Cell signalling may involve the transmission of a chemical signal over a broad range of distances in order to elicit a response

• Examples of intercellular communication within organisms include autocrine, paracrine and endocrine signalling
  

• Additionally, certain chemicals may be transmitted between members of a species (e.g. pheromones) or between different species (e.g. allelopathy)

Autocrine

• Autocrine signalling occurs when the chemicals released by a cell stimulate the cell itself (i.e. ‘self’ signalling)
  

• An example is the proliferation of T lymphocytes following cytokine release from an activated T lymphocyte
  

Paracrine

• Paracrine signalling occurs when the chemicals released by a cell stimulate a neighbouring cell

• An example is the stimulation of post-synaptic neurons by neurotransmitters released from a pre-synaptic neuron

Endocrine

• Endocrine signalling occurs when a chemical released by a cell travels in the bloodstream to activate distant cells

• An example is the release of hormones from endocrine glands to activate distant target tissue