So our immune system uses these foreign antigens to basically stimulate these internal defense mechanisms that ultimately catch these pathogens and destroy these pathogens. So, to see what we mean, let's take a look at the following diagram. So these are two human cells. This is a pathogen, for example, a bacterial cell. Notice that both of these cells actually contain these membrane bound macromolecules. For example, let's suppose they're lipoproteins.