Cellulitis is infection of which tissue layer?

• A. Epidermis only
• B. Dermis and subcutaneous tissue
• C. Muscle tissue
• D. Hair follicles only

Answer: (B)

Cellulitis is an infection that involves the dermis and the subcutaneous tissue, not just the surface layer of skin. When bacteria breach the epidermis and enter the dermis and fat beneath, the area becomes red, warm, swollen, and tender with a spreading border. If the infection were limited to the epidermis, it would be a surface skin infection such as dermatitis or impetigo, which does not involve the deeper tissues. Folliculitis involves hair follicles, a localized follicular infection rather than a diffuse dermal/subcutaneous process. Infections of muscle tissue, such as myositis or necrotizing infections, involve deeper structures and often have severe pain and systemic signs that differ from typical cellulitis. Thus, the tissue layers affected in cellulitis are the dermis and subcutaneous tissue.