Secretor Status and Its Forensic Value

The secretor locus is distinct from the ABO locus. They are on different chromosomes: ABO on chromosome 9, FUT2 on chromosome 19. The ABO genotype determines what antigens are built on red cells and, in secretors, in body fluids. The FUT2 genotype determines whether body-fluid antigen synthesis happens at all.

The FUT2 enzyme works on type-1 chain precursors found in secretory epithelium (gut, salivary glands, reproductive tract). It adds a fucose residue to generate type-1 H antigen. Once H is in place, the ABO transferase enzymes can add A- or B-specific sugars, exactly as they do on red cells. Without the FUT2 enzyme, no H is built in secretory tissues, and therefore no A or B antigen is produced in body fluids regardless of the ABO genotype.

The most common non-secretor allele in European and East Asian populations is a nonsense mutation at codon 428 of FUT2, which introduces a premature stop codon and abolishes enzyme activity. Homozygotes for this allele (se/se or Se428/Se428 in modern notation) are non-secretors. A heterozygote (Se/se) is a secretor: one functional copy of FUT2 is sufficient for normal secretion. Additional non-secretor alleles exist at lower frequencies in African and other populations.

Population surveys of secretor status have been conducted since the 1930s. The classic result is that approximately 80% of European, East Asian, and South Asian populations are secretors and approximately 20% are non-secretors. This proportion is remarkably consistent, though the underlying allele frequencies differ. In West African populations, alternative non-secretor alleles and partial-secretor alleles are more common, and the secretor frequency varies more widely between ethnic groups.

The near-complete secretor status in some Amerindian groups is a textbook example of founder effect and population bottleneck. For forensic purposes, these frequency differences matter: if the reference population for a case has a non-secretor frequency different from the European 20%, using the wrong database overstates or understates the significance of a non-secretor result.

In secretors, blood-group substances appear in all exocrine secretions, but in very different concentrations. Saliva is by far the most concentrated source, with A, B, or H substance detectable at microgram-per-millilitre concentrations in whole saliva from a secretor. This makes saliva stains on cigarette butts, postage stamp adhesive (licked), and bite-mark swabs strong candidates for secretor typing.

• Saliva: highest concentration of blood-group substance; reliable typing from small volumes; primary substrate for bite-mark and discarded-item analysis.
• Semen: secretors have H, A, or B substance in the seminal plasma; concentrations lower than saliva but sufficient for inhibition typing from a single ejaculate stain; the main substrate in pre-DNA rape-case serology.
• Vaginal secretions: present in secretors; useful for corroborating donor typing in mixed stains where male-fraction DNA may be absent.
• Urine: secretors excrete blood-group substance in urine; useful for typed urinary stains at scenes but less concentrated than saliva.
• Sweat and tears: contain trace blood-group substance in secretors; generally too dilute for reliable forensic typing from dried residues.

Non-secretors produce essentially none of these soluble antigens. Their sweat, saliva, and semen carry only background-level non-specific inhibition when tested. An analyst encountering no inhibition from a saliva stain cannot conclude the donor is group O; they must consider whether the donor is a non-secretor of any ABO group. This is why determining whether a stain is from a secretor or non-secretor was a prior step before ABO typing from body fluids.

The inhibition test for secretor typing is a variant of the absorption-inhibition method. The stain extract is mixed with defined volumes of anti-H lectin (from Ulex europaeus, specific for H antigen), anti-A, and anti-B antisera. Each mixture is incubated, then indicator cells are added: O cells for the anti-H test, A cells for anti-A, B cells for anti-B. A titre drop compared to the control confirms the presence of the corresponding substance.

1. 1. Secretor screen
   The anti-H lectin test is run first. A significant titre drop indicates the stain contains H substance, confirming the donor is a secretor. No drop indicates a non-secretor; ABO typing from this stain will not be informative.
2. 2. ABO group typing
   If the secretor screen is positive, anti-A and anti-B inhibition tests are run in parallel. Titre drops identify which specific antigen is present. The combination of inhibition results gives the ABO group of the secretor donor.
3. 3. Cross-checking with blood typing
   Where a bloodstain is also available from the same scene, the ABO type from the blood should agree with the type from the body-fluid stain (if from the same person). Discrepancies flag either mixed stains or typing errors, both requiring investigation before reporting.

The multiplication of discriminating power works through simple probability. If group B occurs in 9% of a reference population and secretors are 80% of the population, then group B secretors are approximately 9% times 80%, or about 7.2% of the population. Non-secretors who are group B are 9% times 20%, or 1.8%. Telling a group B secretor stain from a group B non-secretor stain doubles the resolution within the group B fraction.

In practice, the UK Home Office Forensic Science Service and equivalent laboratories in other countries built multi-system profiles through the 1970s and 1980s, combining ABO with secretor status, Rh blood group, and red-cell enzyme polymorphisms (such as phosphoglucomutase and esterase D isoforms). These profiles could reduce the matching fraction of the reference population to a few percent, sometimes lower. The Narborough murders investigation, which began in 1983 and culminated in the first DNA-based conviction in the UK when Colin Pitchfork was sentenced in January 1988, was preceded by conventional serology that had already identified that the murderer was a group A secretor with a specific PGM enzyme profile, a combination matching approximately 10% of the adult male population.

STR DNA profiling renders secretor typing redundant for most modern casework. A full STR profile from a semen stain or a saliva stain is more discriminating by orders of magnitude than any combination of serological typing results. The FUT2 genotype itself can be determined from DNA, so if secretor status information is ever needed, it is technically possible to derive it from the same extract used for STR typing.

Where secretor biology retains forensic relevance is in two areas. First, historical case review: a large number of convictions in several jurisdictions rest partly on pre-DNA serological evidence including secretor typing. Analysts reviewing these cases need to understand what was tested, how it was interpreted, and whether the interpretation was correct. Second, degraded evidence: in samples so degraded that STR profiling fails, the carbohydrate-based ABO and secretor antigens sometimes survive and can still yield class-level information.

• Historical case review: convictions based on serological secretor evidence require analysts fluent in the methodology and the failure modes to assess whether the original interpretation was justified.
• Triage screening: a rapid secretor screen on a scene exhibit can confirm the presence of a human secretory body fluid before DNA resources are committed.
• Research and educational contexts: FUT2 allele frequency studies continue in population genetics and infectious disease epidemiology, generating data that inform forensic reference databases.