M2

• The function of the immune system is to recognize self from nonself and to defend the body against nonself
• The immune system faces challenges to generate effective protective responses against infectious pathogens
• The immune system meets challenges to optimally perform its protective functions dependent on the rapid and varied responses of immune cells
• Learning Objectives:

  • Contrast non-specific from specific immune responses
  • Explain components/examples/mechanisms of different types of immunity
  • Recall examples of natural and acquired immunity
  • Describe the phylogeny/development of the immune system
  • Construct the architecture of lymphoid tissues and identify sites of lymphocyte storage, antigen processing and presentation, and antibody production
• Immunity is derived from the Latin word "immunitas," historically meaning protection from disease and infectious disease
• Immunity refers to protection from legal prosecution offered to Roman senators during their tenures in office
• Immunity constitutes the immune system, and their collective response to foreign substances is called the immune response
• Body defenses:

  • First line of defense includes unbroken skin and mucosal membrane surfaces
  • Secretions like mucus, sebum, lactic acid, and earwax provide defense against microbial invasion
  • Acidic fluids in the stomach, intestinal tract, and vagina can destroy infectious microorganisms
  • Tears and saliva contain lysozyme and Immunoglobulin A (IgA) antibody for defense
• Natural immunity is nonadaptive, nonspecific, and present in all individuals to resist infection
• External defense system includes skin, mucosal membranes, and secretions discouraging microorganism growth

  • Internal defense system involves lymphocytes and complement to recognize unique molecules of infectious organisms
• Adaptive immunity is specific to individual pathogens, remembers prior exposure, and increases response upon repeated exposure
• Innate and adaptive immunity are essential for good health and operate together for maximal effectiveness
• Humoral immunity:

  • Mediated by antibodies produced by B lymphocytes
  • Antibodies recognize microbial antigens, neutralize infectivity, and target microbes for elimination
• Cell-mediated immunity:

  • Mediated by T lymphocytes
  • Defense against microbes inside phagocytes and infected cells to eliminate reservoirs of infection
• Active immunity is acquired through natural exposure, vaccination, or intentional injection of antigen
• Vaccination stimulates antibody production and memory without contracting the disease
• Passive immunity is acquired by transferring antibodies from an immunized individual to confer immunity on another individual
• Soluble effector molecules of innate immunity bind to microbes, act as opsonins, enhance phagocytosis, and promote inflammatory responses
• Major components of the humoral innate immune system are the complement system, collectins, pentraxins, and ficolins
• Pentraxins like C-reactive protein (CRP) and serum amyloid P (SAP) activate complement and increase during infections and inflammatory stimuli
• Acute phase reactants are plasma proteins elevated in the blood during acute inflammatory reactions
• Liver synthesis and plasma levels of several proteins, including SAP, increase in response to IL-1 and IL-6
• Increased production of acute phase reactants is part of the acute phase response to infection and other insults
• PTX3 is an acute phase reactant produced by several cell types in response to TLR ligands and inflammatory cytokines
• PTX3 is stored in neutrophil granules and released as neutrophils die
• PTX3 recognizes various molecules on fungi, certain bacteria, viruses, and apoptotic cells
• PTX3 activates the classical complement pathway
• Collectins and Ficolins are soluble effector molecules in the innate immune system
• Members of this family include mannose-binding lectin (MBL) and pulmonary surfactant proteins SP-A and SP-D
• MBL functions as an opsonin by enhancing phagocytosis of microbes
• SP-A and SP-D are collectins with lipophilic properties
• SP-A and SP-D maintain alveoli expansion and mediate innate immune responses in the lung
• SP-A and SP-D bind to various microorganisms, act as opsonins, inhibit bacterial growth, and may activate macrophages
• Ficolins are structurally similar to collectins and possess a collagen-like domain
• Ficolins are opsonins and activate complements similar to MBL
• Lymphoid Tissue
• Lymphoid organs are where immune cells undergo maturation, differentiation, and proliferation
• Cells of both innate and adaptive immunity are derived from self-renewing precursor cells
• Myeloid progenitor cells give rise to neutrophils, monocytes, basophils, and eosinophils
• Lymphoid progenitor cells give rise to naïve mature B cells and precursor T cells