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C1Q complex

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Title: C1Q complex  
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Subject: C-reactive protein, Complement system, Decorin, Primary immunodeficiency, Antibody-dependent enhancement, Immune adherence
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C1Q complex

complement component 1, q subcomponent, A chain
Symbol C1QA
Entrez HUGO OMIM RefSeq UniProt Locus p36.3-34.1
complement component 1, q subcomponent, B chain
Symbol C1QB
Entrez HUGO OMIM RefSeq UniProt Locus p36.3-34.1
complement component 1, q subcomponent, C chain
Symbol C1QC
Alt. symbols C1QG
Entrez HUGO OMIM RefSeq UniProt Locus p36.11

The C1q complex is potentially multivalent for attachment to the complement fixation sites of immunoglobulin.

The sites are on the CH2 domain of IgG and, it is thought, on the CH4 domain of IgM.

The appropriate peptide sequence of the complement fixing site might become exposed following complexing of the immunoglobulin, or the sites might always be available, but might require multiple attachment by C1q with critical geometry in order to achieve the necessary avidity.


C1q is a 400kDa protein formed from 18 peptide chains in 3 subunits of 6. Each 6 peptide subunit consists of a Y-shaped pair of triple peptide helices joined at the stem and ending in a globular non-helical head.

The 80-amino acid helical component of each triple peptide contain many Gly-X-Y sequences, where X and Y are proline, isoleucine, or hydroxylysine; they, therefore, strongly resemble collagen fibrils.


It is assumed that the globular ends are the sites for multivalent attachment to the complement fixing sites in immune complexed immunoglobulin. Patients suffering from Lupus erythematosus often have deficient expression of C1q. C1q may also play a central role in the aging of cells.[1]


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