+C5b is a protein fragment, formed from the cleavage of [C5](/wiki/C5) within the [complement system](/wiki/Complement_system). It acts as the cornerstone, initiating the assembly of the [membrane attack complex](/wiki/Membrane_attack_complex) (MAC) on pathogen surfaces, a critical step in immune defense.
+## See also
+- [Complement System](/wiki/Complement_system)
+- [MAC](/wiki/MAC)
+- [Immune System](/wiki/Immune_system)
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