Follicular Homing T Helper (Th) Cells and the Th1/Th2 Paradigm

The Th1/Th2 paradigm is a cornerstone for our understanding of T cell responses 1. It conveniently subdivides T cell immune responses into those specialized for defense against intracellular pathogens including viruses and some bacteria (Th1), and a second for defense against large extracellular pathogens such as helminths. Th1 responses depend on IL-12 and IFN-γ to mediate a range of biological effects designed for antiviral immunity. In contrast, Th2 responses employ the cytokines IL-5, IL-4, and IL-13, which promote the mobilization of eosinophils and cause other inflammatory processes designed to expel large parasites. The Th1/Th2 paradigm also has particular relevance for certain inflammatory diseases, as asthma is essentially a Th2 response gone awry, and many autoimmune diseases depend on a Th1 response to autoantigens. However, this neat scheme had one troubling aspect: where do helper cells for antibody production...