The secreted NGF monomers mature to form tiny homodimers with a high degree of hydrophobic interaction. NGF is comprised of a neurotrophic factor produced from the brain and neurotrophic factors 3 and 4/5 . All of these peptides have a low affinity for the p75 receptor . However, it has a high affinity for the tropomyosin-related kinase (Trk) receptor . NGF interacts with TrkA's extracellular immunoglobulin-like domain (Ig-C2) . NGF may aid with survival. Under acidic circumstances, the signal forms a stable compound with TrkA . TrkA is a plasma membrane protein that interacts with dimeric NGF in the extracellular environment. Without p75, TrkA can entirely mediate NGF endocytosis . NGF stimulates and increases TrkA internalization, while NGF that has been internalized degrades slowly and inefficiently. NGF is reversely endocytosed upon internalization. It supports retrograde TrkA trafficking and sympathetic neuron survival retrogradely.