How does the immune system discriminate between good or bad pathogens? It doesn’t, evolution does.
Immunology, the study of the immune system, is highly complex and understanding its responses to pathogens or cancer is challenging by any stretch of the imagination. To understand how an immune response comes about, we should start with the beginning; the evolution of our species and how we have leveraged the immune system to keep us free of disease.
Sjoerd's thoughts on academic vaccine research 