Proteins similar to alpha2-macroglobulin (alpha (2)-M). Alpha (2)-M is a major carrier protein in serum. It is a broadly specific proteinase inhibitor. The structural thioester of alpha (2)-M, is involved in the immobilization and entrapment of proteases. This group contains another broadly specific proteinase inhibitor: pregnancy zone protein (PZP). PZP is a trace protein in the plasma of non-pregnant females and males which is elevated in pregnancy. Alpha (2)-M and PZ bind to placental protein-14 and may modulate its activity in T-cell growth and cytokine production thereby protecting the allogeneic fetus from attack by the maternal immune system. This group also contains C3, C4 and C5 of vertebrate complement. The vertebrate complement is an effector of both the acquired and innate immune systems The point of convergence of the classical, alternative and lectin pathways of the complement system is the proteolytic activation of C3. C4 plays a key role in propagating the classical and lectin pathways. C5 participates in the classical and alternative pathways. The thioester bond located within the structure of C3 and C4 is central to the function of complement. C5 does not contain an active thioester bond.
Jiyao W et al.(2023). "The conserved domain database in 2023.",