Innate Immunity: The First Functional Division in the Immune System


By Barry C. Fox, M.D., University of Wisconsin

Innate immunity is an intricate collection of specialized cells, tissues, and molecules working together to protect the body from infectious diseases. When the immune system is working well, the body stays healthy; when it’s not, the body may become susceptible to deadly infections, or even develop autoimmune disorders.

An illustration of lymphocytes.
Lymphocytes are a type of white blood cell. They are produced in the thymus. (Image: Design_Cells/Shutterstock)

Components of the Immune System

There are four structural components of the immune system. This includes the thymus, which is a specialized organ in front of the heart and behind the breastbone. It’s responsible for producing T-lymphocyte white blood cells, where T stands for thymus. 

An illustration of lymph nodes.
Lymphocytes pass through the lymph system to reach all parts of the body. (Image: sciencepics/Shutterstock)

The second component is the bone marrow. Three different types of cells are produced in the bone marrow—red blood cells, blood-clotting elements (platelets), and two different types of white cells. The marrow is in the center of the bones. 

The third, component is the spleen on the left side of the abdomen. It synthesizes proteins known as antibodies and also gets rid of antibody-coated bacteria and old red blood cells.

Lastly, there is the lymph system, which is part of the circulatory system. It’s made up of a network of small vessels that carry a clear fluid called lymph. It works in tandem with lymphoid organs, especially lymph nodes. Lymphocyte immune cells are passed through this system and converge in the lymph nodes throughout the body.

This is a transcript from the video series An Introduction to Infectious Diseases. Watch it now, on Wondrium.

The First Innate Line of Defense

There are certain built-in, hence, innate immune features that help protect the body when under attack. First, there are physical barriers to invasion from microbes, including the skin and mucous membranes. These defenses do not target specific germs but are all-purpose. Having intact skin and membranes helps prevent foreign invaders from entering the body. 

For example, if there’s a break in the skin, thousands of normal staphylococci or streptococcal germs on the surface of the skin can breach the skin layer. The germs enter the subcutaneous tissue and cause a skin infection or cellulitis. Or worse, they can reach the blood vessels and cause an infection of the bloodstream.

Another example is when mucous membranes are breached enabling diphtheria to destroy throat membranes. Some viruses disrupt the lining of the respiratory tract causing viral bronchitis or even viral pneumonia. 

Other biological barriers include stomach acid, mucus, and tears. Tears and mucus contain an enzyme called lysozyme that can dissolve bacteria. Another barrier, saliva, also has antibacterial and antiviral properties. There are also mucus and hair cells, known as cilia, which can trap germs and move them away.

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Inflammation, Another Line of Innate Immunity

The body has a common inflammatory response to invaders or injury. Damaged tissues release substances that initiate the inflammatory process. There are four cardinal signs of the inflammatory response (written in 3000 B.C. in original Latin).

An image of a child's leg suffering from bacterial skin infection.
Inflammation is another line of defense against germs. (Image: DonyaHHI/Shutterstock)

Rubor, or redness, is caused by increased blood flow. Dolor, or pain. Calor, or warmth, is also caused by increased blood flow. Tumor, or swelling caused by the movement of fluid into the area. Therefore, for example, if a body part is infected, it turns red, swollen, and painful. 

It swells because the blood supply increases and the walls of the blood vessels become more porous. This allows infection-fighting white cells to enter the infected tissues. With this swelling comes the pain that one feels since the nerves are compressed. This response occurs within seconds, is short-lived, and importantly, does not involve any long-term memory of the invading germ.

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Fever, the Weapon of Heat against Pathogens

Another innate line of defense involves fever—the rise in body temperature, meaning utilizing the weapon of heat against a pathogen. There are many misperceptions about the benefits or dangers of fever. Fever weakens the ability of bacteria and viruses to copy, creating an unfriendly setting for the invading organisms. 

By turning up the heat, the metabolism and reproduction of invading microbes are skewed. Elevation of body temperature is part of the physiological reaction involving the synthesis of special proteins called endogenous pyrogens. The area of the brain located behind the bridge of the nose (hypothalamus) is pivotal as the thermostat involved in body temperature regulation. 

When there is a foreign stimulus for fever, the immune system secretes endogenous pyrogens, for example, a prostaglandin. When this pyrogen reaches the hypothalamus area, the body’s thermostat gets reset to a higher level.

Some individuals such as alcoholics, the elderly or infirm, and the very young are less capable of generating fever. They may even experience a drop in temperature in response to severe infection. In fact, fevers should be considered less worrisome than they appear—they’re actually beneficial in many instances.

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The Complement System and Different Types of White Blood Cells

There are five types of white blood cells helping the body with the innate immune response, with neutrophils being one. Neutrophils are one of the first lines of defense against foreign invaders and make up the largest number of white blood cells in the body. 

They’re produced in the bone marrow, then position themselves along the margins of the circulatory system and in the bloodstream, acting as a reservoir of infection-fighting cells. What remains of dead neutrophils are known as pus.

Moving on, the last component of the innate immune system is the complement system. This is a series of small protein molecules helping recruit inflammatory cells and enhance germ destruction. They work in tandem with neutrophils and proteins (known as antibodies) to engulf invaders.

So, to summarize, the innate immune system is the initial defense against pathogens and utilizes a variety of tools to accomplish this objective.

Common Questions about Innate Immunity—the First Functional Division in the Immune System

Q: What are the immune system’s components?

There are four structural components that make up the body’s immune system. These elements consist of the thymus, the bone marrow, the spleen, and the lymph system.

Q: How does innate immunity help protect the body?

The innate immunity features that protect the body include physical barriers like skin, inflammatory responses, fever, complement system, and neutrophils. However, these features don’t target a specific germ or disease.

Q: How many types of white blood cells are there in the immune system?

There are five types of white blood cells that help innate immunity function well and respond efficiently to germs. A neutrophil is one type of white blood cell produced largely in the bone marrow and then released into the bloodstream.

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