Algebraic immunity AI(f) defined for a boolean function f measures the resistance of the function against algebraic attacks. Currently known algorithms for computing the optimal annihilator of f and AI(f) are inefficient. This work consists of two parts. In the first part, we extend the concept of algebraic immunity. In particular, we argue that a function f may be replaced by another boolean function fc called the algebraic complement of f. This motivates us to examine AI(fc). We define the extended algebraic immunity of f as AI*(f)= min {AI(f), AI(fc)}. We prove that 0≤AI(f)–AI*(f)≤1. Since AI(f)–AI*(f)= 1 holds for a large number of cases, the difference between AI(f) and AI*(f) cannot be ignored in algebraic attacks. In the second part, we link boolean functions to hypergraphs so that we can apply known results in hypergraph theory to boolean functions. This not only allows us to find annihilators in a fast and simple way but also provides a good estimation of the upper bound on AI*(f).