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Ch. 15 - Innate Immunity
Chapter 12, Problem MC4

The alternative complement activation pathway involves:
a. Factors B, D, and P
b. The cleavage of C5 to form C9
c. Binding to mannose sugar
d. Recognition of antigens bound to specific antibodies

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1
Understand that the complement system has three activation pathways: classical, lectin, and alternative.
Recall that the classical pathway is triggered by antibodies bound to antigens, which corresponds to option d.
Know that the lectin pathway is initiated by binding to specific sugar molecules like mannose, which corresponds to option c.
Recognize that the alternative pathway is unique because it involves factors B, D, and P (properdin) and does not require antibodies or mannose binding.
Therefore, identify that the alternative complement activation pathway involves factors B, D, and P, which corresponds to option a.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Alternative Complement Pathway Components

The alternative complement pathway is activated spontaneously and involves complement factors B, D, and properdin (factor P). These proteins work together to form and stabilize the C3 convertase enzyme, which amplifies the immune response without the need for antibodies.
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Complement Cascade and C5 Cleavage

The complement cascade involves sequential activation of complement proteins, including the cleavage of C5 into C5a and C5b. While C5 cleavage is crucial for forming the membrane attack complex (MAC), this step is common to all pathways and not unique to the alternative pathway.
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Review of the Complement System

Pathway Activation Triggers

Different complement pathways are triggered by distinct mechanisms: the alternative pathway is antibody-independent and activated by pathogen surfaces; the lectin pathway binds mannose sugars on microbes; the classical pathway requires antigen-antibody complexes for activation.
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