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Immune System Overview

Notes

Immune System Overview

Sections




Here we will learn about the immune system, which combats disease-causing organisms.

Pathogens

Disease-causing or harmful microorganisms

Antigens

Material that can evoke an immune response

Innate Immune Branch

Non-specific, fast

The innate branch is non-specific (it reacts to a broad range of microbes) and is fast (responses occur within hours of infection).

Actors of the Innate System

Physical barriers:

  • Epidermal cells create a slightly acidic surface and release enzymes and other antimicrobial peptides that make the skin surface inhospitable to microbes.

Chemical barriers:

  • Some body surfaces (such as the respiratory pathways) are also covered in mucus, which can trap microbes before they can infect the body.

Chemokines:
Chemical signal produced by damaged cell to alert the body to danger and act as a homing signal for immune cells

Neutrophils:
First type of phagocytic cell to arrive

Monocytes:
Arrive and mature into macrophages which engulf and destroy pathogens

Inflammation:
Response to tissue damage, four clinical signs: redness, heat, swelling and pain.

Innate System in Action

Pathogens gain entry to the inside of the body through the wound.

Chemokine chemical signals (chemotactic cytokines):
Released by damaged cells to alert the body of trouble.
Act as homing signals that stimulate cells of the immune system to migrate towards the source of the chemokines.

Phagocytic cells are then attracted to the wound site from the blood.
Neutrophils, monocytes, macrophages.

Macrophages phagocytose pathogens:
Phagocytosis is a type of bulk transport into the cell in which large extracellular cargo is brought into the cell and broken down.
In our example, the bacterium will eventually be broken down and the macrophage will go on to engulf and destroy other pathogens.

Clinical signs of inflammation
Result from innate immunity activation.
Redness, heat, swelling, and pain are all classically accepted signs of inflammation.
Importantly, inflammation can result in impairment in function.

If the innate branch is unable to fight off the invading pathogens on its own, the adaptive branch is called in to help.

Adaptive Immune Branch

Specific, slow, systemic, memory

The adaptive branch is:
Specific (it can distinguish specific species of pathogens)
Slow (responses occur within days of infection).
Systemic (NOT restricted to the initial site of infection).
Has memory (it mounts a faster and even stronger attack against repeat pathogens).

Actors of the Adaptive System

Adaptive immunity is further divided into humoral immunity and cell-mediated immunity.

Humoral immunity: B cells (matured into plasma cells) producing antibodies (Y-shaped proteins)

Cell-mediated immunity: Cytotoxic T cells recognize infected cells and kill them while helper T cells act as the general of the immune army and release chemical signals that activate various immune cell types

Lymphoid tissue is the home of many types of immune cells.

Plasma cells reside in the lymphoid tissues:

  • Plasma cells are activated and matured B cells (B cells and T cells are the two adaptive immune cell types).
  • Plasma cells pump out antibodies.

Antibodies are Y-shaped proteins that recognize antigens from the invading pathogen.

The antibodies produced by plasma cells are released into the blood vessel where they travel throughout the body.

In the tissues, we show an antibody binding two viruses.

Neutralizing antibodies are able to bind to viruses and toxins in such a way as they are no longer able to infect or cause damage.

The major cells of cell-mediated immunity are the T cells, which are divided into cytotoxic T cells or helper T cells.

Cytotoxic T cells have receptors on their surfaces that bind infection signals on infected host cells.
The cytotoxic T cell has a receptor on its surface that is able to bind the infected cell's signal.
When this happens, the T cell releases death signals which kill the infected cell.

Helper T cells act as the general of the immune army.
They release activation signals (cytokines and chemokines) that help phagocytic cells to perform their job more efficiently.
They also release signals that activate cytotoxic T cells.
B cells also receive activation signals from helper T cells.

With all of these cells working together, the immune system is usually able to destroy the invading pathogens.

Full-Length Text

  • Here we will learn about the immune system, which combats disease-causing organisms.
  • Start a table, so we can understand some key concepts of immunity.
  • First let's start with some key definitions:
    • Pathogens are disease-causing or otherwise harmful microorganisms.
    • Antigens are material that can evoke an immune response.
    • Inflammation is a response to tissue damage.
  • Finally, denote that the immune system is divided into two branches:
    • The innate branch is non-specific (it reacts to a broad range of microbes) and is fast (responses occur within hours of infection).
    • The adaptive branch is:
      Specific (it can distinguish specific species of pathogens)
      Slow (responses occur within days of infection).
      Systemic (NOT restricted to the initial site of infection).
      Has memory (it mounts a faster and even stronger attack against repeat pathogens).

Now, let's see how the immune system works. Split it into the two branches: Innate immunity and Adaptive immunity and we'll start with the innate branch, which responds in a non-selective way to pathogens.

Begin with the first layer of defense: the physical barriers of the body – the skin and mucosal surfaces.

