Which intercellular chemical signal is secreted into the bloodstream and influences specific activities on target tissues?

The correct answer is hormone. Hormones are intercellular chemical signals that are secreted into the bloodstream by endocrine glands. Once in the bloodstream, they travel to target tissues and influence various physiological activities and processes. This system of signaling is integral to maintaining homeostasis and regulating functions such as metabolism, growth, and mood.

Hormones have specific receptors on their target cells, which is critical for their function, as only those cells with the appropriate receptors will respond to the hormone's signal. This allows for a precise and coordinated response to the hormone’s presence, enabling the body to adapt to changes and maintain balance.

In contrast, neurohormones, while also influencing target tissues, are specifically produced by neurons and released into the bloodstream, making them a subtype of hormones. Paracrines act locally by affecting nearby cells within the same tissue, rather than being distributed through the bloodstream. Pheromones are chemical signals released into the environment and typically communicate with individuals of the same species rather than acting on specific tissues within an individual organism.