The main function of the immune system is defense against microbial infection. The other major functions of the immune system depend on a number of major actions that defend us against threats to our health.These actions help our immune system sort out a system of complicated reactions that our body uses every day to defend us from infection and disease.he other major functions of the immune system depend on a number of major actions that defend us against threats to our health. These actions help our immune system sort out a system of complicated reactions that our body uses every day to defend us from infection and disease.