The primary mechanism of action for bi-specific antibodies in cancer therapy is to recruit immune cells to the vicinity of cancer cells. One arm of the bi-specific antibody binds to a specific antigen on the cancer cell, while the other arm binds to an antigen on an immune cell, such as a T cell. This dual binding can lead to the activation of the immune cells, which then attack and destroy the cancer cells. This approach leverages the body's own immune system to fight cancer more effectively.