TCRs of CD8 cells recognize antigenic peptides that are complexed to MHC class I antigens, and TCR of CD4 cells recognize antigen bound to MHC class II on the surface of antigen-presenting cells. Cell surface expression of MHC class I molecules fails if either of the two transporters of antigenic peptides (TAP1 and TAP2) is lacking. TAP1 and TAP2 help to transfer peptides from the cytosol into the endoplasmic reticulum, for subsequent loading onto newly synthesized MHC class I molecules. In the absence of peptide loading, MHC class I molecules are degraded before reaching the cell surface. In the absence of MHC class I antigen expression, CD8 cell function is deficient, and these cells are not generated within the thymus, The resulting immunodeficiency is milder than SCID and often presents in later life. Paradoxically, viral infections are not a problem in these patients. Some MHC class I deficient patients develop progrssive bronchiectasis, while others develop vasculitis affecting the face and upper respiratory tract. It has been postulated that vasculitis seen in these patients may be due to self-destruction of vascular endothelial cells by the unrestrained cytotoxicity of NK cells.