We now understand that humans exist as a combination of host cells and a vast consortium of bacteria, viruses and fungi, called the microbiota, that overwhelm the host both in terms of cell number and genetic information. The adaptive immune system has evolved alongside the microbiota and the cardinal feature of adaptive immunity, immune memory, may be an effort to ‘remember’ previous responses and shape subsequent host – microbial interactions. Maintaining ‘friendly’ relations with the microbiota is a particular problem for the immune system because of the huge number of bacteria present that can be inflammatory depending upon the context within which they are experienced.
