The Immunology of Pregnancy
Revising for my advanced immunology exam could not have felt more real. While panicking at the plethora of information I had to retain, I stumbled across a stunning concept that reminded me of the reason I had chosen to study biomedical science in the first place.
Those of you who have studied immunology will know that our immune cells present a structure called a major histocompatibility complex (MHC) on their surface. MHCs contain a group of genes that code for several proteins expressed on the surface of a variety of cells; in humans these are known as human leukocyte antigens (HLA). Infection is directly linked to individual immunity and the foreign proteins presented by the MHC which determine susceptibility to disease. The foreign protein comes from an infectious agent and is recognised by a group of white blood cells known as T lymphocytes, which can elicit a cytotoxic response to eliminate the pathogen.
A foetus has MHCs from both the mother and the father, who have different MHCs to each other. This makes the foetus an allograft. In other words, it is genetically distinct from the mother, meaning that it should theoretically be recognised as 'foreign' and trigger an immune response. Yet, the trophoblast- that is, the outer layer of the placenta- was created in such a way that it does not express MHC molecules, making it resistant to recognition and attack by maternal immune cells. The trophoblast was made to act as a protective interface between the foetus and the mother... Genius!
The lack of MHC molecules is not the only protective mechanism of the trophoblast. Another striking feature is the presence of an enzyme called IDO, responsible for depleting an amino acid called tryptophan. T lymphocytes cannot be activated in the absence of this molecule, further repressing an immune reaction. What is more, the trophoblast expresses a special type of HLA, called HLA-G, which has the ability to inhibit the cytotoxic action of natural killer cells. To top it all off, this protective outer layer produces signalling molecules known as cytokines which have an anti-inflammatory role in preventing the recruitment of neutrophils- the initiators of all immune reactions.
I mentioned at the beginning that this reminded me why I had chosen to study the degree that I did. The fantastic detail and complexity of the trophoblast, the way that it allows nutrients to be transported across the placenta while preventing the maternal immune system from recognising the foetus as a foreign body, is just a small fraction of a huge array of finely-tuned mechanisms of the human body. The architecture of our body at a molecular level is more detailed than the latest smart phone, the most complicated machine, the most ancient building. There is a talented designer, a clever engineer, a famous architect behind these entities. People visiting art galleries sing praises at the artists who created the paintings on the walls. It only follows that there must also be an Artist, Engineer, Architect, Designer, A Creator, behind the human body. Undoubtedly, He is the best of all creators.
