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For English language definitions see: Wiktionary:irritation
Wiktionary:irritant
Irritation, in Biology and Physiology, is a state of over-excitation or hyper-reactivity. A stimulus or agent which induces the state of irritation is an irritant. Irritants are typically thought of as chemical agents (for example Phenol and Capsaicin) but mechanical, thermal (heat) and radiative stimuli (for example ultraviolet light or ionising radiations) can also cause irritation.
In higher organisms the allergic response may be cause of irritation. An Allergen is defined distinctly from an irritant, however, as allergy requires a specific interaction with the Immune system and is thus dependant on the (possibly unique) sensitivity of the organism involved while an irritant, classically, acts in a non-specific manner.
In humans, it is a mild form of Suffering, often with Anger about this, in particular, if applicable, anger at the person who caused it. This can also be oneself, e.g. when forgetting something or doing something stupid.
It is a form of stress, but conversely, if one is stressed by unrelated matters, mild imperfections can cause more irritation than usual, one is irritable; see also Sensitivity (human).
In more basic organisms, assigning irritation the status of Pain is the Perception of the being stimulated - which is not observable although it may be shared (see Gate control theory of pain).
We can't say that an oyster feels pain, but we can say it does react to something irritating it, to produce something that is useful to us, and not useful to it. Pearls do not attract mates for the oyster, they attract them for us. It seems impossible to find an evolutionary advantage for the ability to produce the pearl, thus it can be explained only as a reaction to an irritation.
We can also say that an amoeba avoids a pin we poke it with, but not how much it feels this. Irritation is apparently the only universal Sense shared by even single-celled creatures.
It is postulated that most such beings also feel pain, but this is a projection - Empathy. Some philosophers, notably René Descartes, denied it entirely, even for such higher mammals as dogs or primates like monkeys - to him, intelligence was a pre-requisite to even the feeling of pain.
Mating is a fascinating example of the complexity that can arise in considering irritation. Males generally pursue females in mating species - females avoid males that they assume are undesirable, or at bad times to bear young. Thus the physical aspects of sexual pursuit may irritate the female, and the pursuit may itself be an irritation. This changes when a desirable male approaches at the right time - at least insofar as the female considers it to be so.
So something which is an irritation at one time may serve a purpose, or even be a pleasure, at another. For these reasons mating is very often omitted from any discussion of irritation itself, and indeed prevents problems for discussions of Pain and pleasure itself. Mating often involves both - and certainly motherhood also involves a great number of irritations, but with the third party observed advantage of passing on genes - that is, if one accepts that this is of any use.
See also Sensitivity.