The complement system is:

a. a set of proteins involved in innate but not acquired immunity.
b. a set of proteins secreted by cytotoxic T cells and other CD8 cells.
c. a group of antimicrobial proteins that act together in a cascade fashion.
d. a group of proteins that includes interferons and interleukins.

a. a set of proteins involved in innate but not acquired immunity.

Explanation:

The complement system are a set of proteins or protein fragments that play a major role in the innate immune system. The complement system is involved in the defense of the body from foreign pathogen through specific plasma proteins that participate in leukocyte activation, chemotaxis and opsonization. In other words, they increase the ability of phagocytes and antibodies to mop up damaged cells and foreign microorganism, activate inflammation process.

Some of these proteins include complement component 3 (C3), complement component 9 (C9) and  C1Q complex.

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