-Second messengers are molecules that relay signals received at receptors on the cell surface; such as the arrival of protein hormones, growth factors, etc., to target molecules in the cytosol and/or nucleus.
-Many of the chemical messengers such as hormones, that transmit messages from one cell to another never enter the target cell, these molecules are known as first messengers which work by binding to a specific receptor on the cell surface. The receptor then stimulates or inhibits the production of a so-called second messenger within the cell.
-The second messenger are small molecules such as Cyclic AMP, which activate an effector within a cell.
