Immune System: Function, Parts, and Main Organs

2.5.2025 0 comments

Author: Trisha Houghton, CNS, ASIST

You are exposed to countless threats every single day.

From pollutants to irritants, allergens to pathogens, your body feels it is constantly under attack from both inside and out.

Thankfully, you have a defense system built into your body to keep you safe and shield you from infection and illness—but only if it’s in good shape and able to work properly.

In this post, we’re going to take a deep dive into immune system function. We’ll look at what your immune system is, how the immune system protects you, and what other functions it fulfills.

We’ll also examine the various organs in the immune system and what other internal components get involved in protecting your body.

We’ll take time to look at common immune system disorders so you know what threats you’re most likely to face, and what you can do to boost and maintain your immune system.

By the end, you’ll have a better understanding of this critical internal system and you’ll be armed with the knowledge necessary to ensure its optimal function through healthy daily habits.

Introduction to the Immune System

The immune system is a large network of organs, proteins, chemicals, and cells that protect your body from pathogens. 

The term “pathogen” refers to “any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.” [1]

Viruses, bacteria, fungi, prions, and protozoans, are all pathogens that can invade the body and cause disease or infection through various mechanisms. Each has its own method of entry into, and different ways it affects the body. But thankfully, as you’ll see below, our immune system is incredibly adaptable and capable of responding to a wide variety of threats very efficiently.

Primary Functions of the Immune System

Below you’ll see details about the main roles that the immune system plays in your body.

Defense Against Pathogens

The main job of the immune system is to protect the body against pathogens like parasites, bacteria, viruses and fungi that could cause disease or infection.

Not only does it have methods of killing any invading pathogens, but there are also parts of your immune system that act as barriers to keep many pathogens from getting inside your body in the first place.

Detection of Harmful Substances

The immune system also detects harmful substances like toxins or chemicals that could cause infection and triggers immune reactions to eliminate them. It has the ability to detect which cells belong to your body and which don’t.

Those it perceives as “invaders” are eradicated to restore your body to homeostasis.

Identification and Removal of Abnormal Cells

In addition to detecting and destroying pathogens, your immune system can also identify and remove damaged or cancerous cells. 

Damaged cells release certain signaling molecules (for example, histamine or bradykinin) which your white blood cells react to. Phagocytes (one type of white blood cell) consume dead and damaged cells while natural killer cells attack cancer cells. Once these cells finish their job, they break down and can be eliminated from the body.

Prevention of Autoimmunity

While the immune system is designed to protect against harmful invaders; it also must avoid attacking its own healthy cells. This process is known as self-tolerance. This is the way the immune system regulates itself to avoid autoimmunity, or a condition in which the body attacks its own healthy tissues.

A diverse microbiome is one of the ways the body helps train the immune system to distinguish the self vs. the invader or pathogen (non-self) and respond appropriately to the pathogen without harming it’s own healthy cells.

Immune Memory (Adaptive Immunity)

One of the immune system’s most critical functions is adaptive immunity. This is the immune system’s ability to recognize and respond more effectively to pathogens.

When you encounter a new type of sickness, your body produces special antibodies to neutralize and eliminate the sickness. However, the body creates memory B and memory T cells that store the “memory” of that sickness in the form of those antibodies so that if it ever returns, it has the correct immune response to deal with it more efficiently.

Adaptive immunity is the founding principle behind vaccines.

The body’s adaptive immunity is immensely powerful and is largely the reason why we are able to stave off so many illnesses and infections.

Main Components of the Immune System

There are two main parts of your immune defence; these are the innate and adaptive immune system.

Innate Immune System

Your innate immune system is the body’s first line of defense. It provides non-specific protection against invaders.

It’s made up of a few key physical barriers, including:

• The skin, which stops bacteria from penetrating your tissues;
• Saliva and mucus, which contain bacteria-killing substances;
• Tears, which keep bacteria from penetrating your eyes or causing infection;
• The various mucous membranes inside the body, which also play a role in killing off bacteria.

There are also a number of immune system cells that play a role in your innate immunity, including natural killer cells, macrophages, neutrophils, and dendritic cells. These cells are trained to respond to all threats and do so with varying degrees of efficiency.

