M1 is a small molecule of sialic acid derivative, which can be selectively expressed on the glycan end of the surface of tumor cell membrane, and is recognized and phagocytized by macrophages expressing CD169. M1-modified tumor cells are equivalent to being added with a "death" tag that attracts CD169+ macrophages to aggregate and phagocytose. Macrophages phagocytose tumor cells and present tumor-associated antigens to CD4+/CD8+ T cells, activate immune responses in vivo, and clear tumor cells