Editing Irritation (section)

==Irritation in organisms==
In higher organisms, an [[allergy|allergic response]] may be the 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 dependent on the (possibly unique) sensitivity of the organism involved while an irritant, classically, acts in a non-specific manner.

It is a form of [[Stress (medicine)|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, the status of [[pain]] is the [[perception]] of the being stimulated, which is not observable although it may be shared (see [[Gate control theory]]).

[[File:Pearl oyster.jpg|thumb|right|180px|[[Pearl oyster]]]]
It is not proven that oysters can feel pain, but it is known that they react to irritants. When an irritating object becomes trapped within an oyster's shell, it deposits layers of calcium carbonate (CaCO<sub>3</sub>), slowly increasing in size and producing a [[pearl]]. This is purely a defense mechanism, to trap a potentially threatening irritant such as a parasite inside its shell, or an attack from outside, injuring the mantle tissue. The oyster creates a pearl sac to seal off the irritation.

It has also been observed that an amoeba avoids being prodded with a pin, but there is not enough evidence to suggest 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 [[mammal]]s as dogs or [[primate]]s like [[monkey]]s; Descartes considered [[intelligence (trait)|intelligence]] a pre-requisite for the feeling of pain.{{citation needed|date=July 2013}}