IL21 and viral infectious disease: However, there are reports that IL-21 is unable to induce the expression of Bcl-6 [17], a transcription factor critical for TFH cell differentiation, and Il21−/− and Il21r−/− mice have normal or near normal levels of TFH cells following viral infection [18], or immunization with sheep red blood cells, ovalbumin, or 4(hydroxy-3-nitrophenyl)acetyl coupled to keyhole limpet hemocyanin [19], [20], [21].