CD40L, also known as gp39, tumor necrosis factor-related activation protein (TRAP), or T cell-B cell activating molecule (T-BAM), is a 33-kDa type II transmembrane glycoprotein consisting of 266 amino acids. It has a 214-amino acid extracellular domain that shares a high degree of homology with tumor necrosis factor (TNF) at the amino acid level. The primary function of CD40L is to participate in immune responses by binding to CD40, a molecule widely expressed on immune cells. The CD40-CD40L interaction activates various signaling pathways, regulating a broad range of cellular responses, including proliferation, differentiation, and cell death. Additionally, CD40L expression is typically inducible and primarily found on activated CD4+ T lymphocytes, with partial expression on activated CD8+ T cells, basophils, mast cells, and NK cells.