Human milk contains many components that protect the newborn against infection at a time when the infant's own defense mechanisms are poorly developed. Fat is one of the major nutrients in human milk. The fat is contained within milk fat globules composed of a core of triglyceride and a membrane consisting of phospholipids, cholesterol, proteins, and glycoproteins. Both the membrane and the core components can provide protection against microorganisms. The major protective membrane glycoproteins, mucin, and lactadherin are resistant to conditions in the newborn's stomach and maintain their structure and function even at low pH and in the presence of the proteolytic enzyme pepsin. The core triglycerides upon hydrolysis by digestive lipases (especially gastric lipase, which is well developed in the newborn) produce free fatty acids and monoglycerides, amphiphylic substances able to lyse enveloped viruses, bacteria, and protozoa. Therefore, in addition to its nutritional value, the fat in human milk has a major protective function.