What is Signal
Signal is a message those are given to a cell through extracellular or intracellular signalling mechanisms by which target cell response accordingly.
On the basis of cell Signalling molecules, the extracellular cell Signalling are of various types-
Endocrine :
The signal molecules moves to a distantly located target cell therefore signalling molecules move through blood vascular system. Eg. Hormonal signalling
Paracrine :
Signalling molecules secreted in vicinity of target cell. Eg. Neurotransmitter
Autocrine :
signalling molecules acts on its own derived cell
Eg. T cell in immune response