Discovery of the BLyS (also known as BAFF) family of ligands and receptors has yielded a paradigm shift in our view of B-lymphocyte selection, survival, activation, and homeostasis. Previously, the B-cell antigen receptor (BCR) was viewed as the sole mediator of these parameters, in which BCR signals were not only dominant but were also linearly related to consequent outcomes. However, appreciating that BLyS signaling is an equal partner in establishing and maintaining B-cell pools in- cated that additional regulatory complexity apparently based on population density and homeostatic demands...