Fifty Shades of Immune Defense

In their struggle to survive and thrive, all living things must defend themselves from predatory attack. Microbes, in the form of parasites, bacteria, fungi, and viruses, are life's most accomplished predators. Therefore, all living things have evolved mechanisms to defend against them. Historically, biological defense systems have been classified into two broad categories-innate systems that provide nonspecific defense against invading pathogens and adaptive systems that provide long-lasting defense against attack by specific pathogens. Recently, a growing body of literature in comparative immunology has indicated that these categories may not be as distinct as was originally believed. Instead, a variety of immune mechanisms that share properties of both innate and adaptive systems have been recently elucidated. Here, we summarize five key facts about the newly appreciated shades of grey between innate and adaptive defense systems.