All of the following represent functions of the complement system except

• A. Lysis of foreign cells.
• B. Increase in vascular permeability.
• C. Migration of neutrophils to the tissues.
• D. Decreased clearance of antigen-antibody complexes.

Answer: (D)

The complement system defends by directly attacking some pathogens, promoting inflammation, and helping remove immune complexes. Direct lysis of foreign cells is achieved through the membrane attack complex forming pores in microbial membranes. Anaphylatoxins released during activation (like C3a and C5a) drive inflammation by increasing vascular permeability and by attracting neutrophils to sites of infection. The system also flags immune complexes with C3b to enhance their recognition and clearance by phagocytes. Therefore, the statement about decreased clearance is not a function of the complement system; it actually contributes to clearing immune complexes.