Allele
One of the alternative versions of a gene a person can carry — the unit of human-to-human genetic variation.
A gene sits at a fixed locus, but the exact sequence there varies between people; each distinct version is an allele. You inherit one copy from each parent, so you can be homozygous (two identical alleles) or heterozygous (two different ones). Think of the locus as a key and the allele as the specific value stored there.
Alleles matter enormously here because the HLA genes are the most polymorphic in the genome — there are thousands of known HLA alleles (e.g. HLA-A*02:01), and each binds a different repertoire of peptides. A patient's particular set of HLA alleles is exactly what makes neoantigen selection personal: the same mutation yields a usable target in one allele's binding groove and nothing in another's.