Complement component 4 (C4), in humans, is a protein involved in the intricate complement system, originating from the human leukocyte antigen (HLA) system. It serves a number of critical functions in immunity, tolerance, and autoimmunity with the other numerous components. Furthermore, it is a crucial factor in connecting the recognition pathways of the overall system instigated by antibody-antigen (Ab-Ag) complexes to the other effector proteins of the innate immune response.C4d is a degradation product of complement activator. If C4d is high in the tumor of patients, its survival period will be reduced; Complement fragment C4d can be used as a biomarker for early diagnosis and prognosis prediction of lung cancer patients.

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