Perforin is a pore-forming protein that leads to osmotic lysis of the target cells and subsequently enables granzymes to enter the target cells and activate apoptosis, the cell death program. The expression of perforin is upregulated in activated CD8+ T cells, but in NK cells the expression is constitutively very high and stable. Perforin expression can also be stimulated in some activated CD4+ T cells. CD8+ T cells and NK cells use both the perforin/granzyme and the Fas /FasL pathways, while CD4+ T cells preferentially use the Fas /FasL pathway.
