IL17A and inflammatory bowel disease: The antibody ustekinumab, which acts against p40 (the common subunit of IL-23 and IL-12), may be useful in IBD because of its role in blocking the differentiation of naïve T cells to Th1 and Th17 cells; however, other anticytokine reagents show little effect or make disease worse (e.g., secukinumab: anti IL-17A antibody), implying individual cytokines may have proinflammatory and anti-inflammatory effects (128).