Co-stimulatory receptor-ligand pairs in the initiation of a T-cell response. CD27 and CD28 are co-stimulatory molecules that are constitutively expressed. Their ligands CD70, CD80, and CD86 are inducible on dendritic cells upon their activation. The CD154-CD40 interaction represents an important amplification loop that upregulates the expression of CD80 and CD86. MHC, major histocompatibility complex; TCR, T-cell receptor.