What is a true statement regarding HLA-A20?

The assertion that HLA-A20 is not a component of the official World Health Organization (WHO) nomenclature is valid. The WHO has established a standardized nomenclature system to provide consistent naming conventions for human leukocyte antigens (HLAs). HLA-A20, like other alleles that are formally recognized, typically adheres to these nomenclature rules. However, some less commonly studied or rare alleles might not be included in the WHO's official list, which can lead to confusion regarding their classification.

Understanding the context of HLA-A20 in relation to the other statements can highlight its significance. For instance, while HLA-A20 may indeed be rare in certain populations, particularly among Caucasians, this does not necessarily pertain to its nomenclature status. Similarly, the concepts around null alleles or shared epitopes tend to relate to more specialized contexts in immunology and may not directly affect how HLA-A20 is categorized under the nomenclature system. Recognizing the nomenclature is crucial for clear communication and research in the histocompatibility field.