CD154, also known as CD40 ligand (CD40L), is a type II transmembrane protein belonging to the tumor necrosis factor (TNF) superfamily. It is predominantly expressed on activated CD4+ T cells, but can also be found on other immune cells such as monocytes, natural killer cells, and platelets. CD154 interacts with CD40, a receptor expressed on antigen-presenting cells (APCs), B cells, and other immune cells, and is crucial for T cell activation, B cell proliferation and differentiation, and the production of various cytokines. This interaction is essential for the adaptive immune response, including B cell class switching, germinal center formation, and the generation of memory B cells. Mutations in the CD154 gene can lead to X-linked hyper-IgM syndrome, an immunodeficiency characterized by a lack of IgG, IgA, and IgE antibodies. Additionally, CD154 is involved in various inflammatory processes and has been implicated in conditions such as atherosclerosis.