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The fragment crystallizable region （Fc region） is the tail region of an antibody that interacts with cell surface receptors called Fc receptors and some proteins of the complement system. This region allows antibodies to activate the immune system, for example, through binding to Fc receptors. Fc binds to various cell receptors and complement proteins. In this way, it mediates different physiological effects of antibodies （detection of opsonized particles; cell lysis; degranulation of mast cells, basophils, and eosinophils; and other processes）.