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CD80 （B7-1） is a cell surface glycoprotein that belongs to the B7 family of immune regulatory molecules. It is primarily expressed on antigen-presenting cells, such as dendritic cells, macrophages, and activated B cells. CD80 functions as a costimulatory molecule that interacts with CD28 and CTLA-4 receptors expressed on T cells. This interaction plays a crucial role in the regulation of T cell activation and immune responses. Specifically, CD80 binding to CD28 on T cells provides a costimulatory signal that enhances T cell proliferation, cytokine production, and survival. This interaction promotes the development of effective immune responses against pathogens. However, CD80 also binds to CTLA-4, which has higher affinity for CD80 compared to CD28. The CD80-CTLA-4 interaction inhibits T cell activation and proliferation, thus preventing excessive or autoreactive immune responses.