Inflammatory lesions can occur due to infectious or non-infectious causes. Compared to tissue cells, inflammatory cells are readily collected by aspirates and scrapings. Inflammation is characterized by the predominant cell type which helps to predict the underlying etiology.

Neutrophils indicate suppurative or purulent inflammation. Non-infectious causes include immune-mediated disorders, chemical irritation, neoplasia, and acute trauma. Infectious causes include most bacterial infections and some fungal organisms. Degenerative neutrophils (swollen, karyolitic nuclei) are commonly associated with the toxic environment induced by bacterial infections however degenerative changes also occur as cells become exposed on mucosal or external surfaces.

Macrophages may be seen in mixed, granulomatous, and pyogranulomatous inflammatory processes. Causes include yeast and fungal agents, foreign body reactions, certain bacteria (Mycobacteria spp.), hemorrhage, and chronic inflammatory conditions. Epithelioid macrophages have a wispy to epithelial appearance. These, and multinucleated cells, and are typical of granulomas making granulomatous inflammation difficult to distinguish from histiocytic neoplasia.

Eosinophils are associated with tissue parasites, neoplasia (lymphoma, mast cell tumor, melanoma, others), fungal organisms, protozoal organisms, eosinophilic granulomas, collagen necrosis, and hypereosinophilic syndrome.

Mast cells in low numbers are often seen in inflammatory lesions.

Infectious inflammatory conditions

Bacterial agents may be found in abscesses, contaminated surgical sites, and secondary to numerous underlying metabolic, allergic, and neoplastic disorders. Specific agents that may be recognized include Actinomyces/Nocardia (filamentous), Mycobacterium (negative staining), and Yersinia (bipolar 'safety pin'). Scrutiny of diplococci has increased recently, with additional concerns over MRSAs. Bacterial infection primarily induce a suppurative response, however the filamentous bacteria and Mycobacterium sp may induce a mixed to pyogranulomatous response.

Fungal diseases can be present as either cutaneous or subcutaneous lesions. The cutaneous diseases include those induced by dermatophytes (Microsporium, Tricophyton, or Epidermophyton), Candida, and Malassezia. Dermatophytosis typically results in hairless lesions on the head, paws, ant occasionally the tail. The organism are best detected through examination of skin scrapings taken from the periphery of the lesion as they may are often seen in association with hair shafts. Dermatophytes induce a mixed inflammatory response. Cutaneous or mucocutaneous candidiasis is uncommon, but when seen is detected as septate branching hyphae that induce a pyogranulomatous to granulomatous response. Malassezia can be detected by scraping, swab imprints, or impression touch prints of the lesion. As Malassezia can be found on normal dogs and cats and dermatitis is often seen as a secondary disease (e.g. to atopy, pyoderma, hypothyroidism). Subcutaneous mycoses include sporotrichosis, histoplasmosis, cryptococcus, blastomycosis, and coccidioides (see Infectious Disease).

Although Prototheca is often mentioned with the fungi and the organisms have several morphologic features, it is actually an algae (see Infectious Disease section for detailed description). Protothecosis typically produces a strong macrophagic to granulomatous inflammation.

Leishmania is the most commonly seen protozoal organism in skin and subcutaneous lesions (see Infectious Disease section for detailed description).