Which of the following laboratory findings is most suggestive of activation of the alternate complement system rather than the classic complement system?
Serum C2 Serum C3 Serum C4A. Decreased Normal Normal
B. Normal Decreased Normal
C. Normal Normal Decreased
D. Decreased Normal Decreased
Answer
B. Normal _Decreased _Normal
Cascade of plasma
proteins whose basic function is the direct lysis of cells, attraction of
leukocytes to sites of inflammation (chemotaxis), and activation of leukocytes
The complement
system can be activated by one of two basic pathways.
·
The
classic pathway is initiated by antigen-antibody (immune) complexes binding to
C1. The antibodies that are involved in forming these complement-activating
immune complexes are IgM and IgG (subtypes 1, 2, and 3). There are also some
non-immunologic activators of the classic complement pathway, such as urate
crystals, which may be part of the pathophysiologic process of gout.
·
In the
alternate pathway, the early complement components (C1, C4, and C2) are
bypassed and C3 is activated directly by such things as bacterial endotoxins,
cobra venom factor, lipopolysaccharide, and aggregated immunoglobulin (mainly
IgA, but also IgE). C3 nephritic factor is an unusual substance capable of
activating the alternate complement system within the glomerulus, producing
glomerular injury.
Complement
assays can be used clinically to help determine the causes and pathomechanisms
of certain diseases. For example, activation of the complement cascade can
produce local deposition of C3, which can be seen with special histologic
techniques. If a patient has widespread activation of the complement system,
then serum assays of C3 levels might be decreased. In particular, activation of
the classic complement pathway decreases levels of the early complement
components, namely C1, C4, and C2. In contrast, activation of the alternate complement pathway, which bypasses these early complement components, decreases levels of C3, but the levels of the early factors (C2 and C4) are normal
