Immunology and the concept of vaccination

Millions of microorganisms are encountered daily, many of which can cause disease. However, healthy individuals succumb to infection only occasionally, because disease-causing organisms (pathogens) are detected and destroyed before an infection can establish. A wide variety of cellular and secreted components are required for effective immunoprotection, due to the huge variety of pathogens. The first line of defense against foreign invasion consists of physical and chemical barriers. When these barriers are breached and pathogens gain entry into the body, the innate and adaptive immune systems provide robust, protective immune responses. However, the immune system alone is not always able to protect against invading pathogens. That's where vaccination, the most widely practiced form of immunotherapy, comes into play: it is considered one of the greatest triumphs of modern medicine. This chapter introduces the concept of immunity, explaining how vaccines can induce immune responses that protect against infectious organisms.