Bovine CD27 is a type I transmembrane glycoprotein that belongs to the tumor necrosis factor receptor (TNFR) superfamily, which includes related receptors such as CD40, OX40 (CD134), 4-1BB (CD137), and CD30 that regulate lymphocyte activation and survival. In cattle (Bos taurus), CD27 is primarily expressed on T lymphocytes, memory B cells, and subsets of natural killer (NK) cells, where it functions as an important costimulatory receptor in adaptive immune responses. CD27 interacts with its ligand CD70 (TNFSF7) expressed on activated antigen-presenting cells and lymphocytes, triggering intracellular signaling pathways including NF-κB and MAPK that promote lymphocyte activation, proliferation, survival, and differentiation into memory cells. In bovine immunology, CD27 plays a key role in T cell costimulation and the development and maintenance of memory lymphocyte populations, contributing to effective immune responses against viral, bacterial, and parasitic pathogens affecting cattle. Because the CD27–CD70 signaling pathway is critical for adaptive immune regulation and long-term immune memory, bovine CD27 is studied in veterinary immunology, vaccine development, and host–pathogen interaction research, particularly for improving protective immunity and vaccine responses in cattle.