Final answer:
A vehicle that helps to spread agents onto the skin is a nanoparticle. Nanoparticles can be administered subcutaneously or through direct injection into the blood, allowing them to interact with the skin and deliver agents such as vaccines or drugs.
Step-by-step explanation:
A vehicle that helps to spread agents onto the skin is a nanoparticle. Nanoparticles can be administered subcutaneously or through direct injection into the blood, allowing them to interact with the skin and deliver agents such as vaccines or drugs. For example, small nanoparticles can be transported into the lymphatic system to interact with dendritic cells in the lymph nodes, while larger nanoparticles stay in the skin and interact with dendritic cells or monocytes available in the skin.