com·ple·ment • n. / ˈkämpləmənt/ 1. a thing that completes or brings to perfection: the libretto proved a perfect complement to the music.2. [in sing.] a number or quantity of something required to make a group complete: we have a full complement of staff. ∎  the number of people required to crew a ship: almost half the ship's complement of 322 were wounded. ∎  Geom. the amount in degrees by which a given angle is less than 90°. ∎  Math. the members of a set that are not members of a given subset.3. Gram. one or more words, phrases, or clauses governed by a verb (or by a nominalization or a predicative adjective) that complete the meaning of the predicate. ∎  (in systemic grammar) an adjective or noun that has the same reference as either the subject (as mad in he is mad) or the object (as mad in he drove her mad).4. Physiol. a group of proteins present in blood plasma and tissue fluid that combine with an antigen–antibody complex to bring about the lysis of foreign cells.• v. / -ˌment; -mənt/ [tr.] add to (something) in a way that enhances or improves it; make perfect: a classic blazer complements a look that's stylish or casual. ∎  add to or make complete.DERIVATIVES: com·ple·men·tal / ˌkämpləˈmentl/ adj.

Complement

Complement refers to a series of some 30 proteins that enhance the bacterial killing effect of antibodies. This complementation involves facilitating the engulfing of bacteria by immune cells in the process known as phagocytosis , or by the puncturing of the bacterial membrane. Additionally, complement helps dispose of antigen-antibody complexes that form in the body.

The various complement proteins circulate throughout the bloodstream in an inactive form. When one of the proteins is converted to an active form upon interaction with an antigen-antibody complex, a series of reactions is triggered. The activation step involves the cleaving, or precise cutting, of the particular complement protein. The cleavage turns the complement protein into a protease, a protein that is itself capable of cleaving other proteins. In turn, cleavage of a second complement protein makes that protein a protease. The resulting cleavage reaction generates a series of active complement proteins. These reactions, known as the complement cascade, occur in an orderly sequence and are under precise regulation.

The reactions involve two pathways. One is known as the classical complement activation pathway. The end result is an enzyme that can degrade a protein called C3. The other pathway is known as the alternative pathway. The second pathway does not require the presence of antibody for the activation of complement. Both pathways result in the formation of an entity that is called the membrane attack complex. The complex is actually a channel that forms in the bacterial membrane. Under the magnification of the electron microscope , a bacterial membrane that is a target of the complement system appears riddled with holes.

The channels that form in a membrane allow the free entry and exit of fluids and molecules. Because the concentration of various ions is higher inside the bacterium than outside, fluid will flow inward to attempt to balance the concentrations. As a result, the bacterium swells and bursts.

Other reaction products of the complement cascade trigger an inflammatory immune response. In addition, the invading bacteria are coated with an immune molecule (C3b) that makes the bacteria more recognizable to phagocytes. This process is called opsonization . The phagocytes then engulf the bacteria and degrade them.

Tight control over the activity of the complement system is essential. At least 12 proteins are involved in the regulation of complement activation. Defects in this control, or the operation of the pathways, result in frequent bacterial infections.

See also Immune system; Infection and control

complement
1. of a set, S, with respect to some universal set U. The set consisting of elements that are in U but not in S; it is usually denoted by S′, ~S, or S̄. Formally, S′ = {x | (x ∈ U) and (x ∉ S)}

The process of taking complements is one of the basic operations that can be performed on sets.

The set difference (or relative complement) of two sets S and T is the set of elements that are in S but not in T; it is usually written as S – T. Thus S′ = U – S

See also operations on sets.

2. See Boolean algebra.

3. of a subgraph G′, with vertices V′ and edges E′, of a graph G, with vertices V and edges E. The subgraph consisting of the vertices V and the edges in E but not in E′.

4. See radix-minus-one complement. See also radix complement, complement number system.

COMPLEMENT. A constituent of GRAMMAR that completes the meaning of a word. Broadly, complements can be found for every major word class: VERBS (in Canada in They live in Canada); nouns (of their debt in the payment of their debt); ADJECTIVES (that we're late in I'm aware that we're late); ADVERBS (for me in luckily for me); PREPOSITIONS (our place in at our place). Some grammarians use the term complementation for the constituent or the process of complementing the word. More narrowly, complements refer to constituents that complete the meaning of the verb, including direct objects (their debt in pay their debt), indirect objects (me in Show me the way), and adverbial complements (at them in Look at them). In an even more restricted use, two kinds of complements are recognized for verbs: (1) Subject complements, which follow the verb be and other copular verbs, my best friend in Tom is my best friend. (2) Object complements, which follow a direct object and have a copular relationship with the object: I consider Tom my best friend. Complement clauses are subordinate clauses that function as complements of a word: that they were too noisy in She told them that they were too noisy. The subordinator that introduces a complement clause is sometimes known as a complementizer: for example that in the sentence just cited.

complement A group of proteins present in blood plasma and tissue fluid that aids the body's defences following an immune response; the genes encoding it form part of the major histocompatibility complex. Following an antibody–antigen reaction, complement is activated chemically and becomes bound to the antibody–antigen complex (complement fixation). It can cause lysis of certain types of bacteria, or it can render the target cell more susceptible to phagocytosis – a process called opsonization. The complement reactions attract scavenging white blood cells (phagocytes) to the area of conflict in the body.