Sweet Syndrome Histology

• Infiltrate Location: Features a dense, diffuse, and nodular infiltrate of mature, neutrophils strictly localized to the upper and mid-dermis. 
• Hallmark Feature: Characterized by prominent papillary dermal edema (which gives the lesions a swollen, "juicy" clinical appearance). 
• Vasculitis Status: True leukocytoclastic vasculitis (with frank fibrinoid necrosis of the vessel wall) is absent, though neutrophils are often seen clustered around and exiting vessels. 
• Micro-abscesses: Subcorneal or intraepidermal pustules may be present. 

Cellulitis Histology

• Infiltrate Location: Features a diffuse, sparse to moderate inflammatory infiltrate that primarily involves the deeper dermis and subcutaneous fat (panniculitis). 
• Cell Types: The infiltrate is often mixed. While neutrophils are present, it characteristically includes lymphocytes, macrophages, and sometimes mast cells. 
• Hallmark Feature: Shows signs of acute inflammatory edema spreading diffusely between collagen bundles in the deep dermis. 
• Infectious Evidence: Bacterial colonies or structures may be visualized (though often not seen on routine H&E), whereas Sweet syndrome lacks any pathogenic organisms. 

Summary of Key Differences

• Depth: Sweet syndrome affects the upper dermis, whereas cellulitis invades the deep dermis and subcutis.
• Edema Type: Sweet syndrome presents with marked papillary dermal edema, while cellulitis presents with widespread diffuse interstitial edema.
• Bacteria: Cellulitis has a bacterial origin (though sometimes subclinical), while Sweet syndrome is an autoinflammatory, sterile neutrophilic dermatosis.