When considering the environmental and genetic factors that influence the anatomical structure of the brain, most people would not consider blood type (myself included until about a week ago). But recent research has suggested that there may be a link between blood type and the progression of cognitive decline.
To discuss this topic in any significant manner we need to all be on the same page about blood and blood types. Blood is composed of four primary components. Plasma makes up about 55% of blood and is a complex mix of minerals, salts, hormones, and proteins needed in the body. About 1% of blood is made up of white blood cells, which are necessary for a proper immune response, and platelets, which are essential for the clotting of blood. The final 45% is composed of red blood cells, and it is on the surface of these cells that the identifying features of blood type are found.
Blood types are inherited genetic traits just like eye color, hair, height, and some illnesses. The ABO system is one of the most common systems of blood type classification and it is the one most people are familiar with. In this system, individuals can have blood type A, B, AB, or O (along with negative and positive variations of these main types).
The easiest way to understand how these blood types present as phenotypically distinct within the body is with the help of a food analogy. Let’s say that our red blood cells are donuts. On those donuts are sprinkles which represent antigens. People with blood type A have “A” sprinkles (antigens) on their donuts, people with blood type B have “B” sprinkle (antigen) donuts, people with blood type AB have donuts with both “A” and “B” sprinkles (antigens), and people with blood type O don’t have any sprinkles. These antigen sprinkles themselves are not a problem, because they are self antigens that our body recognizes as non threatening. The antigens become important in situations where donor blood is being introduced into a recipients system, like during blood transfusions.
If blood with a foreign antigen is introduced into an individuals system, the body will produce antibodies to fight the foreign antigen. The antibodies will react with the foreign antigen causing agglutination, or clumping, of the blood, a problem that can be fatal. Your blood type determines the type of antigens your body will not recognize as foreign, and thus the types of blood you would be able to receive in an emergency.
This is why the blood type O is considered the universal donor. The absence of antigens on the red blood cells in blood type O means there are no foreign antigens for the recipients body to react to, removing the risk agglutination. Similarly, type AB is considered the universal recipient because there is no antigen you can introduce that wouldn’t already be present on their red blood cells.
All this talk about blood is great, but since we’re all on the same page now, lets discuss how researchers found a link between this genetic trait and the progression of cognitive decline.
A study done by Matteo De Marco and Annalen Venneri at the University of Sheffield used an index of cognitively healthy adults who had previously undergone structural MRI for other research purposes. The researchers called all participants and collected self reports of the individuals blood types. With a final sample size of 189 participants, they then set about correlating structural neuroanatomical differences and blood types in these individuals.
The findings were without a doubt surprising. ‘O’ adults had larger grey-matter volumes in two symmetrical clusters within the posterior ventral portion of the cerebellum. Additionally, non-‘O’ adults showed lower volume values in temporal and limbic regions, including the left hippocampus.
As we age there is generally a reduction in grey matter volume seen in the brain and in many neurodegenerative diseases we see this same pattern exaggerated. Additionally, the regions in which non-‘O’ adults show a significant reduction in grey matter volume compared to ‘O’ adults are regions of the brain that are commonly damages easiest in the progression of Alzheimers.
This research draws an interesting connection between blood types and general neuroanatomical trends that has previously been unexplored. However, much more research will be needed to begin to understand the mechanisms that underlie these differences.
Do the antigens associated with blood type have a larger role in the development of the nervous system than we previously anticipated? Or, rather than exerting a developmental influence, do these antigens effect functioning later in life in a way that manifests these structural differences? Or maybe, the antigens are not directly causing these differences at all, and there is a larger biological chain reaction happening that we don’t understand.
Despite the missing “how?” piece, the “what?” that we have learned from this research is a testament to the possibilities this field still presents. The recent discovery that blood type affects neuroanatomy shows that we still have so much to uncover about what genetic and biological factors that influence brain development and function.
De Marco, M., & Venneri, A. (2015). ‘O’blood type is associated with larger grey-matter volumes in the cerebellum. Brain research bulletin, 116, 1-6.