The invention provides methods for modulating cytokine levels, GM-CSF levels and the immune system using CD83 nucleic acids, CD83 polypeptides, anti-CD83 antibodies and factors that influence CD83 activity or expression. The invention also provides mice having a mutant CD83 gene and mice having a transgenic CD83 gene, which are useful for defining the role of CD83 in the immune system and for identifying compounds that can modulate CD83 and the immune system.