The Immune System: What It Is and How It Protects You

Dr. Sandra El Hajj-MSc, N-MD, DHSc

What is the immune system?

The immune system is a complex and large network of organs, cells, proteins, cellular products and chemicals that function together to provide protection to the body from foreign external invaders, such as bacteria, viruses, fungi and toxins. External defenses prevent the pathogens in the environment from entering the body. Internal defenses fight pathogens that have already invaded the body and therefore require more specific mechanisms of defense (1).

The immune system can be divided into two parts: The innate and the adaptive immune system. Both the innate and the adaptive immune systems work together to eliminate pathogens that may cause disease.

The innate immune system

The innate immune system is a defense system that you are naturally born with. This type of immune defense is the first defense mechanism of the body against invaders. The innate immune system is inherited and works immediately from birth to protect newborns from pathogens. Unlike the adaptive immune system, the innate immune system is non-specific and does not have a memory of previous pathogen encounters.

The innate immune system possesses internal and external defenses. An example of the external defenses of the innate immune system would be the protection provided by the skin to constantly prevent the entry of pathogens. Secretions like sebum, cerumen, mucus, tears and saliva trap the pathogens and contain chemicals that can kill pathogens like bacteria. Normal flora that is naturally found in the mucus membranes, such as the intestines and vagina, provides a layer of protection from harmful microbes and prevents them from thriving by competing for essential nutrients needed for survival.

The internal defenses associated with the innate immune system involve cellular functions from natural killer cells that release toxic chemicals to kill intracellular pathogens, phagocytic cells that engulf the microbes that may cause the disease and other inflammatory cells that play a role in defending the body from pathogens.

The adaptive immune system

The adaptive immune system is also known as the acquired immune system, because an individual acquires and develops this immune defense after an original encounter or exposure to microbes or toxic chemicals produced by microbes. This type of defense is considered specific and requires specific cells and chemicals to react to certain pathogens.

What is unique about the adaptive immune system is its capacity to recognize previously encountered pathogens and produce specific defenses against those pathogens by producing antibodies. These antibodies are synthesized by cells known as B lymphocytes after the body has been exposed to the pathogen. The initial encounter of the body with a pathogen initiates the formation of memory cells that will remember the pathogen if it attacks the body again.

What are the parts of the immune system?

The immune system is made up of a complex collection of cells and organs that are interconnected and work together to protect you from pathogenic microbes that may cause diseases. The cells of the innate and adaptive immune systems are made up of various organs of the body, including the adenoids, bone marrow, lymph nodes, lymphatic vessels (a network of channels throughout the body that carries lymphocytes to the lymphoid organs needed to combat pathogens), Peyer's patches (lymphoid tissue in the small intestine), spleen, thymus and tonsils(2).

White blood cells (WBCs)

White blood cells are produced by the bone marrow but may mature in the thymus or within the bone marrow. White blood cells are considered the soldiers of the defense mechanism of the body because these cells constantly search and attack the invading pathogens.

White blood cells play an important part of the immune system. There are many white blood cell types in the immune system that can be found circulating in the bloodstream and throughout the body; or, residing in a particular tissue, waiting to attack the invading pathogens.

Each cell type has a specific role in the body's defense system. For instance, neutrophils are useful for bacterial defense because they engulf and destroy the bacteria, while lymphocytes are specific for viral pathogens that are intracellular and therefore cannot be detected by neutrophils.

White blood cells can originate from myeloid stem cells or the lymphoid stem cells.

• Myeloid stem cells produce monocytes and the granular leukocytes: Eosinophils, basophils, and neutrophils.
• Lymphoid stem cells give rise to T lymphocytes and B lymphocytes.

T lymphocytes

T lymphocytes, also commonly known as T cells, are cells involved in fighting specific pathogens in the body. T cells may act as helpers of other immune cells as antigen-presenting cells or attack pathogens directly.

B lymphocytes

B lymphocytes, known as B cells, are cells involved in fighting specific pathogens in the body. Once B cells have been activated by contact with a pathogen, they form plasma cells that produce antibodies to neutralize the pathogens until other immune cells can destroy them. After an infection, memory B cells persist in the body to quickly produce antibodies to subsequent infection by pathogens expressing the same antigen.

Natural killer cells, also known as NK cells, are lymphocytes that are able to respond to a wide range of invading pathogens and cancerous cells. NK cells reside in the lymph nodes, spleen and red bone marrow where they fight most types of infection.

Lymph nodes

Lymph nodes are small glands distributed throughout the body that function to filter and destroy pathogens to prevent more serious diseases. Lymph nodes contain immune cells that constantly seek for foreign invaders such as the neutrophils and lymphocytes, to fight off that particular invader. Hundreds of lymph nodes are found in different parts of the body, including the neck, armpits, and groin. Inflammation of the lymph nodes is an indication that a foreign pathogen has invaded and that the immune cells are currently fighting and destroying the pathogens.

Spleen

The spleen is the largest organ of the lymphatic system and is located above the stomach under the ribcage. It plays multiple supporting functions in the body. As part of the immune system, the spleen acts as a filter of blood by destroying old and damaged red blood cells.

