Idk about humans but I study cattle and their immune system is cool. They’re born with essentially no immune system. They get passive immunity from moms colostrum (first thick milk produced after birth) in the form of antibodies/proteins. This passive immunity goes out after about 1-2 weeks and whatever bacteria that happens to be around starts populating the gut. Calves usually get really sick with “scours” when this happens. Their active immunity is kicking in! Then their rumen continues to get populated with microbiota as they eat forage. They’ve got a complex micro biome because they digest/ferment lots of plant material and we are still learning about how this micro biome develops in early life! It’s part of my PhD research :)