Alarm reaction of pilosebaceous apparatus

A series of histologic changes involving the pilosebaceous structures are described under the term "alarm reaction." The follicular manifestation of this phenomenon is characterized by regression of hair matrix, forming a well-defined nest of small basaloid cells found in various parts of the follicular canal. The sebaceous glands either become undifferentiated, forming a solid mass of stratified squamous epithelium, or the sebaceous lobules are taken into the follicular canal and into the surface keratin layer.