Inhibition of immune precipitation by complement

Area of Science:

• Immunology
• Biochemistry

Background:

• Immune precipitation involves antigen-antibody interactions.
• Complement system activation influences immune complex formation and solubility.

Purpose of the Study:

• To investigate the role of complement in inhibiting immune precipitation of bovine serum albumin (BSA) by rabbit anti-BSA antibody (Ab).
• To elucidate the involvement of classical and alternative complement pathways in immune complex solubility.

Main Methods:

• Immune precipitation assays at 37°C.
• Use of normal human serum, normal rabbit serum, decomplemented serum, and complement-activating reagents.
• Sucrose density-gradient ultracentrifugation to assess complex size.
• Analysis of C3 and C4 fragment deposition on immune complexes.

Main Results:

• Complement inhibited immune precipitation.
• Antibody-excess complexes were more soluble than equivalence complexes.
• Human serum was a more potent inhibitor than rabbit serum.
• Classical pathway initially maintained solubility; alternative pathway became essential later.
• Soluble complexes aggregated after 24 hours and bore C3 and C4 fragments.
• EDTA addition induced slow aggregation of soluble complexes.

Conclusions:

• Complement activation, via both classical and alternative pathways, is critical for maintaining soluble immune complexes.
• The dynamics of complement pathway involvement shift over time during immune complex formation.
• Soluble immune complexes are characterized by specific size and complement fragment deposition.