome halts rearrangement and expression of the allele on the homologous chromosome, ensuring single receptor specificity per cell.

Omenn Syndrome

An autosomal recessive disorder caused by missense mutations in $RAG$ genes leading to partial recombinase activity; characterized by a lack of B cells, severe immune deficiency, generalized red rash, diarrhea, and failure to thrive.

Severe Combined Immunodeficiency (SCID) (RAG Deficiency)

An autosomal recessive condition caused by null mutations in $RAG1$ or $RAG2$ genes resulting in complete absence of $RAG$ enzyme activity, total lack of B and T cells, and profound combined immunodeficiency.

Clonal Deletion

The removal of developing self-reactive lymphocytes through induced apoptosis within primary lymphoid organs such as the bone marrow or thymus.

Clonal Anergy

The functional inactivation of self-reactive lymphocytes in peripheral tissues, which in B cells is characterized by high surface expression levels of $IgD$.

Double Negative T Lymphocytes

Early immature T-cell precursors residing in the thymus that lack cell-surface expression of both $CD4$ and $CD8$ co-receptors.

Double Positive T Lymphocytes

Developing thymocytes located in the thymic cortex that co-express both $CD4$ and $CD8$ surface markers alongside the $CD3$ complex and T-cell receptor.

Class I MHC Molecules

Codominantly expressed surface heterodimers found on all nucleated cells and platelets consisting of an $\frac{\text{alpha}}{\text{}}$ heavy chain with three domains ($\frac{\text{alpha}}{\text{}}_1, \frac{\text{alpha}}{\text{}}_2, \frac{\text{alpha}}{\text{}}_3$) associated with $\beta_2\text{-microglobulin}$.

Class II MHC Molecules

Codominantly expressed surface heterodimers present on professional antigen-presenting cells (macrophages, dendritic cells, B cells) composed of two non-identical chains ($\frac{\text{alpha}}{\text{}}$ and $\beta$), each having two extracellular domains.

HLA-DM

An intracellular Class II MHC gene product that acts as a molecular chaperone inside endosomes to facilitate proper peptide loading onto Class II MHC molecules.

Positive Selection

The thymic selection process wherein double positive T cells whose receptors bind self-MHC molecules with low affinity are signaled to survive, proliferate, and mature into single positive T cells.

Negative Selection

The thymic selection process wherein developing T cells that bind self-MHC and self-peptide complexes with excessively high affinity are eliminated by apoptosis to maintain self-tolerance.

Regulatory T Cells (Tregs)

A specialized subset of $CD4^+$ T cells constitutively expressing $CD25$ and the transcription factor $FoxP3$ that produce $IL\text{-}10$ and $TGF\text{-}\beta$ to suppress self-reactive lymphocytes and maintain peripheral tolerance.