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Category: Medical

Topic: Immunology

Level: EMT

Next Unit: Anaphylaxis

9 minute read

The body has three lines of defense against foreign contaminants--three barriers:

1.) anatomic barriers,

And the two main divisions of the immune system:

2.) the inflammatory response, and

3. ) the immune response.


Anatomic Barriers

Anatomic barriers are skin and mucous membranes, as well as a competitive microbiome, that all work very effectively to keep most foreign material from succeeding in getting inside the body. When there are defects in these protective barriers, the underlying tissue becomes vulnerable to contaminants.

SKIN: Intact skin is a formidable barrier to contaminants and invaders. Besides being a physical roadblock (layers of dead epithelial cells, packed with keratin, overlying stratified epithelium of the epidermis, all over the next layer, the dermis), it contains a synergistic population of microbial flora--the microbiome that resists or compete with foreigners.

THE GI TRACT: The gastrointestinal tract is a portal to the outside world: mouth, throat, esophagus, stomach, intestines. However, it has neutralizing defenses:

  • gastric acid (HCl),
  • pancreatic digestive enzymes, and the GI tract's own microbiome--the
  • gut flora.

THE RESPIRATORY TRACT: Breathing (inspiration) is another direct route into the body for infectious organisms, contaminants, irritants, toxic gases (hydrogen sulfide, ammonia), and chemicals. Its defenses include

  • expulsion via sneezing, coughing;
  • removal via cilia transport;
  • entrapment via mucus which acts as an effective filter,
  • phagocytosis (via macrophages); and
  • cellular and humoral responses (IgA, IgG, and IgM antibodies).

Each ciliated cell has ~250 cilia which undulate in an upward direction ("the mucociliary escalator") to clear any contaminants that have made it past the initial barriers. (Smoking paralyzes the cilia, eliminating an important defense.)

Another mechanism is the flushing action of urine and tears to carry contaminants away. Tears also contained secretory antibodies.


There are two main divisions of the Immune System, which provide a 2nd and 3rd barrier of sorts, after the initial (1st) anatomic barriers:

1.) anatomic barriers, as described above, and the two main divisions of the immune system, elsewhere explored in other units of this section:

2.) the INFLAMMATORY response (the "innate" immune system), and3. ) the IMMUNE response ("adaptive" immunity).


The Immune System: Innate vs Adaptive

A. Innate Immune System—born with, rapid, immediate.

a. Anatomic Barriers: skin, mucus, cilia, keratin.

b. Inflammatory Response: 

i. Leukocytes

  • Phagocytic neutrophils, monocytes, and macrophages
  • Dendritic cells
  • Eosinophils
  • Basophils
  • Mast cells
  • Natural killer cells
  • Innate Lymphoid cells


  • Epithelial cells
  • Platelets
  • Adipocytes

ii. Inflammatory Cytokines: small secreted proteins from many cell types, but mainly from helper T-cells and macrophages.

Pro-inflammatory cytokines from activated macrophages and T-cells create the inflammatory response. (Anti-inflammatory cytokines reverse inflammation and serve as a checks-and-balances against runaway inflammation. 

B. Adaptive Immune System

a. Humoral immunity: "non-cellular," relying on antibodies from B-cells and plasma cells in the blood, plasma, and lymphatic fluid. Macromolecules of antibodies, complement, and antimicrobial peptides/proteins (AMPs).

i. Antibodies: from B-cells, they are assisted by T-cells in maturation and function.

ii. Complement: identifies foreigners and eliminates them via adherence and ingestion ("phagocytosis") and promotes inflammatory and immune responses to these targets. 

iii. AMPs: secreted and circulating pattern-recognizing antimicrobial peptides and proteins. They function to enhance phagocytosis and host protection. They also trigger the complement cascade ("ii," above). 

b. Cell-mediated immunity--interactions of helper and cytotoxic T-cells.