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In biology, complement is a group of proteins found in blood serum which act in concert with antibodies to achieve the destruction of non-self particles such as foreign blood cells or bacteria.

The complement is a concept used in naive set theory. See Complement (set theory).

In mathematics, complement is an operation that transforms an integer into its additive inverse. It is useful for subtracting numbers when only addition is possible, or is easiest. negative numbers are one form of additive inverse; subtracting 5 from 8 is the same as adding -5 to 8. Another form of subtraction deals only with positive numbers, and is called the method of complements. One common form of this is the two's complement, which is often used in computer microprocessors which perform calculations using the binary numeral system.

In optics, complement refers to complementary colors.