Basic Immunology

There are four major components of the immune system. These include: 1. mechanical barriers to pathogen entry. 2. the innate immune system. 3. the adaptive immune system. 4. the lymphoid organs. Mechanical barriers include skin and mucous membranes and tight junctions between epithelial cells prevent pathogen entry. Breaches can be iatrogenic, for example, IV lines, surgical wounds, and mucositis, and are a large source of healthcare- associated infections. The innate immune system provides the first internal line of defence, as well as initiating and shaping the adaptive immune response. The innate system comprises a range of responses: phagocytosis by neutrophils and macrophages (guided in part by the adaptive immune system), the complement cascade, and the release of antimicrobial peptides by epithelial cells (e.g. defensins, cathelicidin). The adaptive immune system includes both humoral (antibody- mediated) and cell-mediated responses. It is capable of greater diversity and specificity than the innate immune system, and can develop memory to pathogens and provide increased protection on re-exposure. Immune cells are divided into myeloid cells (neutrophils, eosinophils, basophils, mast cells, and monocytes/macrophages) and lymphoid cells (B, T, and NK cells). These all originate in the bone marrow from pluripotent haematopoietic stem cells. The lymphoid organs include the spleen, the lymph nodes, and mucosal-associated lymphoid tissues—which respond to antigens in the blood, tissues, and epithelial surfaces respectively. The three main ‘professional’ phagocytes are macrophages, dendritic cells, and neutrophils. They are similar with respect to how they recognize pathogens, but differ in their principal location and effector functions. Phagocytes express an array of Pattern Recognition Receptors (PRRs) e.g. Toll-like receptors and lectins (proteins that bind carbohydrates). PRRs recognize Pathogen- Associated Molecular Patterns (PAMPs)— elements which are conserved across species, such as cell-surface glycoproteins and nucleic acid sequences. Though limited in number, PRRs have evolved to recognize a huge array of pathogens. Binding of PRRs to PAMPs enhances phagocytosis. Macrophages are tissue-resident phagocytes, initiating and co-ordinating the local immune response. The cytokines and chemokines they produce cause vasodilation and alter the expression of endothelial cell adhesion factors, recruiting circulating immune cells.