The monoclonal antibody Kat5-18 recognizes T-cell immunoglobulin and mucin domain containing molecule (TIMD-4, also called TIM-4). TIMD-4 is a type I transmembrane protein belonging to the immunoglobulin superfamily, containing one Ig-like V-type (immunoglobulin-like) domain. TIMD-4 is expressed in Mac1+ cells in spleen, lymph nodes and fetal liver. TIMD-4 binds apoptotic cells by recognizing phosphatidylserine via its immunoglobulin domain. TIMD-4 in fibroblasts enhanced their ability to engulf apoptotic cells. TIMD-4 is involved in regulating T-cell proliferation and lymphotoxin signaling. TIMD-4 and TIMD-1 are phosphatidylserine receptors for the engulfment of apoptotic cells, and are also involved in intracellular signaling in which exosomes are involved.The monoclonal antibody Kat5-18 blocks the engulfment of apoptotic cells by thymic macrophages.