Lysozyme (Muramidase, LYZ) is a bacteriolytic and histiocytic enzyme that functions in the immune system as a defense mechanism against certain pathogens. Lysozyme can break down the peptidoglycans that compose gram-positive bacterial cell walls (such as Streptococcus and Bacillus). High levels of lysozyme are present in egg white, and in humans it is found in secreted mucus, tears, saliva, and milk. It is also expressed in macrophages and polymorphonuclear neutrophils. In immunohistochemistry, lysozyme stains positively in histiocytes, monocytes, neutrophils, macrophages, goblet cells, lactating lobules of the breast, some epithelial cells and normal hematopoietic cells. In cancer, it is positive in myeloid leukemias where it can by highly overexpressed, acinic cell carcinomas of the breast and salivary gland, goblet cell carcinoids, myeloid sarcomas and various histiocytic lesions. References: Am J Surg Pathol. 2009. 33:1137, PMID: 19461506; CRC Press. 1994. p. 223. ISBN: 978-0-8493-8935-1; McGraw-Hill Higher Education. 2007. ISBN: 978-0-07-110706-8; Am J Clin Pathol. 1992 Feb;97(2):195-201, PMID: 1546687