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The innate immune response is a general response your immune system has against foreign invaders. It involves a number of different special cells.

In this post, we’ll cover those cells and the roles they play.

Topic Outline:

• Phagocytes
• Macrophages
• Neutrophils
• Monocytes
• Mast Cells
• Eosinophils
• Basophils
• Natural Killer Cells
• Dendritic cells

Types of Immune Responses

When something foreign enters your body, whether it’s a bacteria, virus, or another pathogen, your immune system attempts to get rid of it.

There’s a quick initial response that is a nonspecific response. That is, if it doesn’t belong to you, we want it gone.

Then, there’s a slower response that does care what the specific antigen is. In fact, its response is specifically tailored to specific pathogens.

For the purpose of this post, we’re going to focus on the nonspecific response and the different cells that are involved in this process.

Phagocytes

Phagocytes are cells that can engulf foreign particles. The word phagocyte literally means “eating cell.”

Phagocytes eat the potentially harmful substance, that is, they take it inside themselves to destroy it.

There are two primary phagocytes namely macrophages and neutrophils.

Macrophages

Macrophages are phagocytes that have irregular shapes.

They have the ability to squeeze through capillary walls to leave the circulatory system. Because they can do this, they can roam through the tissues, and when they encounter a pathogen, they eat up the pathogen using a process called phagocytosis.

Depending on where you find these macrophages in the body, you might see them called different names:

• Alveolar macrophages – found in the lungs.
• Kupfer cells – found in the liver.
• Histiocytes – found in the connective tissue.

These cells also have another function. They can release cytokines (signaling molecules), which will attract other immune cells to the area.

Picture the macrophages like security guards in a mall. Let’s say someone’s trying to cause damage in the mall or trying to steal something. They catch that person and try to deal with them. They can also use walkie-talkies to call other guards and even the police if necessary.

That’s like the macrophages first doing what they do – phagocytosis – and then, sending out the cytokines to bring in reinforcements.

Neutrophils

Neutrophils are granulocytes. They have granules inside their cytoplasm. These granules are containers or vesicles that contain specific substances.

In the case of the neutrophils, they have things like histamines and other substances. Neutrophils are very good at dealing with bacteria and fungi.

You can think of these as police officers. Signals from security guards (macrophages) call them in. These neutrophils have receptors on their surfaces that can detect cytokines and other chemicals released from other cells.

Then, they’ll respond by coming into the area, eating up stuff, and releasing histamines to trigger inflammation and other substances like antimicrobials to fight against bacteria.

Monocytes

Monocytes are cells that aren’t fully developed. In fact, they are precursor cells that can differentiate and become either a macrophage or a dendritic cell. They are circulating in the blood and depending on the signals they receive when there is some kind of pathogen, they can become what they need to become to best help the situation and try to get rid of that potentially harmful substance.

Mast Cells

You’ll find Mast cells in connective tissues and also in mucous membranes. They play a crucial role when there’s a cut by helping with the healing process and protecting against pathogens.

When you get a cut, mast cells in that area will activate and release cytokines and other substances that cause inflammation. They release histamine which causes inflammation.

This gets more blood to the area to get more of the immune system cells there to help fight off infection and promote healing.

Eosinophils

Eosinophils are involved in fighting against parasites and bacteria. They are also active when dealing with allergic reactions.

They are granulocytes and have substances in their granules that are toxic to bacteria and parasites.

When eosinophils release their granules into the tissues to fight against a parasite (or something else), those chemicals can also cause damage to our own tissues. This is the reason why the process of releasing granules needs to be regulated.

Basophils

These are also granulocytes. They have histamine in their granules, which causes inflammation, especially when dealing with allergic reactions.

Natural Killer cells (or NK cells)

NK cells are different in that they don’t directly attack the pathogen. Instead, they detect when a cell has been infected with a pathogen, and then they trigger a process called apoptosis (cellular death).

They have granules inside them that release substances like perforins and granzymes.

The perforins will perforate the cell wall making holes throughout the membrane, and the granzymes enter the cell to trigger apoptosis killing the cell.

This works very well when a cell has been infected with a virus.

Dendritic cells

Dendritic cells are also called antigen-presenting cells. They are in tissues that are in direct contact with the external environment like the skin, the lining of the nose and lungs, the stomach, and the intestines.

Since they’re exposed to the external environment, they often identify pathogens and present them to the rest of the immune system, triggering a nonspecific inflammatory response.

It’s like they take up the bad stuff and present it to all the other cells saying, “Hey, this is bad stuff. Kill it.”

But they’ll also present them to B and T cells to trigger a specific immune response.

Because of this, they are often seen as a bridge between innate and adaptive immune responses.

What’s Next?

So those are the cells that are involved in innate immunity.

In the next lesson, we’ll dig into adaptive immunity a little bit more. Specifically, we’ll start by looking at the T cells and how they function.

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