HLA-B18 (B18) is an HLA-B serotype. The serotype identifies the more common HLA-B*18 gene products. (For terminology help see: HLA-serotype tutorial) B*1801, the most common allele is at highest frequencies in Northern Italy and the Balkans, a peak frequency distribution it shares with B*3501.
References
HLA-B18 Wikipedia(Text) CC BY-SA