Immune System

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...