Several subtype of HLA class I deficiency have been described. Symptomatic transporter associated with antigen deficiency occurs when the patients have a very large reduction in the cell-surface expression levels of HLA class I molecules. They display recurrent bacterial infections of the respiratory tract and about half of them also have granulomatous skin lesions. Their HLA genotype is homozygous, and their parents are usually first cousins. The absence of severe viral infection despite the defective presentation of viral antigens to CD8 T cells may be explained, at least in part, by different factors:
- Normal humoral responses
- Presence of significant, although frequently reduced, numbers of T cell receptor-αβ
- Recognition by certain T cell receptor-αβ CD8 T cells of transporter associated with antigen-independent viral antigens
- Expansion of T cell receptor-γδ
- Presence of natural killer
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