A heat-sensitive, complex system in fresh human and other sera which, in combination with antibodies, is important in the host defense mechanism against invading microorganisms.

1. The complement of a number A is another number B such that the sum A + B will produce a specified result.

2. For a subset of a set, the collection of all members of the set which are not in the given subset.

3. For a fuzzy set A with membership function m_A , the complement of A is the fuzzy set Ā whose membership function mA has the value 1 - m_A(x ) for every element x.

4. See <<radix complement>>