The lymphatic system is the anatomical scaffold of the adaptive immune response. Understanding lymph node architecture, lymphocyte trafficking, and the molecular basis of lymphoma is foundational to veterinary internal medicine and oncology.
Lymph nodes are highly organized secondary lymphoid organs that serve as sites of antigen presentation and adaptive immune response initiation. Each lymph node is divided into three functional zones:
Cortex (B cell zone): Contains primary follicles (naive B cells) and secondary follicles with germinal centers (activated B cells undergoing somatic hypermutation and affinity maturation). Germinal center formation is the hallmark of a T cell-dependent humoral immune response.
Paracortex (T cell zone): Contains T lymphocytes and dendritic cells (antigen-presenting cells). Naive T cells enter the paracortex via high endothelial venules (HEVs) and are activated by antigen-presenting dendritic cells. This is the site of T cell-dependent immune responses.
Medulla: Contains medullary cords (plasma cells secreting antibody) and medullary sinuses (macrophages filtering lymph). Efferent lymph exits via the medullary sinuses and returns to circulation via the thoracic duct.
Lymphocyte trafficking is regulated by chemokines and adhesion molecules. Naive lymphocytes express L-selectin (CD62L) and CCR7, which mediate binding to peripheral node addressin (PNAd) on HEVs and migration into lymph nodes. Activated effector lymphocytes downregulate L-selectin and CCR7, upregulate tissue-homing receptors (e.g., alpha4beta7 for gut homing, CLA for skin homing), and migrate to sites of inflammation rather than secondary lymphoid organs.
Reactive lymphadenopathy (lymphadenitis) results from antigen-driven lymphocyte proliferation and macrophage activation within the lymph node. Histologically, reactive nodes show follicular hyperplasia (expanded germinal centers), paracortical hyperplasia (expanded T cell zones), and sinus histiocytosis (macrophage accumulation in sinuses). The node enlarges due to cellular proliferation and increased lymph flow from the site of inflammation.
Lymphoma is a clonal neoplasm of lymphocytes that has escaped normal regulatory mechanisms. In dogs, lymphoma is one of the most common malignancies, accounting for approximately 7-24% of all canine cancers. The molecular pathogenesis involves:
Oncogene activation: Translocation or amplification of proto-oncogenes (e.g., c-myc, bcl-2) that promote cell survival and proliferation. In human Burkitt lymphoma, t(8;14) translocation places c-myc under immunoglobulin heavy chain promoter control -- analogous translocations occur in canine lymphoma.
Tumor suppressor loss: Loss of p53 function (TP53 mutations) removes a critical checkpoint for DNA damage response, allowing accumulation of additional mutations. p53 mutations are found in approximately 20-30% of canine lymphomas.
Clonal expansion: A single transformed lymphocyte undergoes clonal expansion, producing a population of genetically identical neoplastic cells that progressively replace normal lymph node architecture.
Canine lymphoma is classified by immunophenotype (B cell vs. T cell) because this has significant prognostic implications:
B cell lymphoma (approximately 70-80% of canine cases): Typically multicentric distribution. Responds well to CHOP-based chemotherapy. Median survival with treatment: 12-14 months. Immunohistochemistry: CD20+, CD79a+, PAX5+.
T cell lymphoma (approximately 20-30% of canine cases): More variable presentation. Generally poorer prognosis than B cell. Median survival with treatment: 6-9 months. Immunohistochemistry: CD3+, CD4+ or CD8+.
PARR (PCR for Antigen Receptor Rearrangements) can detect clonal lymphocyte populations in cytology samples, providing molecular confirmation of lymphoma and immunophenotyping when histopathology is not available.
Feline lymphoma differs significantly from canine lymphoma in distribution and etiology. The most common form in cats is gastrointestinal (GI) lymphoma, which is further divided into low-grade (small cell) and high-grade (large cell) forms. Low-grade GI lymphoma is the most common feline GI neoplasm and has a favorable prognosis with chlorambucil and prednisolone treatment (median survival 2-3 years). Feline leukemia virus (FeLV) is a significant risk factor for lymphoma in cats, particularly mediastinal lymphoma in young cats.