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...