Which substances carry messages through the body in the blood?

Hormones are chemical messengers that travel through the bloodstream to tissues and organs, regulated by the endocrine system. They are produced by various glands in the body, such as the pituitary, adrenal, and thyroid glands. Hormones affect numerous physiological processes, including growth and development, metabolism, and mood regulation. Their specific roles and target sites allow them to elicit precise responses in the body, making them vital for maintaining homeostasis.

Enzymes facilitate biochemical reactions but do not serve as messengers. Proteins can have various functions, including structural roles, but hormones are a specific subset of proteins dedicated to signaling. Antibodies are involved in the immune response, providing defense against pathogens rather than carrying messages throughout the body. Thus, hormones stand out as the correct answer as they are the substances primarily responsible for conveying messages through the blood.