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The term plaque type parapsoriasis has been used for over a hundred years to describe a variety of different conditions. The literature can therefore appear slightly chaotic and confusing unless care is taken to update the concepts accepted at the time the article referred to was written.
The skin comes into contact with a wide variety of environmentally-borne sources of antigenic stimuli. It is populated by specific lymphocytes which form a line of defence and are part of the body’s adaptive immunity. These are called naive T cells. They are initially found in the circulation, but their role is to protect the skin against aggressions. They migrate from the circulation to the skin where they bind and generate memory T lymphocytes after antigenic stimulation. Thus transformed, these lymphocytes can remain quiescent for up to ten years. Once the naive cell has become a memory cell, the first line of defence in the cutaneous immunological arsenal is located in the skin itself .
Slight and chronic stimulation by certain xenobiotics may cause, at the very least, allergic contact dermatitis or irritant contact dermatitis. Their appearance may change to resemble the macula of mycosis fungoides (MF). In certain cases, true MF may develop . A viral origin, in particular Herpes type VIII (HHV8), has also been suggested [3-5]. This is known to be extremely difficult to diagnose, both clinically and via laboratory tests. At the present time, two clinically different forms of plaque type parapsoriasis have been identified: large-plaque parapsoriasis and small-plaque parapsoriasis. The link between MF and large-plaque parapsoriasis has been clearly established. The link between small-plaque parapsoriasis and MF has been rejected by most authors but is upheld by some [6-9].
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