Lysozyme (muramidase) is an antimicrobial enzyme present in tears, saliva, nasal mucus, breast milk, and serum. It cleaves the β-1,4-glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in bacterial cell wall peptidoglycan, causing cell lysis. It is especially effective against Gram-positive bacteria. Alexander Fleming discovered lysozyme in 1922 before discovering penicillin. It is a key component of innate immunity.