Immunology Overview

Immunity System Overview

• Immunity is a state of protection from infectious diseases.
• The immune system is a versatile defense system that protects animals from invading pathogenic microorganisms and cancer.
• It generates a variety of cells and molecules capable of specifically recognizing and eliminating foreign invaders.

Innate Immunity

• Innate immunity is an immediate response to pathogens and represents the first line of defense.
• It is not directed against any particular pathogen and lacks immunologic memory.
• Components of innate immunity include:
  • Anatomical barriers (skin, epithelium, and mucous membranes)
  • Humoral components (complement, coagulation system, cytokines, lysozymes, and lactoferrin)
  • Cellular components (neutrophils, monocytes, macrophages, NK cells, and eosinophils)

Physiological Barriers

• Skin: few microorganisms can penetrate intact skin, but many can enter through sweat or sebaceous glands and hair follicles.
• Mucous membranes: bacteria tend to stick to a film of mucous, which is constantly being driven upward by ciliated cells toward the natural orifices.

Mechanisms of Innate Immunity

• Cellular components:
  • Phagocytic leukocytes (phagocytosis and intracellular killing)
  • Macrophages (phagocytosis, intracellular killing, extracellular killing, tissue repair, and antigen presentation)
  • NK cells and eosinophils (killing of virus-infected and altered self targets, and killing of certain parasites)
• Humoral components:
  • Complement (lysis of bacteria and some viruses, opsonin, increase in vascular permeability, and recruitment of phagocytic cells)
  • Coagulation system (increase in vascular permeability and recruitment of phagocytic cells)
  • Lactoferrin and transferrin (compete with bacteria for iron)
  • Cytokines (various effects)

Phagocytosis

• Phagocytosis involves:
  • Extension of pseudopodia to engulf foreign material
  • Fusion of pseudopodia to trap the material in a phagosome
  • Fusion of the phagosome with a lysosome to create a phagolysosome
  • Digestion
  • Exocytosis of digested contents
• Phagocytic cells increase in number during infections, and their main functions include chemotaxis, migration, ingestion, and microbial killing.

Adaptive Immunity

• Adaptive immunity is specific, repeated exposure leads to augmented memory, and responses are faster, more vigorous, and longer-lasting.
• Components of adaptive immunity include:
  • Cells (cell-mediated immunity)
  • Soluble factors (humoral immunity)

Cardinal Features of Adaptive Immune Responses

• Specificity: individual lymphocytes express membrane receptors that distinguish subtle differences in structure between distinct antigens.
• Diversity: the lymphocyte repertoire can discriminate 10^9 to 10^11 distinct antigens.
• Memory: exposure to foreign antigen enhances the immune system's ability to respond again to that antigen.

Interactions between Innate and Adaptive Immunity

• Innate immunity holds off disease until adaptive immunity develops specific memory.
• Failure of innate immunity leads to disease, and adaptive immunity provides recovery and specific memory against second infections.