Antigen–Antibody Complexes

InAntigen vertebrates, immunoglobulins (Igs), commonly known as antibodiesAntibody, play an integral role in the armamentarium of immune defenseImmune defense against various pathogens. After an antigenic challenge, antibodiesAntibody are secreted by differentiated B cells called plasma cells. AntibodiesAntibody have two predominant roles that involve specific binding to antigensAntigen to launch an immune responseImmune response, along with activation of other components of the immune system to fight pathogens. The ability of immunoglobulins to fight against innumerable and diverse pathogens lies in their intrinsic ability to discriminate between different antigensAntigen. Due to this specificity and high affinity for their antigensAntigen, antibodiesAntibody have been a valuable and indispensable tool in research, diagnostics and therapyTherapy. Although seemingly a simple maneuver, the association between an antibodyAntibody and its antigenAntigen, to make an antigenAntigen–antibodyAntibody complex, is comprised of myriads of non-covalent interactions. Amino acid residues on the antigenAntigen binding siteBinding site, the epitope, and on the antibodyAntibody binding siteBinding site, the paratope, intimately contribute to the energetics needed for the antigenAntigen–antibodyAntibody complex stability. Structural biology methods to study antigenAntigen–antibodyAntibody complexes are extremely valuable tools to visualize antigenAntigen–antibodyAntibody interactions in detail; this helps to elucidate the basis of molecular recognition between an antibodyAntibody and its specific antigenAntigen. The main scope of this chapter is to discuss the structureStructure and functionFunction of different classes of antibodiesAntibody and the various aspects of antigenAntigen–antibodyAntibody interactions including antigenAntigen–antibodyAntibody interfacesAntigen-antibody interface—with a special focus on paratopes, complementarity determining regionsComplementarity Determining Region (CDR) (CDRs) and other non-CDR residues important for antigenAntigen binding and recognition. Herein, we also discuss methods used to study antigenAntigen–antibodyAntibody complexes, antigenAntigen recognitionAntigen recognition by antibodiesAntibody, types of antigensAntigen in complexes, and how antigenAntigen–antibodyAntibody complexes play a role in modern day medicine and human health. Understanding the molecular basis of antigenAntigen binding and recognition by antibodiesAntibody helps to facilitate the production of better and more potent antibodiesAntibody for immunotherapy, vaccines and various other applications.