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The term immune system is talked about a lot these days in various contexts. On one end of the spectrum people talk about “building” one’s immune system to ward off or overcome diseases. On the other hand, you may have heard conversation about where the disease itself is the result of some “immune disorder”.

Many years ago, when I used to visit my primary physician to deal with my seasonal allergies (which I was able to successfully cure, as I described in another blog post), I would often ask him, what was underlying cause of my allergies and he would say, “It is something to do with immune system.  We don’t understand much about immune system yet.”

Later after doing my own research, I figured out that actually we do know a lot about the immune system and there is a lot we can do about it too. And, that knowledge gave me the impetus to take actions to cure my seasonal allergies. See my post So, did I tell you the story about my allergies?

In this post, I would like to delve into what exactly is immune system and what we can do so it is really a friend and not our foe.

What is Immune System?

Here is a quick summary of some key terms from National Institute of Allergy and Infectious Diseases (NIAID) at NIH website.

The immune system is a network of cells, tissues, and organs that work together to protect the body from infection. The overall function of the immune system is to prevent or limit infection.

Immune system is comprised of multiple organs, cells and response types that include:

Immune System

Skin: The skin is usually the first line of defense against microbes. Skin cells produce and secrete important antimicrobial proteins, and immune cells can be found in specific layers of skin.

Bone marrow: The bone marrow contains stems cells that can develop into a variety of cell types. The stem cells in the bone marrow create two types of immune cells: innate immune cells and adaptive immune cells.

Innate immune cells —neutrophils, eosinophils, basophils, mast cells, monocytes, dendritic cells, and macrophages— are important first-line responders to infection.

Adaptive immune cells—B cells and T cells—that are responsible for mounting responses to specific microbes based on previous encounters since they retain immunological memory.

Natural killer (NK) cells share features of both innate and adaptive immune cells, as they provide immediate defenses like innate cells but also may be retained as memory cells like adaptive cells.

Lymphocytes – B, T, and NK cells also are called lymphocytes.

Bloodstream: Immune cells constantly circulate throughout the bloodstream, patrolling for problems. When blood tests are used to monitor white blood cells, another term for immune cells, a snapshot of the immune system is taken. If a cell type is either scarce or overabundant in the bloodstream, this may reflect a problem.

Thymus: T cells mature in the thymus, a small organ located in the upper chest.

Lymphatic system: The lymphatic system is a network of vessels and tissues composed of lymph, an extracellular fluid, and lymphoid organs, such as lymph nodes. The lymphatic system is a conduit for travel and communication between tissues and the bloodstream. Immune cells are carried through the lymphatic system and converge in lymph nodes, which are found throughout the body.

Lymph nodes are a communication hub where immune cells sample information brought in from the body. For instance, if adaptive immune cells in the lymph node recognize pieces of a microbe brought in from a distant area, they will activate, replicate, and leave the lymph node to circulate and address the pathogen. Thus, doctors may check patients for swollen lymph nodes, which may indicate an active immune response.

Spleen: The spleen is an organ located behind the stomach. Spleen is important for processing information from the bloodstream. Immune cells are enriched in specific areas of the spleen, and upon recognizing blood-borne pathogens, they will activate and respond accordingly.

Mucosal tissue: Mucosal surfaces are prime entry points for pathogens, and specialized immune hubs are strategically located in mucosal tissues like the respiratory tract and gut.

How does immune systems defenses work?

An immune response is generally divided into innate and adaptive immunity.

Immune Types of Response

Innate immunity occurs immediately, when circulating innate cells recognize a problem by expressing genetically coded receptors. Collectively, these receptors can broadly recognize viruses, bacteria, fungi, and even non-infectious problems. However, they cannot distinguish between specific strains of bacteria or viruses. Their key feature is quick and broad.

Adaptive immunity occurs later, as it relies on the coordination and expansion of specific adaptive immune cells. If a B or T cell has a receptor that recognizes an antigen from a pathogen and also receives cues from innate cells that something is wrong, the B or T cell will activate, divide, and disperse to address the problem. B cells make antibodies, which neutralize pathogens, rendering them harmless. T cells carry out multiple functions, including killing infected cells and activating or recruiting other immune cells.

Vaccination, or immunization, is a way to train your immune system against a specific pathogen. Vaccination achieves immune memory without an actual infection, so the body is prepared when the virus or bacterium enters. Saving time is important to prevent a pathogen from establishing itself and infecting more cells in the body.

When immune system is not working as intended?

When the immune system is working well through all the parts and pieces we discussed above, it is a thing of beauty. As a pathogen enters the body, body’s defenses are activated through the appropriate part of the immune system.  The pathogen is recognized and immediately zapped.

When immune system is not working properly there could be different types of issues.

Immune Deficiencies

Temporary Immune Deficiencies: Temporary immune deficiency can be caused by a variety of sources that weaken the immune system. Common infections, including influenza and mononucleosis, can suppress the immune system.

When immune cells are the target of infection, severe immune suppression can occur. For example, HIV specifically infects T cells, and their elimination allows for secondary infections by other pathogens.

Patients receiving chemotherapy, bone marrow transplants, or immunosuppressive drugs experience weakened immune systems until immune cell levels are restored. Pregnancy also suppresses the maternal immune system, increasing susceptibility to infections by common microbes.


Allergies are a form of hypersensitivity reaction, typically in response to harmless environmental allergens like pollen or food. Hypersensitivity reactions can be caused by antibodies, IgE or IgG, which are produced by B cells in response to an allergen. Overproduction of these antibodies activates immune cells like basophils and mast cells, which respond by releasing inflammatory chemicals like histamine. And that is why people take antihistamine to stop allergic reactions such as sneezing.

Allergic reactions can also be caused by T cells, which may either directly cause damage themselves or activate macrophages and eosinophils that damage host cells.

Autoimmune Diseases

Autoimmune diseases occur when adaptive immune cells that recognize host cells persist unchecked. Autoimmunity is either organ-specific or systemic, meaning it affects the whole body. Autoimmune diseases have a strong genetic component, and with advances in gene sequencing tools, researchers have a better understanding of what may contribute to specific diseases.


Some forms of cancer are directly caused by the uncontrolled growth of immune cells. Leukemia is cancer caused by white blood cells. Lymphoma is cancer caused by lymphocytes, i.e., adaptive B or T cells. Myeloma is cancer caused by plasma cells, i.e.,  mature B cells. Unrestricted growth of any of these cell types causes cancer.

In addition, an emerging concept is that cancer progression may partially result from the ability of cancer cells to avoid immune detection.

So, what can we do optimize our immune system?

Things that are in our controls to enhance our immunity and make sure that the immune system does not go haywire are the ones that you have heard about gazillions of time by now and are probably tired of hearing about: Nutrition, Exercise and Lifestyle.

In the next post, we will talk of specifics of nutrition, exercise and lifestyle that can enhance immune system and also prevent it from attacking itself or the body.

In the meantime, what are your thoughts on this subject?

Would love to hear from you and learn from you.

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