Chapter 52: Plasma Proteins & Immunoglobulins

Plasma Proteins & Immunoglobulins summary from Harper’s Illustrated Biochemistry delves into the structural and functional diversity of plasma proteins and immunoglobulins, which are essential for maintaining homeostasis and orchestrating host defense. The text begins by outlining the multifaceted roles of blood, including respiration, nutrition, excretion, and the regulation of water balance and body temperature. A significant portion of the discussion focuses on albumin, the most abundant plasma protein synthesized by the liver, which is critical for transporting ligands like fatty acids and maintaining intravascular osmotic pressure via Starling forces; deficiencies here can lead to edema and analbuminemia. The chapter explores the acute-phase response, where inflammatory cytokines like IL-1 and IL-6 trigger hepatocytes to increase the synthesis of reactants such as C-reactive protein (CRP) and alpha1-acid glycoprotein. A major thematic focus is iron homeostasis, detailing how transferrin transports iron, ferritin stores it intracellularly, and haptoglobin protects the kidneys by binding extracorpuscular hemoglobin. The regulation of iron is meticulously explained through the interaction of iron regulatory proteins (IRPs) with iron response elements (IREs) on mRNA, and the systemic control exerted by the peptide hepcidin, which induces the degradation of the cellular iron exporter ferroportin. Clinical correlations are highlighted, including hereditary hemochromatosis caused by HFE mutations, and Wilson disease, where copper accumulation results from ceruloplasmin deficiency. The summary also covers protease inhibitors like alpha1-antiproteinase, the primary inhibitor of neutrophil elastase, noting how its genetic deficiency or oxidation by cigarette smoke contributes to emphysema. Furthermore, the text distinguishes between the innate and adaptive immune systems, describing the structure and function of immunoglobulins (IgG, IgA, IgM, IgD, IgE). It explains how antibody diversity is generated through combinatorial gene rearrangement, junctional diversity, and somatic hypermutation driven by activation-induced cytidine deaminase (AID). Finally, the chapter elucidates the complement system, describing how the classical, lectin, and alternative pathways converge to form the membrane attack complex (MAC) to eliminate pathogens, while also touching upon amyloidosis and the production of monoclonal antibodies via hybridoma technology for research and therapy.