What is the defining characteristic of a secretor's blood group?

The defining characteristic of a secretor's blood group is the presence of Lewis antigens. Secretors, individuals with a specific genotype, have the ability to secrete blood group antigens into body fluids, such as saliva and other secretions. This is significant because Lewis antigens are not directly encoded by the ABO blood group system but are instead influenced by the secretor status of an individual. In secretors, the secretor gene encourages the expression of Lewis antigens (specifically, Lea and Leb) in their secretions.

In contrast, the H antigen is present in all individuals regardless of secretor status and does not define whether someone is a secretor. The absence of A or B antigens applies to individuals with blood type O but does not necessarily correlate with being a secretor or not. Lastly, universal donor capability pertains to type O blood, which lacks A or B antigens entirely, but this feature does not relate to secretor status or the presence of Lewis antigens. Therefore, the presence of Lewis antigens is the definitive characteristic that correctly identifies a secretor's blood group.