CD27 protein is a member of the tumor necrosis factor （TNF） receptor superfamily. It is expressed on the surface of various immune cells such as T cells and B cells. CD27 can interact with its ligand CD70, which is mainly found on antigen - presenting cells （APCs） and activated T cells. This interaction plays a crucial role in the regulation of immune responses. It can promote T cell proliferation and differentiation, enhance the cytotoxic activity of T cells against infected or cancerous cells, and also participate in the regulation of B cell functions such as immunoglobulin production. Moreover, the CD27 - CD70 pathway is involved in the development and maintenance of immune memory, helping the immune system to remember and respond more effectively to pathogens it has previously encountered. Abnormal expression or function of CD27 has been implicated in certain immune - related diseases and cancers, making it a potential target for therapeutic interventions in these conditions.