This gets tricky and difficult to explain but stay with me on this one.
All mammals have special parts of the cell wall called neuraminic acid. These are on the outside of the cell like millions of hairs that are used by the body for identification (analogous to a bar code). The tip of the neuraminic acid is something called sialic acid and is very important for cellular binding. Almost all animals have a specific one called Neu5Gc (N-Glycolylneuraminic acid, we will stay with the shorter name). Millions of years ago, a small change occurred creating a new sialic acid called Neu5Ac. This can be found in Humans (of course), shellfish, chickens, mollusks, elephants, ferrets and New World monkeys. If anyone can figure out what the connection is, please let us know. It is unknown why this happened, but we do know animals with Neu5Ac are resistant to a form of malaria that was rampant millions of years ago; so this afforded some degree of protection. The Neu5Ac is also known to improve brain function and may have been responsible for the expansion of the brain size. Our brain is very rich in the amount of sialic acids, more than any other cell type in our body. We also have multiples of the amount of sialic acid in our brains than other animals. Neu5Gc may be toxic to our brains. Human breast milk is very rich in Neu5Ac and appears to be very important for human brain development and the communication between brain cells. Unfortunately, many baby formulas do not have a lot of sialic acid and the ones that use cow milk have a preponderance of Neu5Gc which may impair the brain function of formula-fed infants.
Some pathogens (viruses) use sialic acid to attach to the cell, and then gain access. The ancient malaria, Plasmodium reichenowi, uses Neu5Gc to gain access to the cell. So, humans were ok since we did not have that one. However, about 10,000 years ago another malaria type, Plasmodium falciparum, changed and now uses the Neu5Ac to gain access. We are not sure how long we are going to have to wait till the next evolutionary change to protect us.
Neu5Gc appears at very high levels in tumor cells, especially in malignant metastatic cells. These cells somehow ingest them from our diet and concentrate them. Some think this is a way for metastatic cells to hide themselves from our immune system. This is some sort of stealth technology. Science is currently trying to figure out how to use this to our advantage to kill the cancer cells.
We also have a relatively high concentration of Neu5Ac in the lining of our heart, arteries and intestine. Lectins and especially WGA bind to Neu5GC and are identified by our immune system as invaders and we make antibodies to them. Since Neu5Ac and Neu5Gc are so close, the antibodies to the Neu5Gc cross react, bind and injure Neu5Ac. This leads to autoimmune heart and arterial disease. It is interesting that African elephants have no known coronary artery disease since they feed on grass. Now, over 50% have severe coronary artery disease thanks to the lectins in their diet (corn and soy).
Dietary sources for Neu5Gc are red meat (beef and pork). Dairy contains some Neu5Gc but not fish and chicken. So, we should be cautious how much red meat we consume. It is not the cholesterol.