In Immune complex formation, which structure of a protein is involved?

The modular nature of protein synthesis and folding are embodied in the concept of orders of protein structure. Primary structure is the sequence of the amino acids in a polypeptide chain. Secondary structure is the folding of short (3- to 30-residue), contiguous segments of polypeptide into geometrically ordered units. Tertiary structure is the assembly of secondary structural units into larger functional units such as the mature polypeptide and its component domains. Quarternary structure is the number and types of polypeptide units of oligomeric proteins and their spatial arrangement. In some cases, the proteins are assembled from more than one polypeptide or protomer. Quarternary structure defines the polypeptide composition of a protein, and for an oligomeric protein, the spatial relationships between its subunits or protomers. Monomeric proteins consist of a single polypeptide chain, and are hence, devoid of a quarternary structure. Dimeric proteins consist of two polypeptide chains. Homodimers contain two copies of the same polypeptide chain, while in heterodimers, the polypeptides differ. Ref: Harper’s ‘Illustrated Biochemistry’; 27/e, pg 30, 34