Distinguishing human blood from that of other species:
Too much monkey business?
The term “human blood” is consistently used in discussions of the bloodstains on the Shroud. Just how is human blood distinguished in the laboratory? And where exactly does the data on the Shroud stand? These questions are briefly discussed below.
Human blood versus animal blood
A scientist cannot just look at a bloodstain with the naked eye and tell that it’s human blood. For fresh blood, microscopic analyses may allow one to distinguish mammalian red blood cells from non-mammalian red blood cells due to the absence and presence of a nucleus, respectively. If sufficient numbers of (white) blood cells are present, chromosomal characterization (karyotyping) may be performed, at significantly higher magnification. Among primates, only humans contain 46 chromosomes; chimpanzees, gorillas, and orangutans contain 48. The chromosome number of other species is quite variable, for example pigs have 38, sheep have 54, dogs have 78, and cows have 60.
In aged bloodstains, such microscopic tests are not practical because blood cells become dehydrated and rupture within hours of drying. Species characterization of dried, aged bloodstains relies on serological (immunology) tests or molecular (DNA) analysis. Chemical tests cannot distinguish human from animal blood. For serological studies, human blood components (usually albumin or immunoglobulin proteins) are detected using antibodies that are generated in another species, for instance, rabbits. When utilized in the laboratory, such antibodies would not react with blood components from other animals, for example chickens or cows, because enough difference exists from their human counterpart proteins that the antibodies fail to recognize them.
A positive reaction in such tests usually results in the conclusion that human blood is present. However, this is where things get somewhat tricky. Even though such reagents are designated as “anti-human”, this only refers to the species in which they were generated. Cross-reactivity (or the lack thereof) must be independently verified. Species that are closely related to humans (i.e. non-human primates) express blood components (albumin and immunoglobulin) that are similar enough to those of humans to also react positively in such tests (see picture above). In forensic settings this is typically disregarded unless special circumstances warrant that such possibilities be considered (at a crime scene within a zoo, for example, or if someone were known to keep apes or monkeys as pets). In most situations, when it is stated that bloodstains tested positive for human blood, this underlying supposition exists. Strictly speaking, such serological tests do not distinguish human blood from the blood of other primates (monkeys or apes). If sufficient DNA is intact for molecular biology analysis, specific regions of certain genes may be targeted that have sequences unique to humans, which allow them to be effectively distinguished from those of closely related species.
Human origin of the blood on the Shroud
In the vast majority of discussions of the human nature of the bloodstains on the Shroud, the studies that are typically referenced are the experiments of Adler and colleagues and Baima Ballone and coworkers, utilizing serological tests for detection of common major blood components: albumin, immunoglobulin, ABO antigens. In fact, such studies do not distinguish human blood from the blood of other primates.
With the Shroud, primate may imply human but this is an extension beyond what the data actually show. Adler was appropriately cautious in concluding that the data only demonstrated that the blood was of primate origin, and even conducted experiments to evaluate the cross-reactivity of such “anti-human” reagents. Adler would also comment, “If you choose to think that the image you see is that of a chimp or an orangutan, you’re perfectly welcome to believe that…”
To date, the only study that directly addresses the human nature of the Shroud bloodstains is an often overlooked report by Baima Ballone et al. that evaluated the expression of additional blood components found on red blood cells, specifically the M,N, and S antigens. (Such antigens have also been studied in the blood analysis of King Tut). The conclusion was that the bloodstains on the Shroud are characterized as MNS positive. What is most significant about these studies is that unlike M and N antigens, which are shared between certain primates and humans, the S antigen is exclusive to humans only. No S counterpart exists in other species, including apes or monkeys. This point was not emphasized (or mentioned) in the report, as the significance of this relationship among primates was not fully elucidated until several years later (in non-Shroud related studies). Of the six serological analyses of blood components on the Shroud, this brief study remains the single most definitive piece of serological evidence that directly addresses the human origin of the blood on the Shroud. For a more detailed discussion see: “Empirical evidence that the blood on the Shroud of Turin is of human origin: Is the current data sufficient?” recently published on shroud.com. The full-length manuscript is available at http://www.shroud.com/pdfs/kearse1.pdf.
Taken together, the current serological data indicate the blood on the Shroud is of primate, i.e. human origin. Could more work be done in the laboratory to strengthen the conclusion that the blood is indisputably human? The answer is yes. Should there be sufficient doubt that the blood is in fact, human, and may represent the blood of a monkey or ape? The MNS studies say no. All other data are consistent with this finding. Moreover, Adler has effectively commented on the difficulties a forger would encounter in trying to apply clotting blood [of any species] to various regions on the Shroud. Since the original blood studies were performed some thirty years ago, significant advancements have been made in the development of serological and molecular tools that could prove useful in advancing previous information. For example, within the past few years, monoclonal antibodies have been generated that effectively distinguish human blood from that of closely related species. Similar to blood typing data, additional analyses would cross-check and verify previous findings on the bloodstains of the Shroud.