S3C18VA2-1: In the characterization of epidermal responses, emphasis is generally placed on cytologic features with little attention to the alterations in the inter-cellular matrix. The epidermal matrix, being normally scanty and inconspicuous, is easily ignored. Conceptually, the lichenoid reaction, as defined by the variations encountered in lesions of lichen striatus, can be represented as if they are expressive of sequential phases. Initially, the response is focal edema in the basal unit of the epidermis (the zone in which the interstitial spaces are open and mildly mucoid); the inter-cellular spaces appear to be widened ( spongiosis). In sequence, the basal unit undergoes hyperplasia with accentuation of the mucoid, interstitial spaces in a hyperplastic basal unit. The epidermal pattern becomes psoriasiform; often this alteration is accompanied by a migration (exocytosis) of lymphocytes and histiocytes into the mucoid spaces of the altered basal unit. This reaction characterizes the interstitial alterations in a primary lichenoid reaction (a psoriasiform lesion with a rich infiltrate of lymphoid cells in the altered basal unit; the combination of lichenoid and psoriasiform patterns might be characterized as a pityriasic quality.

The psoriasiform phase is followed by a change in the relationships between basal keratinocytes and lymphoid cells. The lymphoid cells induce lysis and coagulation of keratinocytes (target cells); lytic defects are produced in the basal unit of the epidermis. The superficial unit is more resistant and may undergo hyperplasia and hypertrophy. In the hyperplastic superficial unit, the interstitial spaces have been closed by the formation of a lipid-rich membrane. This alteration in the epidermal interstitium is characteristic of the changes seen in an established lichenoid reaction.

In the reparative (senescent) phase, the lytic defects are inlaid with fibrous tissue; colloid bodies of the defects tend to be entrapped in the fibrous tissue. Duplications of basement membrane material provide a marker for the sequential nature of the inlay of fibrous tissue.

S3C18VA2-2: In this drawing, the stages of the lichenoid reaction as encountered in the setting of lichen striatus are illustrated. It is a simple matter to accommodate the follicular changes by simply transposing the reactions from the epidermal domain to the follicular domain. For the sweat gland component, the concept of a primary lichenoid reaction is most appropriate.

Lichenoid Reaction

(appendageal patterns)

1. lichen plano-pilaris

a. concentration of lymphocytes in perifollicular connective tissue sheaths

b. transgression of follicular basement membranes by infiltrates of lymphoid cells

S3C18VA2-3: The reaction in lichen plano-pilaris is a common expression of a lichenoid reaction. It does not define a specific disease; it is a common pathway.

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