MMP9 and Alzheimer disease: The impact of colitis on AD is supported by the following findings from our study: (1) Aβ accumulation in the brain is exacerbated by acute colitis, unchanged by chronic colitis, and tends to decrease with food allergy-induced enteritis; (2) acute colitis alters the percentage and number of immune cells in the brains of AD mice, particularly by increasing neutrophils in the blood and brain; and (3) removal of neutrophils and inhibition of MMP-9 suppressed the accumulation of Aβ in the brains of AD mice induced by acute colitis.