Immunology Overview

Problems with Immunology:

Immunology is not something you can learn quickly or cram before an exam. You need to dedicate time to understanding each aspect for the whole thing to make sense. It is a bit like reading a murder mystery book, lots of the information seems unusual or irrelevant until it is all tied neatly together at the end.

One of the big problems with learning immunology is that each new thing you learn requires an understanding of previous information, that may have seemed irrelevant at the time. What you will find something might not make sense when you are initially learning it, but once you have the context of something you are learning later, it will start to make sense.

So take your time, learn the details that don’t seem to be relevant in the faith that they will become relevant once you understand later information and don’t be afraid to back up and revisit earlier information to piece everything together. I suggest starting with this overview, then going through each topic in detail, then returning to the overview to put it back in to context.

Physical and Chemical Barrier to Infection

The invading pathogen must first get past the physical and chemical barriers.

• Skin
• Mucosa
  • Respiratory tract (with cilia)
  • Gastrointestinal tract
  • Genital tract and urinary system
• Chemical barriers
  • Hydrochloric acid (stomach)
  • Lysozyme (sweat and tears)

Initial Response to Pathogen Invading Tissue

• Complement system activation by pathogen
  • Membrane Attack Complex destroys pathogen
  • C3b is an opsonin
  • C5a attracts cells (neutrophils, monocytes and eosinophils)
  • C3a, C4a and C5a activate mast cell degranulation.
  • C3a and C5a activate eosinophil degranulation.
• Macrophages recognise pathogen and activate the innate immune system
• Dendritic cells pick up antigens and head off to activate the specific immune system

Innate Immune System

• Macrophages recognise pathogen and are activated
  • Start phagocytosis
  • Release cytokines
  • Release interferons
• Interferons inhibit viral entry and replication within cells
• Cytokines recruit help
  • Monocytes are recruited and become macrophages
  • More macrophages are activated
  • Neutrophils are recruited
• Cytokines initiate an Inflammatory Response
  • Vasodilation
  • Increased vascular permeability
  • Localised endothelial cell activation
• Cytokines activate other systems
  • Mast-cell degranulation
  • Clotting system activation
  • Kinin system
• Inflammation leads to an acute phase response, which is where macrophages and neutrophils secrete more cytokines, notably interleukins
  • IL-1 causes fever, lethargy and anorexia
  • IL-6 simulates the liver to produce acute phase proteins (opsonins)
  • IL-8 recruits and activates neutrophils
  • IL-2 and IL-12 activate natural killer cells
  • Tumour necrosis factor alpha (TNF-α) also does all of the above
• Natural Killer Cells are recruited to the tissues
  • Kill virally infected cells (and tumour cells)
  • Produce interferon-gamma (IFN-γ) that further stimulates macrophages

Specific Immune System

• Dendritic cells present antigens to CD4 cells
  • CD4 cells become T helper cells
• T helper cells
  • Activate CD8 cells to become cytotoxic T cells
  • Activate B cells
  • Activate macrophages
• Cytotoxic T cells help destroy infected cells via
  • Cytokines
  • Activating Fas molecule
• B cells when activated either:
  • Differentiate and multiply into plasma cells
    • Secrete antibodies
  • Become memory B cells
    • Wait to be activated during a subsequent infection