Adaptive Immune System

When your body encounters a pathogen or threat that your innate immune system can’t handle, that’s when your more active adaptive immune system kicks in. The adaptive immune system’s purpose is to provide a targeted, precise response to specific pathogens.

B lymphocytes in your bloodstream produce antibodies in response to fungal, bacterial, or viral infections. They tap into your “immune memory” and produce more of the specific antibodies that have proven effective in the past at curbing or curing infectious diseases.

T cells are produced to attack infected cells in your tissues. They purge the infected cells, kill off the pathogens that cause the infection, and restore the body to homeostasis.

Main Organs of the Immune System

Before we see how the immune system works, let’s first take a look at its main components.

Bone Marrow

The soft, fatty tissue inside your bones is called bone marrow. This is where the bulk of blood cell production takes place.

Both the red blood cells that transport oxygen and nutrients, the white blood cells that support your immune response, and platelets which aid in blood clotting are produced in your bone marrow.

Thymus

The thymus is a small organ with a big responsibility, one of the most critical parts of the immune system.

Its job is to make sure the “maturing” T cells that attack infected tissue cells can recognize the self vs. non-self. This helps ensure that once they mature, they are highly effective disease-killing cells that can respond appropriately to an infection or invasion.

Spleen

Your spleen plays the crucial role of storing monocytes, a type of white blood cell while they’re inactive—while your body is healthy.

Certain white blood cells may not be needed at all times, but the spleen ensures monocytes, a specific type of white blood cell, is stored and ready for action the moment any infection or disease is detected.

The spleen also plays a crucial role in filtering your blood to eliminate old red blood cells, detect pathogens, as well as recycle iron from damaged cells to support the production of new, healthy blood cells.

Lymph Nodes

Your lymph nodes are small, bean-shaped organs located in various parts of your body, including your neck, armpits, and groin. They play the crucial role of filtering lymph fluid that is drained from your cells and tissues, while ensuring vital nutrients remain in the body.

These filters ensure that bacteria, viruses, and any other invaders or foreign substances present in the lymph fluid are trapped and eliminated.

Lymphatic Vessels

The lymphatic vessels are tubes that make up a highly complex network that allow lymph fluid to flow through your body, to and from the lymph nodes.

These vessels transport lymph fluid,which carries immune cells that aid in detecting and fighting pathogens.

Tonsils and Adenoids

Located at the back of your throat and nasal passage, your tonsils and adenoids work together to keep certain invaders from entering your body through your mouth and nose.

They trap harmful germs like bacteria and viruses to prevent infections.

Peyer’s Patches

Peyer’s patches are a collection of lymphoid tissues in your small intestines, particularly in the ileum. They help immune cells recognize and respond to high levels of intestinal bacteria.

They help regulate immune responses for healthy intestinal bacteria balance. They also prevent pathogenic bacteria from flourishing out of control as your food is digested.

How the Immune System Works

Your body’s first line of immune defense is your innate immune system response—i.e. your skin, mucus, saliva, tears, natural killer cells, and mucus membranes. This is always active, always protecting your body, and capable of dealing with the majority of the threats you face every day.

However, when a particularly insidious or potent pathogen slips past your innate defenses, your adaptive immune response has to get involved. The adaptive immune response activates when immune-triggering substances called antigens on the surface of invading cells attach to receptors on immune cells.

If the body has detected this invader in the past, its memory B cells quickly produce the specific antibodies that worked in the past to eradicate it. If it’s a new invader and there is no “immune memory” (in the form of previously created antibodies), the body relies on antigen-presenting cells (APCs) including dendritic cells to identify the invader as best it can and trigger the most effective immune response.

T lymphocytes and B lymphocytes work together to destroy (T cells) and neutralize (B cells) the pathogens. During the process of eradicating the pathogen, new antibodies are produced by B cells and the immune memory stores the specialized cells that proved most effective at eliminating them.

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Immune System Disorders

Immune system disorders occur when the bodys defenses do not work properly.

An underactive immune system can make the body more vulnerable to infections and diseases, while an overactive immune response may attack healthy tissues. Clinical immunology helps diagnose and treat these conditions to maintain immune balance.

Below, you will find some common diseases of the human immune system.

