The best answer is Hormone.
Hormones are chemicals secreted by one tissue and affect the functioning of other tissues and cells.
Hormones are secreted by glands of the endocrine system. They are secreted into the blood and also extracellular fluid. They then circulate in the blood and come into contact with essential all cells, but a specific hormone will only affect a limited number of cells, namely the cells that bear a receptor for that hormone.
Red blood cells for example have receptors for the insulin hormone that is produced by the pancreas. They will therefore be directly affected by insulin while all other cells in the blood remain oblivious of this hormone..
