This is an old story, but new to me, now that I started to dig a little deeper. And the story is very simple: humans can't synthesise Neu5Gc, a kind of sialic acid found on a surface of all mammalian cells. Humans don't make Neu5Gc, but they do have Neu5Gc from eating meat from other mammals. Sialic acids do get transported and incorporated into human cells!
As a consequence, Neu5Gc is seen by human immune system as a foreign antigen and response is mounted. This can lead to autoimmune diseases. Whether it does, there is no evidence, but Neu5Gc is being closely looked at now. Another possible implication of presence of Neu5Gc on human cells could be increased risk of infection, e.g. E. coli. This might have contributed to survival advantage in our ancestors who lost the Neu5Gc gene for the human race.
The reason it worries me it that instead of a typical winter cold, this year I have been having a strange kind of infection/inflammation in my chest. CRP is slightly increased, but no symptoms of infection other than occasional mild cough and abundant mucus in the morning. It does not look allergic either.
Also, I remember reading somewhere that higher vitamin D can lead to exaggerated immune response. It is supposed to be anti-inflammatory, but also strengthens immunity, which can be not so good if my immune system is fighting Neu5Gc in my chest.
But the whole Neu5Gc story is not very convincing. Neu5Gc has been found in foetal tissues. If Neu5Gc is present when natural tolerance is being developed, then the antigen should be seen as own, not foreign, regardless of whether if came from dietary meat/milk, or synthesised from scratch. It makes perfect sense though, that antibodies against Neu5Gc can form when children of vegetarian (or nearly vegetarian) mothers start eating meat or drink cow milk. This can be quite common on some ethnic groups. Also, vegetarians who decide to eat meat after many years may have lost natural tolerance and can react to the mammalian meat/milk antigen vigorously. On the other hand, it makes sense to recommend to expectant mothers to eat more red meat, so that more Neu5Gc gets transported to the foetus, thus helping induce natural tolerance.
One might try to use the "leaky gut" argument in addressing the fact that Neu5Gc gets into human cells in the first place, but it appears that pinocytosis is used as transport mechanism. Actually, this is the first example of such transfer of complex compounds from gut to nearly all cells.