Immunopathology

• Diagnostic Hallmark of Canine IMHA: Spherocytes are the hallmark erythrocyte morphological abnormality pathognomonic for Immune-Mediated Hemolytic Anemia (IMHA) in dogs.
• Mechanism of Spherocyte Formation: When circulating erythrocytes are opsonized with anti-erythrocyte autoantibodies (IgG) and/or complement ($C3b$), splenic and hepatic macrophages partially phagocytose portions of the cell membrane without lysing the whole cell. The damaged RBC reduces its surface-area-to-volume ratio, transforming into a small, rigid, densely staining spherical red blood cell that lacks central pallor.
• Contrasting Erythrocyte Morphologies:
• Acanthocytes: Irregularly spaced, blunt, spurlike projections seen in hemangiosarcoma and liver disorders.
• Schistocytes: Fragmented RBCs formed by mechanical shear stress in DIC.
• Echinocytes: Uniformly scalloped, crenated spikes associated with uremia or artifactual in vitro drying.

Correct option: (4) Spherocytes

• Adaptive Antitumor Immunity: CD8+ Cytotoxic T Lymphocytes (CTLs) are the principal effector cells of the adaptive immune system that specifically recognize and eliminate tumor cells.
• Mechanism of Action: CTLs recognize foreign tumor-associated antigens (TAAs) or neoantigens presented as peptide complexes on cell surface MHC Class I molecules via their clonotypic T-cell receptors (TCRs). Upon immunological synapse formation, CTLs trigger directed exocytosis of perforin and granzymes or engage the Fas/FasL axis, driving targeted apoptotic demise of neoplastic cells.
• Innate Antitumor Effectors: Natural Killer (NK) cells and macrophages recognize stressed cells lacking MHC-I or coated with antibodies via innate germline-encoded receptors, NOT through antigen-specific adaptive immunity.

Correct option: (4) CD8+ Cytotoxic T cells

• Cutaneous Round Cell Tumors in Dogs: The canonical canine round cell tumor group comprises five classic neoplasms characterized by discrete round-to-oval cells exfoliating individually on cytology:
• Mast Cell Tumor (MCT)
• Malignant Lymphoma (Lymphosarcoma)
• Cutaneous Histiocytoma
• Transmissible Venereal Tumor (TVT)
• Plasmacytoma
• Classification of Malignant Melanoma: Melanocytic neoplasms are taxonomically classified as neuroectodermal / mesenchymal-epithelioid tumors. Histologically and cytologically, melanoma cells display marked pleomorphism—ranging from spindled to epithelioid sheets containing melanin granules—and are not classified as round cell tumors.

Correct option: (1) Malignant melanoma