Transplant physicians can select units with higher cell counts instead of a closer HLA match to increase the chances of a successful engraftment. However, some degree of HLA compatibility is necessary, although the minimum acceptable level is somewhat controversial (see also Appendix G). Most cord blood transplants are done with units mismatched for a maximum of two HLA antigens (HLA-A and HLA-B) using low-resolution typing techniques and for DRB1 antigens by using high-resolution typing techniques.
The detection of a suitable cord blood unit for a given patient is difficult. First, significant differences in HLA types exist among various ethnic populations. For example, HLA-Bw53 is found in 5.4 percent of the haplotypes in African Americans but only 0.05 percent of the haplotypes in Caucasians (Margolis and Casper, 2000). Because of the number of possible HLA combinations, which are enhanced by the differences inherent in the various ethnic populations, the likelihood of finding an exact match is extremely small.
HLA-types have traditionally been identified with serologic typing (also called “low/intermediate resolution” typing) methods. Sequencing (one of the many methods referred to as “high resolution typing”) of HLA genes revealed that each of the serologically defined HLA antigens represent many different protein structures. Thus, a unit typed at the low resolution level