Cytokines in human milk

Epidemiologic studies conducted in the past 30 years to investigate the protective functions of human milk strongly support the notion that breastfeeding prevents infantile infections, particularly those affecting the gastrointestinal and respiratory tracts. However, more recent clinical and experimental observations also suggest that human milk not only provides passive protection, but also can directly modulate the immunological development of the recipient infant. The study of this remarkable defense system in human milk has been difficult because of its biochemical complexity, the small concentration of certain bioactive components, the compartmentalization of some of these agents, the dynamic quantitative and qualitative changes of milk during lactation, and the lack of specific reagents to quantify these agents. However, a host of bioactive substances, including hormones, growth factors, and immunological factors such as cytokines, have been identified in human milk. Cytokines are pluripotent polypeptides that act in autocrine/paracrine fashions by binding to specific cellular receptors. They operate in networks and orchestrate the development and functions of immune system. Several different cytokines and chemokines have been discovered in human milk in the past years, and the list is growing very rapidly. This article will review the current knowledge about the increasingly complex network of chemoattractants, activators, and anti-inflammatory cytokines present in human milk and their potential role in compensating for the developmental delay of the neonate immune system.