 How is it our immune system is able to distinguish between self and non-self? How does our immune system know what to attack and what not to attack? As I described, the clonal selection theory of immunity suggests that we start out able to fend off basically every cellular structure in the known universe, because we have a billion different types of antibody-producing B cells, each capable of recognizing a different molecular signature. Why then do we tend not to attack ourselves? It's because before we're even born, we kill off each and every B cell that recognizes us. It's called clonal deletion, the killing off of forbidden clones. That's what our thymus gland is for, when we're still a fetus. Our body lines up all our immune cells, our B cells, our T cells, holds up a picture of ourselves and asks them one by one, do you recognize this person? And if any of our immune cells say yes, they're shot in the head, killed on the spot, death by apoptosis and good riddance. Turns out this process of clonal deletion, ridding our bodies of self-recognizing immune cells, happens throughout our lives, mostly in our bone marrow. If you remember though, as I talked about before, IGF-1, cancer-promoting growth hormone boosted by animal protein consumption, prevents apoptosis, prevents our body's killing of cells it wants to get rid of. That may be why IGF-1 levels are linked to cancer. So it is of great interest to determine whether IGF may be contributing to the inappropriate survival of lymphocytes in autoimmune or inflammatory conditions. Maybe that's why people who eat plant-based diets appear protected from autoimmune diseases, explaining, for example, the extraordinary rarity of most autoimmune diseases among, for example, sub-saharan rural blacks following a traditional vegan lifestyle. Before they changed their diet, evidently not a single case of multiple sclerosis had been diagnosed among a population of 15 million.