What are the common themes of receptor signaling during B-cell activation that requires cross-linking of surface Ig?
1. On the surface of a given cell, there are many copies of the same (transmembrane) receptor.
2. Due to transmembrane proteins diffusing laterally in the cell membrane, many copies of the same receptor can be distributed all over the cell surface; polyvalent Ag can induce diffusion of several copies to the same region of the cell membrane (called cross-linking or clustering).
3. Leads to phosphorylation of cytoplasmic tails of Igα and Igβ, which initiates intracellular signaling cascade that leads to changes in gene expression in the nucleus.