Complex sentence: By definition an immune system is specific to a single antigen which creates it, however, many naturally occurring'antigens' are a mixture of macromolecules (e.g. bacteria, toxins, proteins, pollen, fungi, viruses, etc) which contain several epitopes.
Split into simpler sentences: By definition an immune system is specific to a single antigen which creates it. However, many naturally occurring'antigens' are a mixture of macromolecules (e.g. bacteria, toxins, proteins, pollen, fungi, viruses, etc) which contain several epitopes.