Do most lymphocytes in humans really reside in the gut?

It is widely believed that the gut, and particularly the lamina propria (LP) of the gut, contains most of the lymphocytes in humans. The strong depletion of CD4+ T cells from the gut LP of HIV-infected patients was, therefore, suggested to be such a large, irreversible insult that it could explain HIV disease progression. However, reviewing data from different mammalian species, we found that only 5%–20% of all lymphocytes reside in the gut, and that only 1%–9% of the total lymphocyte number is located in the gut LP. Our findings suggest that spleen and lymph nodes, rather than the gut, are the largest immune compartments in mammals.