Let's suppose we have the surface of our skin as shown in the diagram. Then we have the skin tissue and below that we have our blood vessel that carries our blood plasma from this side to this side of our diagram. Now let's imagine that we have some type of cut that is formed on the surface of our skin as shown in this diagram. And let's imagine that in close proximity to our cut we have some type of pathogen. Now the pathogen could be some type of parasite. It could be some type of viral agent.