A nonsecretor is an individual of blood group A, B, or AB who does not secrete the antigens characteristic of these blood groups in bodily fluids such as saliva. Secretor status refers to the presence or absence of water-soluble ABO blood group antigens in a persons bodily fluids, such as saliva, tears, breast milk, urine, and semen. People who secrete these antigens in their bodily fluids are referred to as secretors, while people who do not are termed non-secretors. Secretor status is controlled by the FUT2 gene, and the secretor phenotype is inherited in an autosomal dominant manner, being expressed by individuals who have at least one functioning copy of the gene. The non-secretor phenotype is a recessive trait.

Non-secretors have reduced susceptibility to the most common strains of norovirus, and they seem to be less likely to get some stomach illnesses, like stomach flu and ulcers. On the flip side, non-secretors seem to be more likely to get yeast infections. They also seem to be more likely to get pneumonia and meningitis. Being a non-secretor has also been connected to some autoimmune diseases such as inflammatory bowel disease, psoriasis, or type 1 diabetes.