The phrase "defense system" refers to a set of strategies and structures put in place to protect or defend against potential threats or attacks. It is like a shield that helps to keep something or someone safe. Full definition
He opposes the construction of missile defense systems largely on the ground that they can not be made to work under nearly any circumstances. (blogs.reuters.com)
The cells have natural defense system against free radicals and can repair themselves from the damage caused by the dangerous substances. (giendoscopypractice.com)
The usually well developed defense system in healthy people ensures that deposited particles are rendered ineffective as quickly as possible and removed from the lungs. (sciencedaily.com)