A @RT06841@ is a protein or a protein complex in or on a cell that specifically recognizes and binds to a compound acting as a molecular messenger (neurotransmitter, hormone, lymphokine, lectin, @D01863@, etc). In a broader sense, the term @RT06841@ is often used as a synonym for any specific (as opposed to non-specific such as binding to plasma @P04898@) @D01863@ @B00649@, also including @N04245@ such as @D01597@.