Chapter 54: White Blood Cells & Immune Defense

White Blood Cells & Immune Defense from Harper’s Illustrated Biochemistry provides an in-depth biochemical analysis of white blood cells (leukocytes), the essential components of the immune system responsible for defending the host against infectious pathogens and managing acute inflammatory responses. It categorizes leukocytes into myeloid and lymphoid lineages, exploring the specific functions of neutrophils, eosinophils, basophils, monocytes, and lymphocytes. The text elucidates the process of hematopoiesis, where stem cell differentiation is regulated by specific growth factors and colony-stimulating factors. A major focus is placed on the molecular mechanisms of leukocyte motility, specifically describing how cells migrate from the circulation into tissues via chemotaxis and diapedesis, a process dependent on interactions between endothelial selectins and leukocyte integrins such as LFA-1 (CD11a/CD18). The summary details the biochemistry of phagocytosis, highlighting the respiratory burst wherein the NADPH oxidase system generates potent reactive oxygen species (ROS) like superoxide and hydrogen peroxide to destroy engulfed microbes. It also explains the role of myeloperoxidase in creating hypohalous acids and the formation of Neutrophil Extracellular Traps (NETs), which utilize decondensed chromatin and citrullinated histones to immobilize parasites. Significant clinical correlations are discussed, including the genetic basis of Chronic Granulomatous Disease, Type 1 Leukocyte Adhesion Deficiency, and the tissue damage resulting from protease-antiproteinase imbalances, such as the unchecked activity of elastase in alpha-1-antitrypsin deficiency. Additionally, the chapter covers cell signaling via cytokines (interleukins, interferons, chemokines) and lipid mediators (leukotrienes, prostaglandins), concluding with an overview of adaptive immunity involving antigen presentation by the Major Histocompatibility Complex (MHC), antibody production by B cells, and the cytotoxic actions of T cells and Natural Killer cells.