  • Draw a layer of epithelial skin cells.
  • Write that they form a physical, protective layer.
  • Draw a layer of protective chemicals above the skin cells.
  • Write that the epidermal cells create a slightly acidic surface and release enzymes and other antimicrobial peptides that make the skin surface inhospitable to microbes.
  • And that some body surfaces (such as the respiratory pathways) are also covered in mucus, which can trap microbes before they can infect the body.
  • Now, draw pathogens above the skin.
  • Indicate that these can be viruses or bacteria.
  • Show a set of pathogens get blocked from entry by the chemical layer; they CANNOT survive on the skin.
  • Then, show a set get blocked by the physical layer (the skin cells); they can survive on the skin but NOT breach it.

But what happens when the physical barriers are breached?

  • Draw a break in the skin: a wound.
  • Show that pathogens gain entry to the inside of the body through the wound.
  • Then, show a cluster of chemokine chemical signals (chemotactic cytokines).
  • Write that:
    • Damaged cells release them to alert the body of trouble.
    • They act as homing signals that stimulate cells of the immune system to migrate towards the source of the chemokines.
  • To show how phagocytic cells are then attracted to the wound site, draw a capillary within the tissue.
  • Draw a red blood cell flowing through it.
  • Then, draw a representative phagocytic cell flowing through as well: a neutrophil.
    • They are the first phagocytic cells drawn to a wound site by chemokines.
  • Show a monocyte, another type of phagocyte, pass through the capillary wall.
    • Once within tissue they begin to mature into macrophages.
  • Next, draw a macrophage, which patrol and remove cell debris and microbes, and form residence in select body tissues: such as the liver or lung alveoli.
  • Write that phagocytosis is a type of bulk transport into the cell in which large extracellular cargo is brought into the cell and broken down.
  • Now, show a macrophage phagocytosing a bacterium.
    • The bacterium will eventually be broken down and the macrophage will go on to engulf and destroy other pathogens.

Next, let's make a table to illustrate the clinical signs of inflammation, which result from innate immunity activation.

  • Write that redness, heat, swelling, and pain are all classically accepted signs of inflammation.
    • Importantly, inflammation can result in impairment in function.
  • If the innate branch is unable to fight off the invading pathogens on its own, the adaptive branch is called in to help.
  • Indicate that adaptive immunity is further divided into humoral immunity and cell-mediated immunity.
    • We will give a general description of these two concepts here and will discuss them in detail elsewhere.
  • First, for humoral immunity, draw some lymphoid tissue.
    • This type of tissue is the home of many types of immune cells.
  • Within the tissue, draw a plasma cell.
    • Plasma cells are activated and matured B cells (B cells and T cells are the two adaptive immune cell types).
    • Plasma cells pump out antibodies.
  • Draw an antibody, which is a Y-shaped protein that is able to recognize antigens from the invading pathogen.
  • Now draw a blood vessel running alongside the lymphoid tissue.
  • Indicate that the antibodies produced by plasma cells are released into the blood vessel where they travel throughout the body.
  • Now, introduce the antibody into the capillary, as well as in the tissue.
  • Draw an antibody binding two viruses.
    • Neutralizing antibodies are able to bind to viruses and toxins in such a way as they are no longer able to infect or cause damage.
  • The major cells of cell-mediated immunity are the T cells, which are divided into cytotoxic T cells or helper T cells.
  • Draw a body cell infected with viruses.
  • Now draw a cytotoxic T cell close by.
  • Show that the infected cell has a signal on its surface that shows it is infected.
  • Show that the cytotoxic T cell has a receptor on its surface that is able to bind this signal.
    • When this happens, the T cell releases death signals which kill the infected cell.
  • Draw these death signals.
  • Finally, draw a helper T cell.
    • These cells act as the general of the immune army.
  • Indicate that they release activation signals (cytokines and chemokines) that help phagocytic cells to perform their job more efficiently.
  • Indicate that they also release signals that activate cytotoxic T cells.
    • B cells also receive activation signals from helper T cells.

With all of these cells working together, the immune system is usually able to destroy the invading pathogens.

References

  1. Abbas, A. K. & Lichtman, A. H. Basic Immunology: Functions and Disorders of the Immune System, 2nd ed. (Saunders Elsevier, 2006).
  1. Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H. & Matsudaira, P. Molecular Cell Biology, 6th ed. (W. H. Freeman and Company, 2008).
  1. Campbell, N. A. & Reece, J. B. Biology, 7th ed. (Pearson Benjamin Cummings, 2005).
  1. Marieb, E. N. & Hoehn, K. Human Anatomy & Physiology, 10th ed. (Pearson, 2016).
  1. Stewart, G. J. The Immune System. (Infobase Publishing, 2009).
  1. Parham, P. The Immune System, 4th ed. (Garland Science, 2014).
  1. Abbas, A.K., Lichtman, A. H. & Pillai, S. Basic Immunology: Functions and Disorders of the Immune System, 5th ed. (Elsevier, 2016).