Symbiotic bacteria helps the immune system of vertebrates, animals who are germ-free have immune system difficulties. This is caused by low intestinal activity, a decrease in lymphocytes, low vascularity, decrease of digestive enzyme activity, and muscle wall thickening. Even with all of these negative, they also are more susceptible to infections. Shown in mice, this can affect T cell production, once the microorganisms are introduced, T cell population changed.