And once it binds only then can it begin some type of defensive mechanism as we'll see in just a moment. So the three types of antigen presenting cells that we're going to focus on in this lecture are macrophages dendritic cells as well as B lymphocytes, the cells of our humoral immunity system. So let's begin with macrophages. So let's suppose we have some type of infection taking place, let's say a bacterial cell infects our tissue. Now as soon as that infection takes place the innate immune system will try to prevent that infection from spreading. It will try to localize that infection via the process of inflammation.