How do they do tissue typing for bone marrow if they just swab your cheek?

DNA methodology. Tissue typing is based on hla (human leukocyte antigen) typing originally performed by serologic methods using different known sera to be mixed with white cells (lymphocytes) obtained from the blood. Today, using dna sequencing or PCR (polymerase chain reaction) technology, any tissue in the body (easiest to get is buccal mucosa cells in saliva) could be used as source of dna for study.