1) The first signal in the activation of a naive B-lymphocyte by a T-dependent antigen is the binding of an epitope of that antigen to a corresponding B-cell receptor (sIg) on the cell's surface.

2) The second signal is provided when a component of the complement system called C3b binds to the microbial surface. C3b is subsequently degraded to C3d which, in turn, binds to a complement receptor called CR2 on the surface of the B-lymphocyte.

These events activate the naive B-lymphocyte and enable it to produce increased amounts of MHC-II, co-stimulatory molecules such as B7 and CD40, and receptors for T-lymphocyte derived cytokines.