endocytosis

Endocytosis is a fundamental cellular mechanism used to internalize extracellular substances by enveloping them in an invagination of the plasma membrane, which then pinches off to form a vesicle. Three major types exist: (1) Phagocytosis - primarily performed by professional phagocytes (macrophages, neutrophils, dendritic cells) to engulf large particles like bacteria, cellular debris, or apoptotic cells, forming a phagosome that fuses with lysosomes for degradation; this is actin-dependent and critical to innate immunity. (2) Pinocytosis - nonspecific uptake of extracellular fluid and dissolved solutes, occurring continuously in most cells. (3) Receptor-mediated endocytosis - a highly specific process where cargo (e.g., LDL, transferrin, hormones, growth factors) binds cell-surface receptors that cluster in clathrin-coated pits; the clathrin lattice assists membrane curvature, and dynamin GTPase pinches off the vesicle. After internalization, clathrin is shed and the vesicle fuses with early endosomes, where acidification triggers ligand-receptor dissociation, allowing sorting for degradation (lysosome), recycling (back to membrane), or transcytosis. Clinical relevance: LDL receptor endocytosis defects cause familial hypercholesterosterolemia (LDLR mutations impair clathrin-coated pit binding or internalization, leading to elevated LDL-C and premature atherosclerosis). Diphtheria and cholera toxins exploit receptor-mediated endocytosis to enter cells. Certain viruses (e.g., influenza, HIV) use endocytic pathways for cell entry. Dynamin inhibitors and clathrin-disrupting drugs are studied as antiviral/anti-inflammatory agents. Endocytosis also underlies antigen presentation by APCs (MHC II pathway), synaptic vesicle recycling at neuromuscular junctions, and iron uptake via transferrin receptor. Distinguishing endocytosis from exocytosis (opposite process, releasing vesicle contents) is important for understanding cellular trafficking and secretory pathways tested on boards.