Innate Immunity: Inflammation, Neutrophils & Natural Killer Cells

Innate Immunity: Inflammation, Neutrophils & Natural Killer Cells
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  • 1:16 Inflammation
  • 3:10 Neutrophils
  • 4:31 Marcrophages
  • 5:17 Complement
  • 7:00 Natural Killer Cells
  • 8:00 Eosinophils
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Lesson Transcript
Instructor: Heather Higinbotham
You might already know how your body handles known infectious agents but how about unknown? Watch this video to explore your innate immune system - your body's defense against new bacteria and viruses.

The Innate Immune System

When people think of the immune system, the first thing that comes to mind is often antibodies or vaccines or some other aspect of acquired immunity to an infectious agent that the body has already seen. But how does the body protect itself against potentially dangerous bacteria and viruses that it has never seen before? The immune system needs a way to quickly respond to a variety of unknown invaders, whether they've been encountered before or not, and all of the aspects of the immune system that respond to these unknown threats contribute to our bodies' innate immunity or the ability of the immune system to respond to unknown threats.

The innate immune system defends the body against malignant cells, viruses, and other invaders

The innate immune system is capable of mounting a defense against a variety of different threats, including malignant or cancerous cells, viruses that have already infected a cell, multicellular parasites and any other potential pathogens or foreign invaders that are capable of causing disease that might enter the body through an open wound or body orifice. So, let's take a look at how the innate immune system works.


Let's say that our friend Phil is rock climbing when he cuts himself on a sharp edge. It's not a bad cut, so Phil ignores it, but his immune system certainly doesn't! As soon as Phil sustains even the smallest injury, the potential for infection by a foreign invader dramatically increases. In fact, even with the most minor cuts, thousands, if not millions, of foreign entities, most of them bacteria, will enter the wound. So, whenever there is an injury, it is in the body's best interest to mount an immediate immune response to destroy all foreign invaders before they can establish a foothold.

The first response of the body to a localized injury or infection is inflammation or the immediate response of the body to injury or infection that includes an influx of blood plasma and immune cells to a localized area. Injured tissues release a number of signaling molecules, which cause blood vessel dilation, increase blood vessel permeability, and causes endothelial cells that line the blood vessels to present adhesion molecules on their surface. These adhesion molecules are recognized by immune cells in the blood or white blood cells, which bind to the adhesion molecules and stick to the blood vessel wall. The white blood cells then squeeze themselves between endothelial cells of the now more permeable blood vessels and enter the tissue surrounding the vessel.

The signaling molecules are immune cell chemoattractants that cause the immune cells to exit the blood vessel and migrate to the site of injury. One of the best known inflammatory signaling molecules is histamine, which is capable of dilating blood vessels, increasing blood vessel permeability, and causing immune cells to exit the bloodstream and enter the damaged tissue.


The first immune cells that arrive at an injured site are mostly neutrophils, which are quick-response immune cells that recognize and destroy bacteria. Neutrophils are the most numerous type of white blood cell in the blood, and their job is to immediately respond to inflammation and kill bacteria by phagocytosis, which is the process by which a cell engulfs, destroys and digests another cell, bacteria or cellular debris.

Neutrophils are the first immune cells to arrive at an injured site
Neutrophils and phagocytosis

Neutrophils have several different types of receptors that recognize several different pathogen-associated molecular patterns that are specific to bacteria and not found on human cells. When the receptors bind to the bacteria-specific patterns, the neutrophil captures the bacteria and engulfs it, surrounding it with its plasma membrane. This intracellular compartment is called a phagosome because it is created during the process of phagocytosis. Lysosomes then fuse with the phagosome to add their digestive enzymes, which kill the bacteria and break it down into its component amino acids, nucleic acids and monosaccharides. These nutrients can then be used by the neutrophil or released into the tissue for use by other cells.


Macrophages are another type of immune cell that phagocytizes foreign organisms, dead cells and cellular debris. Macrophages are a little bigger than neutrophils. They live longer than neutrophils and stay in the tissue even after all of the pathogens are gone. They act like a clean-up crew that phagocytizes left over debris, bacteria and dead cells.


So far, most of the components of the innate immune system that we've talked about have been different types of cells, but there are non-cellular components of the innate immune system, too. Complement is a group of serum proteins which mark and destroy foreign microorganisms. Complement proteins are capable of forming a membrane attack complex on plasma membrane surfaces through a series of interactions between the different complement proteins. If the complement proteins are allowed to complete the membrane attack complex, it creates holes in the plasma membrane of its target and destroys it.

Complement proteins form a membrane attack complex on foreign microorganisms
Complement proteins

Complement proteins are very non-specific; they will attempt to form membrane attack complexes on any plasma membrane they can. Now this might cause a problem if complement proteins formed membrane attack complexes on the body's own cells and destroyed them, but our cells don't let this happen.

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