B cell activation

TI-I antigens
Provide 2 signals by engaging both BCR and TLRs

TI-2 antigens
Contain repetitive units (epitopes) which facilitate cross-linking of BCRs and co-receptors.

T cell dependent activation
3 signals are required for B cell activation:

Signal 1 = Ag : BCR → TCR : MHC II-peptide

 * Involves engagement of co-receptor complex CR2/CD19/CD81
 * CD21, a co-stimulatory receptor, greatly enhances B cell activation to low levels of Ag when Ag is decorated with C' fragments of C3
 * FDCs can bind Ags opsonized by C' fragments to present to B cells