Clinical meaning
The immune system operates through three sequential lines of defense. The first line consists of intrinsic (innate) barriers: physical barriers (skin epithelium, mucous membranes, cilia), mechanical barriers (peristalsis, urinary flow, mucociliary escalator), chemical barriers (gastric acid pH 1.5-3.5, lysozyme in tears/saliva, defensins, sebum), and microbiological barriers (commensal flora competing with pathogens for nutrients and adhesion sites).
The second line of defense comprises innate immunity: phagocytes (neutrophils, macrophages, dendritic cells) that recognize pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (PRRs) including Toll-like receptors (TLRs). Natural killer (NK) cells destroy virus-infected and tumor cells via perforin-granzyme pathway and antibody-dependent cellular cytotoxicity (ADCC). The complement system (classical, lectin, and alternative pathways) generates opsonins (C3b), anaphylatoxins (C3a, C5a), and the membrane attack complex (MAC, C5b-9). Interferons (IFN-alpha, IFN-beta) establish antiviral states in neighboring cells. Fever enhances immune cell function and inhibits pathogen replication.
The third line of defense is adaptive (acquired) immunity, characterized by specificity, diversity, memory, and self/non-self discrimination. Humoral immunity is mediated by B lymphocytes that differentiate into plasma cells producing antigen-specific antibodies. Cell-mediated immunity is driven by T lymphocytes: CD4+ T-helper cells coordinate immune responses through cytokine secretion, while CD8+ cytotoxic T cells directly kill infected cells via perforin-granzyme pathway and Fas-FasL apoptosis signaling.
T-helper cell subsets drive distinct immune responses: Th1 cells (IL-2, IFN-gamma, TNF-beta) activate macrophages and promote cell-mediated immunity against intracellular pathogens. Th2 cells (IL-4, IL-5, IL-13) promote B-cell antibody class switching to IgE and eosinophil recruitment, driving allergic and antiparasitic responses. Th17 cells (IL-17, IL-22) recruit neutrophils and are critical for mucosal defense against extracellular bacteria and fungi. Regulatory T cells (Tregs, CD4+CD25+FoxP3+) suppress excessive immune activation and maintain self-tolerance.