In blood bank terms, a “secretor” is someone who is capable of making ABO antigens in their secretions and plasma. Roughly 80% of the population carries at least one allele called “Se.” This allele encodes an enzyme that allows that individual to make the H antigen on long carbohydrate-rich chains called “type 1 chains” found in secretions and plasma (not necessarily on red cells; that is a different gene). Once the H antigen is made, then the person can make either A or B antigens (or both) on the type 1 chains.