The Bcell Antigen Receptor Complex

The B-cell antigen receptor complex consists of the membrane Ig glycoprotein and a heterodimer of nonvariant transmembrane proteins, referred to as Iga (CD79a) and IgP (CD79b). The Igs are tetramers constituted by pairs of disulfide-linked heavy (H) and light (L) chains. In normal B-cell precursors, production of Iga and IgP molecules precedes IgH gene rearrangement. Appearance of cytoplasmic pHC without LC

characterizes the Pre-B differentiation stage (see above). A small fraction of p chains does surface at the cell membrane of Pre-B cells after interaction with a pseudo- or surrogate LC chain complex (yLC), consisting of two proteins, VpreB and X5, and association with Iga and IgP, forming the Pre-B cell receptor (Pre-BCR), which appears to be essential for the further advancement of B-cell maturation.256,264 This differentiation stage can be termed transitional Pre-B cell. Subsequently, IgL K chain genes rearrange, leading to surface expression (s) of HC and LC together to form the B-cell receptor (BCR), first IgM/K (immature B cell) and subsequently coexpression of IgM/K and IgD/K chains with identical variable domains on the majority of mature, resting circulating B cells. If the K rearrangement is not functional, the IgX genes will rearrange and IgH/X chains will be expressed on the cell membrane, which in humans is the case in about 10 to 30 percent of mature B cells. Each lymphocyte will only express either K or X LC, and monoclonality of a given B-cell population for IgLC expression suggests a malignant process. Deduced from the reactivity pattern of antibodies to distinct epitopes of the Pre-BCR, a novel classification system for B-lineage ALL has been proposed based on the intracyto-plasmic expression of surrogate light chains, pH chains, Pre-BCRs, and light chains.266

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