LAMP1 (lysosomal-associated membrane protein 1) is a multifunctional membrane protein with primary roles in lysosomal homeostasis and immune regulation. LAMP1 directly inhibits the TMEM175 cation channel to regulate lysosomal pH acidification, a critical function for optimal hydrolase activity and cellular degradation 1. Beyond its classical lysosomal marker role, LAMP1 functions in immune cytotoxicity: it localizes to cytolytic granule membranes and surface expression (CD107a) marks natural killer and T cell degranulation and antitumor activity 2. LAMP1+ astrocytes expressing TRAIL limit CNS inflammation by inducing T cell apoptosis in a microbiome-dependent manner 3. LAMP1 mediates lipid transport and trafficking independent of autophagy in some organisms 4. Recent evidence identifies cell-surface LAMP1 as a senescence marker associated with cellular aging and idiopathic pulmonary fibrosis, correlating with elevated p16/p21 expression 5. LAMP1 localizes to late endosomes, exosomes, and cytolytic granules rather than exclusively to degradative lysosomes 6. Dysregulation of LAMP1-mediated pathways contributes to lysosomal dysfunction underlying neurodegenerative diseases and cancer progression.