Autoimmune Diseases

Autoimmune disorders are medical conditions in which your immune system mistakenly attacks your own healthy tissues. It’s often the result of dysregulation of the immune system, causing dysfunction or incorrect recognition of “invading” cells.

Common autoimmune diseases include lupus, multiple sclerosis, rheumatoid arthritis, psoriasis, Crohn’s disease, vitiligo, and ulcerative colitis.

Experts don’t know exactly what causes autoimmune disorders, but a great deal of ongoing research suggests genetic, environmental, and hormonal factors contribute to this immune system disorder..

Allergies

Allergies could be considered an “overreaction” on the part of your immune system to otherwise harmless substances like animal dander, mold, pollen, dust, or certain foods. The immune system mistakenly sees these substances as a threat and produces antibodies to attack and neutralize them.

The immune response to allergens can cause a number of allergic reactions, including:

• Itching;
• Sneezing;
• Swelling of your throat or tongue;
• Tightness in your throat;
• Feeling warm;
• Dizziness;
• Pain or cramps;
• Abnormal heart rate;
• and the list goes on…

In some cases, the response to foreign cells can be severe. Anaphylactic shock is an extreme overreaction that leads to significant swelling in the tissues—particularly in the throat—that impairs breathing. If untreated, it can be life-threatening.

Immunodeficiency

Immunodeficiency is a condition in which your immune system’s ability to attack and neutralize infections is compromised, or even entirely absent. Immunodeficiency can lead to more frequent viral or bacterial infection, or infections that grow serious or last longer than in people with “normal” immune systems.

In some cases, people are born lacking certain immune defenses or with a malfunctioning or misfiring immune system. In other cases, immunodeficiency is the result of some other condition, such as a chronic infection or disease, prolonged use of medications, HIV, etc.

There are more than 300 primary immunodeficiency disorders currently known, ranging from mild to severe.

Inflammatory Diseases

Inflammatory diseases are conditions in which your body suffers from excessive or prolonged inflammation triggered by some immune response. The long-term inflammation can lead to tissue damage and is typically considered a “chronic disease”.

Common inflammatory diseases include inflammatory bowel disease (IBD), psoriasis (which is considered both an inflammatory and an autoimmune condition), Type 2 diabetes, non-alcoholic fatty liver disease (NAFLD), certain types of cardiovascular diseases, and endometriosis.

Boosting and Maintaining Immune System Health

Supporting immune system health strengthens the body’s ability to fight infections and diseases. B cells and T cells play a key role in developing acquired immunity, which helps the body recognize and respond to harmful pathogens.

Healthy habits and proper nutrition are essential for keeping these immune cells functioning effectively.

Balanced Diet

Eating more immune-boosting nutrients like Vitamin C, zinc, and antioxidants can help to maintain good immune health. Not only will these nutrients ensure the production and function of more crucial white blood cells, but will also combat inflammation and protect against oxidative stress which can damage cells.

Spices, brightly colored fruits and vegetables, whole grains, and nuts and seeds should be central to your healthy, immune-friendly diet.

Regular Exercise

Exercise stimulates your immune system by increasing circulation of white blood cells throughout your body. This ensures your white blood cells are more actively seeking out pathogens to identify and eradicate.

Exercise will also help balance stress hormones which, as you’ll see below, can reduce immune function and contribute to inflammation.

Even just 20 to 30 minutes of moderate exercise per day is enough to keep your immune system functioning optimally.

Adequate Sleep

Yes, sleep does affect the immune system. It facilitates the production of antibodies that respond to invading threats, increases cytokine production, and reduces your susceptibility to illness.

Sleep deprivation, on the other hand, makes you much more vulnerable to sickness and disease.

Prioritize your sleep and make sure to get a solid 7-8 hours per night in order to keep your immune defenses strong.

Hydration

Lymph fluid is made up of 96% water, so proper hydration is crucial for effective lymph drainage. Drinking enough water supports your lymphatic system so it can collect the lymph fluid from around your body and filter out wastes and pathogens before they can attack your body.

Water consumption varies by individuals; however, in a healthy person, the goal is to drink between 2-3 liters of water every day for optimal immune function .

Stress Management

Chronic stress suppresses your immune response, causes inflammation, and makes you more susceptible to disease and infection.

