Chapter 4: Blood, Lymphoid Tissues & Haemopoiesis

The clear, yellowish plasma contains essential components like ions, glucose, hormones, and various plasma proteins, including clotting factors (e.g., prothrombin) and immunoglobulins. Haemostasis, the process of clotting, involves the precipitation of the protein fibrin, initiated by damaged cells and platelets in the presence of calcium ions. Postnatally, all blood cells are produced in the red bone marrow through haemopoiesis, originating from pluripotent stem cells. Erythrocytes, or red blood cells, constitute the vast majority of cells and are anucleate, biconcave discs, containing haemoglobin, a protein tetramer vital for oxygen transport. Variations in haemoglobin chains are responsible for pathologies like sickle-cell disease and thalassaemia. Leukocytes, or white blood cells, are divided into granulocytes (neutrophils, eosinophils, basophils) and mononuclear cells (monocytes and lymphocytes). Neutrophil granulocytes are the most abundant and function as highly motile phagocytes during acute inflammation, relying primarily on glycolytic metabolism and utilizing hydrolytic enzymes in their primary and secondary granules to kill microbes; they can also deploy Neutrophil Extracellular Traps (NETs). Monocytes, the largest leukocytes, are motile phagocytes that differentiate into macrophages and certain dendritic cells upon entering extravascular tissues. Lymphocytes drive adaptive immunity: B cells develop in the bone marrow, mediating humoral immunity by maturing into plasma cells that secrete specific antibodies (immunoglobulins like IgG, IgM, and IgA). T cells, which mature in the thymus, execute cell-mediated immunity, with helper T cells (CD4+) coordinating responses through cytokine secretion, cytotoxic T cells (CD8+) destroying target cells via perforin and granzymes, and regulatory T cells dampening immune activity. Lymphoid tissues are organized into primary organs (bone marrow, thymus) for cell generation, and secondary organs (lymph nodes, spleen, Mucosa-Associated Lymphoid Tissue or MALT) where mature lymphocytes interact with antigen-presenting cells (APCs) to mount a response. Dendritic cells, including Langerhans cells, are considered the most potent APCs, processing antigens and presenting them via MHC molecules to activate naïve T cells, thereby serving as a critical bridge between the innate and adaptive immune systems.