Human anti-human IL-21 monoclonal antibodies and the hybridomas that produce them are presented. Certain of these antibodies have the ability to bind native human IL-21, a mutant recombinat IL-21 protein and/or peptide regions of human IL-21. These human anti-IL-21 antibodies are useful in therapeutic treatment of autoimmune and inflammatory diseases, particularly diseases mediated by T follicular helper cells, B cells TH cells or TH17 cells.