Interactions between the immune system and the hypothalamo-pituitary-interrenal axis in fish

Endocrine-immune interaction in mammals is realized through a bi-directional communication network functioning through shared receptors and signal molecules to ensure physiological homeostasis. Maintenance of physio-logical homeostasis in a changing and challenging environment is pivotal to the survival of every organism. This review summarises knowledge on immune-endocrine interactions in fish, with focus on the central role of the hypothalamo-pituitary-interrenal axis. Teleosts have well-developed immune and endocrine systems that require communication to realize homeostasis. The head kidney of fish takes a particular place in the study of immune-endocrine interactions. It combines in one organ key-features of both systems: haematopoiesis, antibody production and cortisol and catecholamine production. The head kidney, therefore, offers the opportunity for direct paracrine communication between the immune and endocrine systems in a specific microenvironment. We consider this interaction of relevance durig basal conditions as well as during situations of stress.