A recent commercial for a particular brand of yogurt claims, among other things, that 70% of your immune system is located in your gut. I have seen this claim repeated here (the emphasis is mine):
Most people don’t know that 60-70% of their immune system is located in the gut as a vast network of lymph tissue referred to as GALT (gut associated lymphatic tissue).
And repeated without explanation in a few other places, e.g.:
Understanding Digestive Health ("Since 70% of the body’s immune system dwells in the digestive tract, maintaining balanced digestive health is crucial to the body’s overall well-being.")
All of these sources appear to be pushing some form of edible immune boosting product, and they use the 70% claim in a strange form of logical deduction that says "70% of your immune system is in your gut; our food goes into your gut; thus, our food is good for your immune system."
I'm not interested in how and/or whether these products work, as there are other skeptics questions dealing with that claim. I'm just asking about the "70%" claim. In particular, I would have guessed that most of the cells that make up our immune system would be located in our lymph nodes, blood stream, and bone marrow.
Before anyone asks, I have no idea what they are measuring 70% of, so I'm not even sure that the claim makes sense as written. If someone can explain what measurement makes the 70% figure true, that would be a very helpful answer. Otherwise, I'm mostly interested in actual medial science that could explain how the immune system is "distributed" through the body and how much of that can be attributed to the digestive system.