What histologic finding best differentiates inflammatory bowel disease from small cell lymphoma in feline intestinal tissue?

• A. IBD shows heavy monomorphous CD3+ infiltration; LSA shows lymphoplasmacytic infiltrate
• B. IBD shows granulomatous infiltrate; LSA shows neutrophilic infiltration
• C. IBD shows neutrophilic infiltrate; LSA shows lymphoplasmacytic infiltrate
• D. IBD demonstrates lymphoplasmacytic infiltrate with mixed CD3/CD21-CD79a positivity; LSA demonstrates heavy monomorphous CD3+ infiltration with elevated intraepithelial lymphocytes and villous blunting

Answer: (D)

Distinguishing IBD from small cell lymphoma in feline intestine relies on both the cellular makeup of the infiltrate and the immunophenotype of the cells. Inflammatory bowel disease is a chronic inflammatory process that characteristically shows a lymphoplasmacytic infiltrate, meaning a mix of T cells, B cells, and plasma cells. Immunostaining reflects this diversity, with a combination of CD3-positive T cells and B-cell markers (such as CD79a) present in the infiltrate. The tissue architecture can be relatively preserved apart from inflammatory changes, and you don’t see a uniform population of small, mature lymphocytes.

In contrast, small cell lymphoma is a neoplastic process with a monomorphic population of small, mature lymphocytes that are predominantly T cells. Immunohistochemistry typically shows strong CD3 positivity with a homogeneous infiltrate, and the lymphoma can cause mucosal architectural disruption, including increased intraepithelial lymphocytes and villous blunting due to lymphomatous invasion of the mucosa. The presence of a heavy monomorphous CD3+ infiltrate, rather than a mixed inflammatory infiltrate, is the key differentiator.

So the best differentiating finding is the combination of a lymphoplasmacytic infiltrate with mixed T and B cell immunophenotypes in IBD versus a dense, monomorphic CD3+ T-cell population with mucosal architectural changes in small cell lymphoma.