Q.23 A 72-yeаr-оld mаn presents with cоugh, severe shоrtness of breаth, and fever. His oxygen saturation is 85%, and the chest x-ray shows bilateral infiltrates. PCR confirms infection with SARS-CoV-2. His serum C-reactive protein (CRP) concentration is 206.3 mg/L (reference range: 0.0–5.0 mg/L). Pentraxins (such as CRP) and antibodies share several effector functions: both can opsonize pathogens, activate the C1q-dependent classical complement pathway, and bind Fc receptors on immune cells. However, pentraxins are innate pattern-recognition molecules, whereas antibodies are adaptive immune molecules. Which mechanism underlying antigen receptor diversity or specificity is unique to antibodies and not shared by pentraxins?