Great biological significance of the HLA complex and its impact on practical medicine impels researchers to study it, the result of which is an annual expansion of our knowledge on the system. The polymorphism of the HLA complex has increased--altogether 664 alleles are officially recognised. The genetic defect leading to the bare lymphocyte syndrome was elucidated, too--the mutations in genes coding transcription factors RFX5 and CIITA are responsible for. Also the role of HLA-DM antigens in the exogenous pathway of antigen presentation was elucidated. They are principle molecules which dislodge CLIP from the groove of HLA class II molecules leaving it free to accommodate more suitable immunogenic peptide. It was also discovered that HLA class I antigens are target structures for NK-cells. NK-cell receptors recognising them transduce negative signals switching their cytotoxic activity off. (Fig. 8, Tab. 1, Ref. 34.)