Blood Group Systems in Forensic Science: ABO, Rh, MNS and Beyond

In 1901, Karl Landsteiner, working at the Pathological-Anatomical Institute in Vienna, mixed sera and red cells from six of his colleagues and noticed that some combinations agglutinated and others did not. From those reactions he deduced the existence of two antigens, A and B, and the rule that every individual carries plasma antibodies against the antigen they lack. He named three groups (A, B, C; later renamed O). The fourth, AB, was identified in 1902 by his students Decastello and Sturli. Landsteiner received the Nobel Prize for the discovery in 1930.

The ABO gene sits on chromosome 9q34 and codes for a glycosyltransferase that adds either N-acetylgalactosamine (A allele) or galactose (B allele) onto the underlying H antigen. The O allele is a non-functional version that leaves H unmodified. Genotype to phenotype runs as: AA or AO gives A, BB or BO gives B, AB gives AB, OO gives O. For Indian frequencies the standard textbook spread is roughly B (~37%), O (~32%), A (~22%), AB (~9%); B is the modal group in most North Indian populations, which is itself a frequent MCQ trap because it inverts the European pattern where O leads. For body-fluid grouping the same antigens appear in saliva and semen of secretors, which is what made ABO useful before DNA. For the techniques that turn these antigens into a result on a bloodstain, see the next topic in this unit: blood-grouping techniques from bloodstains.

The Rh system gets its name from the rhesus macaque. In 1940 Karl Landsteiner and Alexander Wiener injected rabbits and guinea pigs with rhesus monkey red cells and obtained an antibody that also agglutinated about 85% of human red cells. The clinical significance had been demonstrated a year earlier, in 1939, by Philip Levine and Rufus Stetson, who described a haemolytic transfusion reaction in a woman who had just delivered a stillborn child, and traced it to a maternal antibody against a paternal antigen the baby had inherited. That antibody, eventually called anti-D, is what makes Rh incompatibility the classic cause of haemolytic disease of the newborn.

The Rh locus on chromosome 1 carries two closely linked genes, RHD and RHCE. The presence of the D antigen defines Rh-positive; its absence defines Rh-negative. The Fisher-Race nomenclature lists five principal antigens: D, C, c, E, e (there is no lower-case d because no anti-d antibody has ever been demonstrated). In Indian populations roughly 93 to 95% are Rh-positive and 5 to 7% Rh-negative, which is the opposite ratio to the well-known European 85:15 split, and another standard distractor on Paper 2. A weak D variant (formerly Du) expresses a reduced D antigen that needs an indirect antiglobulin test to detect; it is reported as Rh-positive for donor purposes and as Rh-negative for recipient purposes, which is the kind of distinction the syllabus rewards. Rh remains the second criterion (after ABO) for any disputed-paternity exclusion conducted in an Indian forensic-biology division, and is one of the markers cross-checked against the Bharatiya Sakshya Adhiniyam 2023 expert-evidence regime when the lab report is tendered in court.

The minor systems are MCQ territory: one or two lines each, learnt as a set, deployed as a one-mark fact.

MNS was reported by Karl Landsteiner and Philip Levine in 1927 (the S antigen was added by Walter and Sanger in 1947, and the full Walter and Race articulation came later). The system is controlled by the GYPA and GYPB genes on chromosome 4 and codes for glycophorins A and B. M and N are the main antigens; S and s are the second pair. Forensic value lies in paternity exclusion and in marker linkage during anthropological work on Indian populations.

Kell was described by Coombs, Mourant and Race in 1946, named after the patient Mrs Kelleher. The K antigen is highly immunogenic; anti-K is the most common irregular antibody (after Rh) implicated in haemolytic transfusion reactions and haemolytic disease of the newborn. Roughly 9% of Europeans are K-positive; in Indian populations the figure is much lower.

Duffy (Fy) was reported in 1950, named after the patient Mr Duffy. The two main antigens are Fya and Fyb. The Fy(a-b-) phenotype, common in West African populations, confers resistance to Plasmodium vivax malaria, which is the cited reason for its near-absence in malaria-endemic regions of South Asia.

Kidd (Jk) was described by Allen, Diamond and Niedziela in 1951 in Mrs Kidd. Anti-Jka is notorious for causing delayed haemolytic transfusion reactions because the antibody titre falls below detection limits between exposures.

Lewis (Le) is unusual: Lewis antigens are not synthesised by red cells but are adsorbed onto them from plasma. Their expression depends on the Le gene and on the Se (secretor) gene, which links Lewis directly to the secretor-status MCQ pattern.

In 1952, Dr Y. M. Bhende and his colleagues at K.E.M. Hospital, Bombay (now Mumbai), reported three patients whose red cells were not agglutinated by anti-A, anti-B or anti-H sera, and whose own serum agglutinated red cells of all four ABO groups including group O. They had inherited two copies of the rare h allele at the H locus, so they could not synthesise the H precursor, and without H neither the A nor the B transferase has anything to act on. The phenotype is written Oh or hh, and is informally called the Bombay phenotype after the city of discovery.

Three points NTA likes to test:

1. On a routine ABO test, an Oh individual types as group O. Only the addition of anti-H serum reveals the difference: ordinary group O cells agglutinate with anti-H; Oh cells do not.
2. An Oh individual can only receive blood from another Oh individual. Even matched-group O donor blood will be destroyed by their anti-H antibody, which is why the Bombay phenotype is a transfusion-medicine emergency.
3. The phenotype is most concentrated in western India and remains rare globally (roughly 1 in 10,000 in parts of Maharashtra and Karnataka, against 1 in a million in Europe), which is why the syllabus tags it as an Indian discovery.

This one paragraph is worth a guaranteed mark in most cycles. Memorise the year, the city, the discoverer and the receiver-restriction.

Before DNA fingerprinting reached Indian forensic-biology divisions in the late 1980s and 1990s, blood-group typing was the workhorse of three case categories:

1. Disputed paternity. ABO + Rh + MNS combined could exclude (never confirm) an alleged father in roughly 50 to 60% of false claims. Indian courts accepted these exclusions under the old Indian Evidence Act regime, and the same data is now corroborative against the DNA profile under Section 51 of the BSA 2023.
2. Bloodstain individualisation. A stain typed as group AB, Rh-negative narrowed the donor population to under 1% in India. Combined with secretor status from a saliva sample on a cigarette butt, it could be powerfully exclusionary.
3. Body-fluid identification on sexual-assault evidence. Semen, saliva and vaginal-fluid stains from secretors carry ABH antigens, allowing the analyst to type the source without a blood sample.

After DNA STR profiling became routine in CFSL Hyderabad, CFSL Kolkata and the larger state SFSLs, blood grouping moved into a supporting role. It still matters in three Indian-context situations: when degraded samples cannot yield a DNA profile but a partial ABO group survives, when transfusion-medicine context is needed (AIIMS blood bank protocols around recipient-donor compatibility), and when the lab is asked to corroborate a DNA result with a cheaper orthogonal marker. Every CFSL serology division still keeps the ABO + Rh + Lewis panel on its accreditation scope under the ISO/IEC 17025 framework.