Cytokines— small secreted proteins that mediate and regulate immunity, inflammation, and hematopoiesis. They are produced in response to an immune stimulus. They act by binding to specific membrane receptors, which then transmit the signal into the cell via secondary messengers. Responses to cytokines include increased or decreased expression of membrane proteins (including cytokine receptors), and proliferation and secretion of effector molecules.
Chemokines— a family of small cytokines secreted by cells to induce chemotaxis, which is critical in cellular movement and in multicellular organism development.
Growth factors— substances (usually proteins) that act as signaling molecules between cells, by interacting with receptor molecules on the surfaces of target cells.
All three of these types of molecules function as indicators of inflammation or disease and provide a means of manipulating cellular responses in vivo and in vitro.
The ability to detect these factors has become increasingly important to researchers and clinicians and can be used to study and diagnose disease. For example, in healthy individuals, cytokine levels are expected to be low or undetectable, whereas they are elevated in a number of disease states.