Abdominal aortic aneurysms are common and life threatening. Although CD4⁺ T cells are abundant in aneurysm tissue, their role in disease progression remains unclear. A new study shows that mouse aortic allografts placed in animals lacking IFN-γ receptors develop a Th2 inflammatory response with aortic aneurysms, whereas Th1 responses promote intimal hyperplasia. It is expected that these surprising findings will stimulate further efforts to clarify whether adaptive cellular immunity in aneurysm disease is detrimental or potentially beneficial.