In the complement system, which statement is correct?

In the complement system, which statement is correct?

• A. A C3 deficiency would likely be asymptomatic
• B. Antibody is required for the activation of the lectin pathway
• C. C3b is an important opsonin
• D. C9 initiates the membrane attack complex

Answer: (C)

The main concept being tested is how opsonization by C3b tags microbes for phagocytosis and how this fits into complement function. C3b binds to the surface of pathogens and is recognized by receptors on phagocytes, such as CR1, which marks the invader for engulfment and destruction. This tagging is a central, efficient way the immune system smooths immediate clearance and links innate and adaptive responses. If C3 is deficient, the entire cascade that leads to opsonization, inflammation, and MAC formation is impaired, so infections are frequent and serious rather than asymptomatic. The lectin pathway, in contrast, is activated by mannose-binding lectin binding to microbial sugars and activating MASPs without needing antibodies, so antibodies are not required for its initiation. The membrane attack complex is formed when C5b initiates the assembly with C6–C9 to create a pore in the pathogen membrane; C9 is a pore-forming component but does not start the complex on its own. Therefore, C3b as an opsonin is the best-supported statement.

The main concept being tested is how opsonization by C3b tags microbes for phagocytosis and how this fits into complement function. C3b binds to the surface of pathogens and is recognized by receptors on phagocytes, such as CR1, which marks the invader for engulfment and destruction. This tagging is a central, efficient way the immune system smooths immediate clearance and links innate and adaptive responses. If C3 is deficient, the entire cascade that leads to opsonization, inflammation, and MAC formation is impaired, so infections are frequent and serious rather than asymptomatic. The lectin pathway, in contrast, is activated by mannose-binding lectin binding to microbial sugars and activating MASPs without needing antibodies, so antibodies are not required for its initiation. The membrane attack complex is formed when C5b initiates the assembly with C6–C9 to create a pore in the pathogen membrane; C9 is a pore-forming component but does not start the complex on its own. Therefore, C3b as an opsonin is the best-supported statement.