Daclizumab is a humanized monoclonal antibody that selectively binds to the alpha subunit of the interleukin-2 receptor (IL-2Rα, also known as CD25) on the surface of activated T cells. Its primary application lies in modulating the immune system by inhibiting the IL-2 signaling pathway, which plays a critical role in T-cell proliferation and activation. It has been used clinically to prevent organ transplant rejection, particularly in kidney transplantation, by suppressing the immune response against the transplanted tissue. By targeting CD25, daclizumab reduces the risk of acute rejection episodes without the broad immunosuppressive effects associated with other agents. Additionally, daclizumab was approved for the treatment of relapsing forms of multiple sclerosis (MS). In this context, it helps reduce the frequency of relapses by modulating immune cell activity in the central nervous system. It selectively targets overactive immune cells believed to contribute to neuroinflammation and demyelination in MS patients. Due to safety concerns, including severe immune-mediated adverse events, its use has been discontinued in many markets. However, it remains an important reference antibody in research settings for studying T-regulatory cells, immune activation, and IL-2 receptor biology.