DNA-based testing for HLA alleles has now completely replaced serological phenotyping and antibody detection/characterisation involves non-cell-based methods. This HLA section is constructed of three main parts, concerning reagents (section 16.4), testing (sections 16.5, 16.6 and 16.7), and application to donor and patient investigations (section 16.8). For certain patient or donor investigations there is, of course, an overlap with the guidance given in this and the granulocyte/platelet immunology chapters (Chapters 17 and 18 respectively). This is particularly relevant to the laboratory investigations of platelet refractoriness and transfusion-related acute lung injury (TRALI), so diagrams are included (Figures 16.1 and 16.2) to indicate how the different guidelines relate to each other.