This is Part 1 of our two-part series on the immune system. After living through a global pandemic, we are all a bit wiser about pathogens, infections, and how the immune system can help us combat illness. Still, the MCAT goes deeper. In Part 1, we will cover the many divisions of the immune system, from innate vs. adaptive to cell-mediated immunity vs. humoral immunity. We will also discuss the immune system’s cells and signaling pathways, such as the complement system, MHC Class I, and MHC Class II.