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Chapter 48

The Immune system

The functions of the immune system

  • Provides defense against pathogens
    • Pathogens are disease causing organisms
    • Prokaryotes and eukaryotes
  • Defense against cancer by recognizing and eliminating grossly abnormal cells: immune surveilance

Cells of the immune system

  • Leukoctyes - general term for the cells of the immune system
    • White blood cells
    • Derived from a common precursor in the bone marrow
    • Circulate in the blood and lymphatic fluids to tissues all around the body
  • Granulocytes
    • Contain visible granules in their cytoplasm
    • Secrete chemicals that attack pathogens or that change the behavior of host tissues near a site of infection
    • Includes basophils, mast cells
    • Also includes eosinophils and neutrophils, which are also phagocytic
  • Phagocytes
    • Phagocytose foreign marterial or debris from dead infected cells
    • Includes macrophages, monocytes, basophils, and dendritic cells
    • Neutrophils and eosinophils are also phagocytic, but technically eosinophils
      • Technically granulocytes if you categorize based on appearance, but highly phagocytotic
  • Lymphocytes
    • Recognize specific pathogens or cells that have been infected by a pathogen
    • Includes B cells, T cells, and NK cells

The immune system

Innate immunity

  • First line of defense
    • Anatomical barriers (e.g. epithelial coverings)
    • Physiological barriers (e.g. secretions)
    • Promotion of non-pathogenic microbes
  • Second line of defense
    • Granulocytes, phagocytes, and NK cells
    • Inflammatory response
    • Complement system
      • A series of proteins secreted the liver that circulate in the blood and lymphatic fluid and get activated by the signals that lymphocytes secrete near sites of infection
  • Not specific to the type of infection

Adaptive (acquired) immunity

  • Third line of defense
    • More targeted, more specific
    • B cells produce and secrete antibodies that target extracellular pathogens
    • Cytotoxic T cells target and kill infected cells to limit the ability of pathogens to reproduce
    • Helper T cells are necessary for the activation of the cells above

There is lots of cross over between the two "divisions" of the immune system, including NK cells and dendritic cells. NK cells and dendritic cells are technically part of the adaptive immune system because they activate B cells and T cells, but they work with the innate immune system to help determine what B cells and T cells to activate.

Innate immune system

Always on, but very generic.

Components of the innate immune system

First line of defense

  • Anatomical, physiological, structural aspects of tissues that act as a barrier to infection

  • Tissues in our body that are exposed to the environment are all epithelial tissues
    • Epithelial tissues are composed of lots of cells packed closely together with tight cell-cell contact, tight junctions, gap junctions, desmosomes, etc sitting on the basal lamina
      • These are multi-layered
    • The epidermis is a multi-layered epithelium.
    • All the outermost cells are dead in skin
      • This is why we are constantly shedding
      • A virus can't infect a dead cell, therefore, this helps as a barrier to infection
  • Physiological barriers to infection include different secretions that are secreted by epithelial tissues that make the environment unfavorable to any pathogens that might be nearby
    • Mucus traps any potential pathogens before they reach tissue, and this mucus is either expelled from the body or degraded in the digestive system
    • Tears include lysozyme, an antibacterial enzyme
    • The nasal passages are lined with mucous secretions and hairs that trap pathogens
    • The digestive tract helps dispose of pathogens that are trapped in saliva and mucus and then swallowed. Most are destroyed by the low pH of the stomach.
Second line of defense
  • Mast cells are a type of granulocyte that cause more blood to be delivered to the site of infection
  • Macrophages are very good at recognizing bacterial and viruses in the environment and then phagocytosing those bacteria and viruses
  • Neutrophils will attack pathogens both inside and outside the cell
  • The inflammatory response is one way these innate immune cells work together in response to infection
  • Platelets try to clot to seal cuts in the skin, which are activated via exposure to the air
  • If pathogens sneak in before the clotting process finishes, they are in an environment that is attractive to immune cells, which secrete more chemicals that attract more immune cells
    • These cells attack the pathogens
    • Mast cells secrete histamine
      • Cause nearby blood vessels to dilate and become more permeable
  • How do the phagocytic leukocytes carry out phagocytosis
    • The bacterium is attached to the membrane evaginations called psudopdia
    • Bacterium is ingested, forming phagosome
    • Phagosome fuses with lysosome
    • Bacterium is killed and then digested by lysosomal enzymes
      • Low pH and hydrolases from the lysosome are used
    • Digestion products are released from cell
  • How do phagocytic leukocytes recognize bacteria and viruses?
    • PAMPs = Pathogen-Associated Molecular Patterns = molecules found on broad categories of pathogens and that are recognized by cells of the innate immune system
      • Bacterial PAMPs include components of bacterial cell walls (e.g. peptidoglyan), bacterial cell membranes (e.g. lipopolysaccharide) or bacterial proteins (e.g. flagellin)
      • Viral PAMPs include dsRNA and some capsid proteins
    • PRRs = Pattern Recognition Receptors = classes of receptors found on innate immune cells and that recognize a particular category of PAMP.
      • Example: Toll-like receptors (TLRs)
  • Activation of the complement system
    • Composed of about 30 different proteins (most secreted by cells of the liver) that circulate in the blood and are activated in specific regions in response to signals from the innate or adaptive immune cells that have encountered pathogens
    • When activated, the complement system proteins:
      • promote more efficient phagocytosis of viruses or bacteria by phagocytes
      • trigger an inflammatory response by attracting neutrophils and macrophages to the area
      • form pores in the cell walls of bacteria, rupturing them
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