The Phenotype List String Grammar for Enhanced Protein and Antigen Reporting in the Immunogenetic Context

Standardised data representation is fundamental to the integrity, interoperability and reproducibility of immunogenetics and histocompatibility testing. While well-established standards such as the genotype list (GL) String, histoimmunogenetics markup language (HML) and minimum information for reporting next-generation sequence genotyping exist for encoding HLA genotyping results, no equivalent syntax currently exists for describing phenotype- or antigen-level information that underlies antibody testing and immunological risk assessment. We introduce the phenotype list (PL) String grammar and the phenotype list string code (PLSC) syntax, structured, machine-readable grammars designed to represent antigen- and protein-level data across three contexts: (1) the HLA phenotype composition of assay reagents, (2) test results derived from those reagents and (3) clinical interpretations of HLA antibody specificities. PL String extends the hierarchical logic of the GL String grammar, while adapting a subset of GL String delimiters and rules to describe ambiguity, heterodimeric relationships and phenotypic composition. PL String incorporates explicit namespace definitions (e.g., World Health Organization HLA Nomenclature Committee for Factors of the HLA System, Organ Procurement and Transplantation Network, Eurotransplant and NMDP) to ensure traceability and compatibility with existing regulatory and operational frameworks. By harmonising how HLA phenotypic and antibody data are encoded, PL Strings enable automated data exchange between laboratories, registries and information systems, reducing transcription errors and improving computational interpretation. This new grammar establishes a foundation for interoperable representation of serologic and functional HLA data, facilitating reproducible research and enhancing clinical decision support in transplantation and immunogenetics. Technical specifications for PL Strings and PLSCs are available at plstring.org.