Now, on the other side of things, if we examine a pathogen, for example, a bacterial cell, bacterial cells also contain their own unique biological molecules, macromolecules on the membrane. And our immune system can ultimately use these foreign macromolecules to stimulate the immune system to initiate a response that ultimately destroys and kills off that pathogen. And in general, anytime we have a substance such as the macromolecule found in the membrane of the bacterial cell that stimulates any type of immune response, this substance is known as an antigen. 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.