When you experience stress, your body releases cortisol, a hormone that suppresses immune function. Your white blood cell count decreases, along with antibody production, lymphocyte proliferation, and the activity of natural killer cells.

To make matters worse, stress can impair sleep, impact digestive system health, and raise your risk of chronic and inflammatory disease.

Curb your stress through daily exercise, meditation, and relaxation to keep your immune system healthy.

Vaccination and Immune Response

While talking about our body’s immune response, we also have to take a moment to discuss vaccines.

Though a subject of great controversy, the rationale behind vaccines and how they affect our body’s immune memory is sound.

Simply put, vaccines inject a small amount of certain pathogens into your body to trigger an immune response. The dose is small enough that it triggers the immune system to produce antibodies, without causing an infection.

Basically, vaccines mimic an infection to train your body’s white blood cells to recognize and destroy the invaders before a greater-scale invasion can cause more serious infections and disease.

There are a number of types of vaccines in use today:

• Inactivated vaccines, which use “killed” versions of the germs.
• Live-attenuated vaccines, which use live but weakened forms of the germs.
• Messenger RNA vaccines, or mRNA vaccines, which make proteins that trigger a specific immune response.
• Subunit, recombinant, polysaccharide, and conjugate vaccines, all of which use specific pieces of a germ to train your body how to react to and eliminate key parts of an invading pathogen.
• Toxoid vaccines, which use toxins made by a germ to build immunity to the damage caused by a pathogen rather than the pathogen itself.
• Viral vector vaccines, which use modified versions of a different virus to trigger the immune system’s protective response.

Vaccines are essentially a “pre-emptive strike” but use a dose of the same germ small enough for your body to overcome and defeat. By doing so, they strengthen your body’s defenses against future invasions.

Conclusion

Your body is under constant threat from outside bacteria, viruses, toxins, chemicals, pollutants, and allergens. Most of the attacks that occur go entirely unnoticed because your immune system is so effective at dealing with them.

Your innate immune system forms an effective first line of defense, while your adaptive immune system kicks in when needed to fight infection of a more serious nature or when a pathogen sneaks by the innate defenses.

But no matter who you are, there will be times when your natural immunity just isn’t enough, and you’ll need to seek medical treatment. But if you’ve worked to include the lifestyle, diet, exercise, and sleep habits suggested above to maintain a strong immunity, those more serious infections and diseases hopefully are fewer and farther between and pose less risk to your health.

FAQs

What is the strongest immune system booster?

Vitamin C is a potent immune system booster. It protects the cells against oxidative damage, curbs inflammation, stimulates the production of immune cells, and facilitates B and T cell function. The vitamin can also promote healing by supporting collagen production.

Can you fix a weak immune system?

You can strengthen a weak immune system by exercising daily, sleeping more, quitting smoking, reducing alcohol intake, managing your stress, practicing good hygiene, eating a balanced diet, and consuming more immune-boosting nutritious foods.

What foods are bad for the immune system?

Highly processed foods and foods that contain a lot of artificial and chemical ingredients weaken your immune system. Alcohol can suppress your immune function, while fried and fatty foods can place excess stress on your immune system. Salty foods can trigger inflammation and contribute to autoimmune disease risk [2].

What is the complement system? 

The complement system is a vital part of the innate immune system, made up of over 30 proteins that work together to fight infections. It supports antibodies and immune cells by marking harmful germs for destruction, promoting inflammation, and directly killing pathogens through membrane attack complexes. The system activates through three main pathways—classical, lectin, and alternative—all working to protect the body and connect innate and adaptive immunity.

How does the immune system protect the body from diseases?

The immune system defends the body using different types of cells. Cells called phagocytes engulf and destroy harmful pathogens, while cells called lymphocytes, such as B cells and T cells, target specific threats and coordinate immune responses. Some lymphocytes become memory cells, allowing the body to respond faster to future infections.

Resources

Immune System

Immune system explained

In brief: How does the immune system work?

The Immune System

How the immune system works

How to boost your immune system

In brief: The innate and adaptive immune systems

Autoimmune Diseases

Allergies and the Immune System

Immunodeficiency disorders

Some common inflammatory diseases

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