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Localized cell-to-cell communication mediated by intentionally cell-produced molecules.
Paracrine signaling can be loosely viewed as hormone-like signaling though, unlike hormones, the signalling molecules do not enter the blood and therefore have only local immediate effects rather than systemic effects. Hemostasis, that is, various mechanisms that can lead to the clotting of blood, is a process that involves paracrine signaling as so too are various signals exchanged between leukocytes (i.e., white blood cells) such as during antigen presentation.
A special case of paracrine signaling occurs when the cell that releases the signal is same cell that receives the chemical signal, a process described as autocrine signaling.
Another special case of paracrine signaling is that which occurs within synapses as mediated by neurotransmitters. This is paracrine-like because neurotransmitters are hormone like – indeed, some can serve also as hormones – but the primary effects of neurotransmitters are only immediately local, i.e., from one neuron (presynaptic) to another (postsynaptic).
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