PIC1 PA, a peptide consisting of 15 amino acids, serves as an effective analog of PIC1 that inhibits complement activation mediated by the classical pathway. Functionally, PIC1 PA interferes with the interaction between the C1s-C1r-C1r-C1s/MASPs and the collagen-like region (CLR) of C1q/MBL. It specifically binds to the CLR of C1q, with an average equilibrium dissociation constant (KD) of 33.3 nM when binding purified C1q.