Tonsils and adenoids

Tonsils and adenoids can trap foreign invaders that enter the body through the mouth and nose. They contain immune cells that produce cellular defenses to prevent throat and lung infections. An inflamed, swollen, and painful tonsil is an indication of an active current throat infection. Adenoids are two glands located at the back of the nasal passage

Thymus

The thymus is a special tissue that shrinks or atrophies as you grow older. It serves as the maturation site of T cells, which help in the process of adaptive immunity for the production of antibodies.

Bone marrow

The bone marrow is the soft, spongy tissue found in bone cavities. Bone marrow plays very important roles in the immune defenses. Most of the defenses of the innate and adaptive immune systems involve white blood cells and other cellular components. Stem cells originate in the bone marrow and develop into different cells such as the red blood cells, plasma cells and a variety of white blood cells and other types of immune cells. Without the proper function of the bone marrow, immune defenses will be significantly suppressed.

Skin, mucous membranes and other first-line defenses

The skin is the first line of defense in preventing and destroying microbes before they enter the body. The skin produces oils and secretes other protective immune system cells. Mucous membranes line the respiratory, digestive, urinary and reproductive tracts, which lubricate and moisten surfaces so pathogens are easily trapped in the mucus. The tiny hairs in the nose to trap microbes and enzymes found in sweat, tears, saliva. Mucous membranes as well as secretions in the vagina defend and destroy germs and are considered the first line of defenses.

Stomach

The acid produced by the stomach kills many bacteria soon after they enter the body through the oral cavity. Foreign microbes usually cannot withstand acidic pH and are therefore destroyed by the acidity of the stomach.

How does the immune system work?

The unique characteristic of the immune system is its capacity to recognize substances that are naturally found in your system and differentiate them from foreign substances that have invaded the body. It defends the body through the activation of different defenses, which attack and kill foreign invaders that may cause harm. Its recognition capacity can be attributed to the interaction of certain protein molecules found on the surface of the immune cells and the proteins found on the surface of microbes. The immune system builds up defenses through unique encounters with the pathogens. The defenses provided by the immune system almost always start with the innate immunity defense, which may further activate the adaptive immunity defense if the pathogen is very harmful(3).

However, when the immune system is suppressed and cannot mount an effective attack against pathogens, which leads to the development of diseases. Unfortunately, in some cases, the immune system mounts an attack when there is no invader, or doesn’t stop an attack even after the invader has been killed. These activities may result in serious problems such as autoimmune diseases and allergic reactions.

What conditions affect the immune system's functions?

Many deficiencies and disorders can damage or disrupt the normal function of the immune system(4).

Allergies

Allergic reactions are responses of the immune system to certain chemicals or allergens found in food and other materials. An allergic reaction occurs when the body overreacts to a normally harmless substance such as food or pollen, resulting in a heightened response. This reaction is usually aided by the release of histamines that cause allergy symptoms. An allergic reaction can range from mild reactions manifested by sneezing or stuffy nose, to a more severe condition, manifested by breathing problems and death. Antihistamine medications help calm the symptoms.

Autoimmune disorders

Autoimmune disorders occur when the immune system mistakenly attacks its own healthy cells, primarily due to suppression of its recognition capacity. Autoimmune disorders cannot be treated by medication. There is an occurrence of continuous inflammation that damages the cells or tissues. Lupus, diabetes, Hashimoto's disease and rheumatoid arthritis are all examples of common autoimmune diseases.

Primary immunodeficiency disorders

These disorders are inherited and passed along in families. There are more than 100 primary immunodeficiency diseases (PIDD) that prevent the normal function of the immune system leading to the development of diseases.

Infections

Disease-causing pathogens can weaken the immune system by attacking certain components of the immune system. HIV and infectious mononucleosis are well-known infections that weaken the immune system because the causative agent for such diseases attacks the white blood cells.

Cancer

Cancer is a result of uncontrolled cell growth because of DNA mutations. Certain types of cancer, like leukemia, lymphoma and myeloma affect the immune system directly, causing immune cells to grow uncontrollably.

Sepsis

Sepsis is a result of the uncontrolled invasion of pathogens which were not successfully eliminated because of a weaker immune response. In this case, the pathogen invades the tissues and eventually invades even the bloodstream. Once the pathogen reaches the bloodstream, it is easier for such pathogens to reach other organs of the body. The body's response to the infection triggers widespread inflammation and causes organ damage, organ failure and death.

Medications

Certain medications, such as corticosteroids, and immunosuppressant medication can weaken and suppress the function of the immune system.

In summary

The immune system is a complex and large network of organs, cells, proteins, cellular products and chemicals that function together to provide protection to the body from foreign external invaders, such as bacteria, viruses, fungi and toxins. The immune system comprises innate and adaptive immunity. Innate immunity is nonspecific and usually functions as part of the first line of defense. If the innate immunity was not effective in eliminating the offensive pathogen, the adaptive immunity will be activated, especially when invasion has already occurred. Conditions that may affect the immune system are allergies, autoimmune disorders, cancer, sepsis, infections, primary immunodeficiency disorders, and medications.