What is the name of the extracellular signal molecule that is transported through the bloodstream in animals?

The extracellular signal molecule that is transported through the bloodstream in animals is called a hormone. Hormones are chemical messengers produced by glands in the endocrine system and released into the bloodstream, allowing them to travel to distant target organs or tissues to elicit specific physiological responses.

This system of hormone signaling is crucial for regulating a variety of bodily functions, including growth, metabolism, reproduction, and homeostasis. Hormones can affect numerous biological processes by binding to specific receptors on target cells, resulting in changes to cell function and behavior.

In contrast, neurotransmitters are signaling molecules that transmit signals across synapses in the nervous system, enzymes facilitate biochemical reactions but do not serve as signaling molecules, and antibodies are involved in immune responses rather than acting as signaling agents in the bloodstream. Thus, the role of hormones as systemic, long-distance communicators highlights their importance in coordinating complex physiological processes in animals.