Microbes that slip past the innate (non-adaptive) immune response contend with the adaptive immune response, a tailor-made, specific response to individual pathogens. There are two arms of adaptive immunity—humoral immunity and cell-mediated immunity, which are carried out by B cells and T cells. B cells and T cells that are activated during an infection can remain in circulation for years, ready to spring into action if the pathogen is encountered again. It is amazing that a person’s immune system can recognize and respond to virtually any molecular structure, yet avoid responding to self antigens. This chapter explains how millions of different antibodies can be generated by a finite genome, how the humoral and cell-mediated pathways interact, how the immune system avoids recognizing self antigens and how serious diseases can occur when these processes go awry. Right now, your immune system is helping to keep you alive.