In the alternative pathway, in the presence of C3b, factor B is cleaved by factor __
into two f____nts, Bb and Ba. The Bb fragment forms a complex with C3_ and the
resultant C3b, Bb complex is known as C3 c_____e. This complex has enzymatic
properties and is capable of cleaving and activating more C_. Properdin (P) acts
as a s___r for the C3b, Bb complex by p___g it from decay and control
mechanisms. Large amounts of C3b are generated and r____y the reaction cycle.
The C3b may release to the fluid phase as an o____n, bind directly to the
activating s____e, or attach to C3 c____e forming the complex, C3b, Bb, 3b. This
complex is known as C5 convertase and is capable of c____g and a___g C5, the
first component of the membrane a____k complex.
The membrane attack complex (MAC), beginning with the cleavage and activation
of C5, is common to both complement p____s. The activation of C5 results in two
f____s. The smaller C5a fragment is released into the fluid phase as an a____xin
or c____ctic factor. The larger C5b fragment binds directly to the a___g s____e,
followed by the binding of C6 and C7 (Figure 2-7). The C5b67 complex provides a
b___ing site for C8 which initiates some m____e d___e. However, the
subsequent binding of multiple molecules of C9 is required for efficient cell l___s.
There are control mechanisms in the complement system that preclude
uncontrolled a____n and c____n of its protein components. For example, binding
sites on activated proteins d___y rapidly, causing dissociation of c____s. This
restricts the complement activation to a l____l a___a.
Several substances are released to produce the biological effects of c____t
For example, anaphylatoxins cause the release of h___e from mast cells and
basophils. Histamine in turn enhances vascular p___y and causes smooth muscle
c_____s, resulting in edema and inflammation.
Many cells, including polymorphonuclear cells, B lymphocytes, and macrophages,
have receptor sites for o____s (C3b and C4b). Also many of the
nonimmunological activators have receptors for these o____s. Opsonin fragments
bind to their receptors and act to facilitate adherence and p____s of the target cell.
The function of the chemotactic factor (C5a) is to induce and direct the migration
and a____tion of phagocytic cells at the site of the immune reaction.
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