The best answer would be the integumentary system. Cardiovascular is only responsible for circulating blood throughout the body, but there’s no shielding or filtrating process used for protection. The nervous system might help by triggering the lymphatic system to send white blood cells out, but that’s after the antigen is already in the body. The respiratory is just responsible for switching CO2 for O2, but doesn’t actively filter antigens. The integumentary system (or the skin) includes the mucous membranes, cilia, hairs, and microvilli that line the nasal and respiratory canals and actively filter the air as it travels into the lungs. The integumentary system also includes the thin layer of bacteria that lines the epidermis and acts as a first line of defense for any antigens.
