Now, such recognition is actually possible because the cells of our body contain special unique macromolecules. For example, proteins or lipoproteins that are found on the membrane of the cells of the body. And our immune system can use these unique biological molecules on the membrane to recognize its own cells. 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.