Training Immune Cells

Your gut bacteria spend your first years educating your immune system. They teach immune cells to distinguish between harmless substances and genuine threats — a calibration process that, when disrupted by antibiotics or sterile environments, leads to allergies and autoimmune diseases later in life.

Children raised on farms with high microbial exposure tend to develop far fewer allergies than children raised in cleaner urban homes. This is the hygiene hypothesis: modern cleanliness deprives the immune system of the microbial training data it evolved to expect.