And once it engulfs the antigen it takes that antigen and it places it onto a special protein complex found on the membrane of the antigen presenting cell. And this protein complex is known as the major histocompatibility complex class Two or simply a MHC Class Two. And only when the antigen is bound to this MHC Class Two complex can the lymphocyte actually see and interact with the antigen and bind onto this antigen. 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.