Our Jack Russell was aggressive with both my husband and myself. He is an alpha male and very hard headed. It took us two years to knock some sense into him. He no longer bites us but he attacks all 3 of my Rottweilers. They steer clear of him. When he bites he locks on and it is very difficult to pry him loose.
One of these days, those Rott's are going to decide that they have had enough.