Which term describes a signaling molecule released by cells that travels through the bloodstream to act on distant target cells?

• A. Hormone
• B. Enzyme
• C. Neurotransmitter
• D. Cytokine

Answer: (A)

Endocrine signaling uses hormones that travel through the bloodstream to reach distant target cells. These signaling molecules are released by glands into the blood and circulate until they encounter cells with specific receptors for them, triggering physiological responses such as metabolism, growth, or homeostasis. This long-range communication distinguishes hormones from other signaling molecules. Neurotransmitters, for example, typically act across synapses on nearby cells for rapid, local signaling rather than through the bloodstream. Cytokines are immune-related signals that can act locally or systemically but are usually discussed in the context of immune communication rather than classic hormonal regulation. Enzymes are catalysts for chemical reactions and do not function as signaling messengers.