Memory cells doesn't undergo apoptosis due to presence of which growth factor:

Nerve growth factor [Ref: Survival of memory B lymphocytes is tightly linked to the integrity of the Bel-2 protein and is regulated by a nerve growth factor (NGF) autocrine circuit. Nerve growth factor inhibits apoptosis in memory B lymphocytes inactivation of p38 MAPK, prevention of Bcl-2 phosphorylation, and cytrochrome c release. NGF is a classical survival factor, essential for a large number of cell types, including neurons, keratinocytes, and memory B lymphocytes.