Acanthosis involves-
Acanthosis is diffuse epidermal hyperplasia (thickening of the skin). It implies increased thickness of the Malpighian layer (stratum basale and stratum spinosum). Acanthosis nigricans is often associated with type 2 diabetes, and is usually observed in the back of neck, axilla, and other folded regions of the skin. Ref Harrison20th edition pg 1206