Rat CD27 is a type I transmembrane glycoprotein belonging to the tumor necrosis factor receptor (TNFR) superfamily, which also includes receptors such as CD40, OX40 (CD134), 4-1BB (CD137), and CD30 that regulate lymphocyte activation, survival, and differentiation. In rats (Rattus norvegicus), CD27 is primarily expressed on T lymphocytes, subsets of B cells, and natural killer (NK) cells, where it functions as an important costimulatory receptor in adaptive immune responses. CD27 interacts with its ligand CD70 (TNFSF7), which is expressed on activated antigen-presenting cells and lymphocytes, triggering intracellular signaling pathways such as NF-κB and MAPK that promote lymphocyte activation, proliferation, survival, and differentiation into effector and memory cells. In rat immunology, CD27 plays a key role in T cell costimulation, regulation of immune activation, and formation of long-lived immune memory, contributing to host defense against viral, bacterial, and parasitic infections. Because rats are widely used as experimental models for immunology, autoimmune disease, and transplantation research, rat CD27 has been studied in models of immune regulation, inflammatory disease, and T-cell–mediated immunity, helping to clarify mechanisms of costimulatory signaling and adaptive immune responses relevant to both veterinary and human biomedical research.