@DeepWhaleSlumber I am in the same boat as you (considering Harvard, Yale and Rockefeller). I am leaning towards Yale or Rockefeller because for me Boston isn't the best location and I am somewhat concerned that there may be some truth to Harvard's competitive reputation. However, I am less sure of my research area of interest (though I am leaning towards neuroscience or genetics). One thing I would say is that if you are sure about pursuing immunology, you may want the wider breadth of choice that Harvard or Yale would provide in terms of PIs.
Good luck with your decision if you haven't already made it!