Clinical meaning
The immune system is a complex network of cells, tissues, organs, and chemical mediators that protects the body from pathogenic organisms (bacteria, viruses, fungi, parasites), abnormal cells (cancer), and foreign substances. It is broadly divided into two interconnected systems: innate (nonspecific) immunity and adaptive (specific) immunity. Innate immunity provides the first and second lines of defense and is present from birth. The first line of defense consists of physical and chemical barriers: the skin (intact epidermis with its acidic pH of 4-6 and antimicrobial peptides called defensins), mucous membranes (trapping pathogens in sticky mucus), secretions (lysozyme in tears and saliva, hydrochloric acid in the stomach, sebum on the skin), and normal flora (commensal bacteria that compete with pathogens for nutrients and attachment sites). The second line of defense activates when pathogens breach the physical barriers and includes phagocytic cells (neutrophils, monocytes/macrophages), natural killer (NK) cells, the inflammatory response, the complement system, and fever. Inflammation is a nonspecific protective response characterized by the five cardinal signs: rubor (redness), calor (heat), tumor (swelling), dolor (pain), and functio laesa (loss of function). The inflammatory cascade begins when tissue damage triggers the release of chemical mediators including histamine (from mast cells and basophils), prostaglandins, leukotrienes, and cytokines. These mediators cause vasodilation and increased capillary permeability, allowing plasma proteins and white blood cells to migrate to the site of injury (diapedesis). The complement system consists of approximately 30 plasma proteins that activate in a cascade fashion, resulting in opsonization (marking pathogens for phagocytosis), chemotaxis (attracting phagocytes), and formation of the membrane attack complex (MAC) that lyses pathogen cell membranes. Fever is initiated when pyrogens (from pathogens or from the body's own immune cells) stimulate the hypothalamus to raise the thermoregulatory set point, creating an environment less favorable for pathogen replication and enhancing immune cell function.