Okay. So which part of the immune system is suppressed by the activation of the HPA axis? From what we know now, it seems like lymphocyte production is very suppressed when we have prolonged activation of the HPA axis. Specifically, it seems like the presence of cortisol in the bloodstream for very prolonged periods of time leads to suppressed development of new lymphocytes. So, we just kind of have fewer lymphocytes in our bloodstream and there is a decreased ability to fight off invading bacteria or viruses.

Now, both macrophages and granulocytes are types of white blood cells, but we have not seen the same consistent relationship found in research where if we have activation of the HPA axis we do not necessarily see consistent patterns of suppression of those two types of cells. And then nephrons are not part of your immune system. They are parts of your kidney. So those are just kind of irrelevant here. Alright.

So our answer is D, lymphocytes.