Structure of the B-cell and of the pre-B-cell receptor. The BCR and the pre-BCR contain the membrane form of the Ig HC that is noncovalently associated with a heterodimer of Igα and Igβ, and the Ig LC (BCR) or the surrogate light chain, which is composed of λ5 and VpreB (pre-BCR). Proximal signaling is initiated by SRC-family protein tyrosine kinase (PTK)–mediated phosphorylation of the Igα and Igβ immunoreceptor tyrosine-based activation motifs. Recent evidence suggests that the pre-BCR spontaneously dimerizes,^75,76  whereas the BCR needs antigen to be aggregated and to give the antigenic signal.