A chemical messenger that signals to specific cells to change their behavior in specific ways. Hormones are secreted into the bloodstream through endocrine glands, part of the endocrine system. Ensuing are a few significant examples: | A chemical messenger that signals to specific cells to change their behavior in specific ways. Hormones are secreted into the bloodstream through endocrine glands, part of the endocrine system. Ensuing are a few significant examples: |