Minecraft wiki entry for Defeating_a_Nether_fortress, associated text: A shield, when used, can help with deflecting a few of the threats of nether fortress, such as mob melee attacks, blaze fireballs, and the explosions caused by ghast fireballs. Successfully blocking a blaze fireball also prevents it from setting the player on fire (but it will not protect the player should the fireball set the ground on fire).