The immune system is a complex set of specialized cells and proteins dedicated to identifying and destroying foreign invaders and altered host cells.
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1.1 Innate Immune System The immune system’s first line of defense against invading pathogens is a set of barriers: cellular, anatomical, and chemical. Collectively referred to as the innate immune defenses, these barriers have: • No specificity: They produce the same response to a wide variety of stimuli. • No regulatory control: They act reflexively, with or without regulation by other cells. • No memory: Their performance is not improved by previous stimulation. • Limited diversity: They have a limited repertoire of functions. 1.2 Acquired Immune System Once innate barriers have been breached, the immune system’s second line of defense is activated. Acquired (or adaptive) immunity, as this second line of defense is ca lled, consists of lymphocytes and antigen-presenting cells. In contrast with the innate immune defenses, acquired immune responses have: • Specificity: They produce a specific response to specific stimuli.
• Regulatory control: Their response to stimuli is tightly regulated. • Immunologic memory: Previous stimulation improves their performance. • High diversity: They possess a wide variety of specificities. Although historically it was popular to divide innate and acquired immunity into separate systems, it is clear that t hey work together as an orchestrated whole in the protection of the body from disease. While innate immunity responds to foreign invaders first, it does not operate in a vacuum. Instead, it receives important amplifying signals from acquired immunity through the following: • The cytokines of innate phagocytes are necessary for the activation ofT lymphocytes (the “generals” of the acquired immunologic “army”). • The cytokines ofT lymphocytes stimulate phagocytes to become “angry phagocytes” which are more effective at phagocytosis and intracellular killing. • The products of differentiated B lymphocytes, ant ibodies, increase the speed with which phagocytes engulf foreign invaders via a process known as opsonization. • Antibodies secreted by plasma cells activate complement (a serum protein) to enhance phagocytosis and attract inflammatory cells.
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