Section 1 Introduction I. Concept of Hormone a hormone --chemical substance secreted into the interstitial fluids by one specialized cell or a group of cells --exert physiological control effect on other cells of the e body2 Section 1 Introduction I. Concept of Hormone A hormone – --chemical substance --secreted into the interstitial fluids by one specialized cell or a group of cells --exert physiological control effect on other cells of the body