CD43, also known as leukosialin, is a sialomucin transmembrane molecule that is highly expressed on all leukocytes except most resting B lymphocytes. It plays a multifaceted role in the immune system, including functions related to adhesion, anti-adhesion, movement, cell activation, differentiation, proliferation, and apoptosis. CD43 is involved in mediating repulsion among leukocytes, thus functioning as an anti-adhesion molecule. It has also been implicated in the activation of T cells and is capable of binding to ICAM-1, which suggests its role in cell-cell interactions. Additionally, CD43 is constitutively phosphorylated in T lymphocytes and monocytes, and it becomes hyperphosphorylated upon activation, indicating its involvement in cellular signaling. The molecule has been shown to promote cell growth in cooperation with β-catenin in non-hematopoietic cancer cells, highlighting its potential role in cancer progression. Furthermore, CD43's abnormal expression has been associated with poor prognosis in non-hematopoietic malignancies, suggesting its contribution to tumorigenesis and tumor microenvironment remodeling. In summary, CD43 is a critical molecule in leukocyte biology with implications in immune cell function and cancer development.