JAK3, or Janus kinase 3, is an enzyme that plays a crucial role in the signaling pathways of the immune system. It is part of the Janus kinase family, which helps transmit signals from various cytokine receptors, influencing cell growth, survival, and differentiation. JAK3 is particularly important for the function of T cells and B cells, which are essential components of the adaptive immune response. Mutations or dysfunctions in JAK3 can lead to immune-related disorders, such as severe combined immunodeficiency (SCID). This condition results in a weakened immune system, making individuals more susceptible to infections. Targeting JAK3 with specific inhibitors is also being explored as a therapeutic approach for autoimmune diseases and certain types of cancer.