Which substance can bind to specific receptors on target cells and is secreted into the blood?

• A. Neurotransmitter
• B. Vector
• C. Hormone
• D. Ligand

Answer: (C)

The substance that can bind to specific receptors on target cells and is secreted into the blood is a hormone. Hormones are signaling molecules produced by endocrine glands, such as the pituitary gland, thyroid gland, and adrenal glands. Once secreted into the bloodstream, they travel to various parts of the body to exert their effects by binding to specific receptors on target cells. This binding triggers a response in the target cells, leading to a variety of physiological changes and effects that help regulate processes such as metabolism, growth, and mood.

The term "ligand" refers to any molecule that can bind to a specific receptor, which includes hormones, neurotransmitters, and other signaling molecules, but it is broader and does not specifically refer to the ones secreted into the blood. While neurotransmitters do bind to receptors, they are typically released at synaptic clefts in the nervous system and operate over shorter distances, rather than being circulated in the bloodstream. A vector generally refers to an organism or agent that transmits disease or genetic material, and is not related to the signaling mechanisms used by hormones or other signaling molecules. Thus, the specificity and mode of action of hormones validate them as the correct answer to the question.