Methods are disclosed herein for specifically inducing proliferation of CD4+ T cells. The methods are of use in treating immunodeficiencies, such as an immunodeficiency produced by infection with an immunodeficiency virus, such as infection with a human immunodeficiency virus (HIV). The methods include contacting isolated mammalian CD4+ T cells with an effective amount of a thymic stromal derived lymphopoietin (TSLP) polypeptide or a therapeutically effective amount of nucleic acid encoding the TSLP polypeptide, thereby inducing proliferation of the T cells. Methods are also disclosed for treating an IgE mediated disorder, such as asthma. The methods include administering to a subject a therapeutically effective amount of a TSLP antagonist. Transgenic mice are also disclosed herein. The somatic and germ cells of these mice include a disrupted thymic stromal lymphopoietin receptor (TSLP) gene, the disruption being sufficient to inhibit the interaction of TSLP with its receptor, and a disrupted γc gene, the disruption being sufficient to reduce signaling through the γc. The mice exhibit diminished thymic cellularity. Methods of using these mice for drug screening are also disclosed.