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Immune responses in MHC class II-deficient mice.

Major histocompatibility complex (MHC) class II molecules are heterodimeric cell surface proteins that are critically important for the development and function of cells in the immune system. In particular, the maturation of CD4+ T cells is dependent on the expression of MHC class II molecules on thymic epithelium, while the activation of these cells requires the expression of class II molecules on specialized antigen-presenting cells in the periphery. The importance of class II molecules is especially evident in humans who are afflicted with MHC class II-deficient combined immunodeficiency, as these individuals die at an early age unless provided with a bone marrow transplant. Here we discuss the functional consequences of MHC class II deficiency in a mouse model generated by gene targeting in embryonic stem (ES) cells. These mice have proved to be valuable reagents for dissecting the mechanisms by which MHC class II molecules control the maturation and activation of lymphocytes as well as for elucidating the role of these cells in various immune responses.