What are the chemical messengers secreted into the bloodstream that act on cells far away?

Hormones are the chemical messengers that are secreted into the bloodstream and can act on target cells located far away from their site of origin. They are produced by various glands in the endocrine system, such as the pituitary, thyroid, adrenal glands, and pancreas, and their primary function is to regulate physiological processes throughout the body, including metabolism, growth, and mood.

Unlike neurotransmitters, which transmit signals across synapses in the nervous system and act locally, hormones travel through the bloodstream to reach their distant targets. Cytokines, on the other hand, are involved in the immune response and typically act in a more localized manner, although some can have systemic effects. Enzymes are biological catalysts that facilitate biochemical reactions but do not function as messengers in the manner that hormones do. This distinction highlights why hormones are considered the correct answer in this context.