Pathophysiology
Clinical meaning
The immune system is a complex network of cells, tissues, and organs that protects the body from infectious agents (bacteria, viruses, fungi, parasites), abnormal cells (cancer), and foreign substances. Immunity is classified into two interconnected systems: innate (non-specific) immunity and adaptive (specific) immunity. Innate immunity provides immediate, non-specific defense through physical barriers (skin and mucous membranes), chemical barriers (stomach acid, lysozyme in tears, antimicrobial peptides), cellular components (neutrophils as first responders, macrophages for phagocytosis, natural killer cells for virus-infected and tumor cells), and the inflammatory response (vascular changes, cellular migration, complement activation). Adaptive immunity develops over days and provides targeted, long-lasting protection through two branches: humoral immunity (B lymphocytes producing antibodies -- immunoglobulins IgG, IgA, IgM, IgE, IgD -- that neutralize pathogens, activate complement, and opsonize targets for phagocytosis) and cell-mediated immunity (T lymphocytes: CD4+ helper T cells that coordinate the immune response, CD8+ cytotoxic T cells that directly kill infected cells, and regulatory T cells that prevent autoimmunity). Immunological memory, mediated by memory B and T cells, provides rapid, enhanced secondary responses upon re-exposure to previously encountered antigens --...