Abstract: A transgenic mouse is disclosed herein whose somatic and germ cells comprise a disrupted IL-21 receptor gene, the disruption being sufficient to inhibit the binding of IL-21 to an IL-21 receptor, and a disrupted IL-4 gene, the disruption being sufficient to inhibit the production of IL-4 or the binding of IL-4 to the IL-4 receptor. A mouse homozygous for the disrupted IL-21 receptor gene and homozygous for the disrupted IL-4 gene has diminished B cell function. A method is disclosed for altering a B cell activity. The method includes administering a therapeutically effective amount of an agent that interferes with the interaction of IL-21 with an IL-21 receptor, thereby altering the B cell activity. A method is also disclosed for of treating a subject with Job's disorder or atopic disease. A method is also disclosed for treating or preventing an allergic reaction in a subject. A method is also disclosed for treating a subject with an autoimmune or antibody mediated disorder.