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Immune System

Immune System: 

 

The immune system is a complex network of cells, tissues, and organs that function as the body’s primary defense system against foreign pathogens such as viruses and bacteria as well as abnormal cells in our body. 

 

To truly understand how the immune system works it is important to understand the mechanisms of defense and a few key concepts. 

 

1. Antigens 

Antigens are proteins or carbohydrates that are found on the surface of pathogens. These are used as markers to the immune system for foreign or dangerous molecules. 

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2. Immunogens

Immunogens are a type of antigen that can trigger the immune system. 

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3. Antibodies

Proteins produced by B cells which will bind to antigens. This will neutralize or mark it for destruction. 

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4. Major Histocompatibility Molecules

Proteins that are key to presenting these antigens to immune cells. 


 

The two key mechanisms of defense the immune system displays are: innate and adaptive immunity. 

 

Innate Immunity: 

  • First line of defense

  • Rapid but nonspecific

 

Adaptive immunity: 

  • Slow yet specific

  • Memory cells

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Organs

Bone marrow:

 

The bone marrow is essential for the formation of blood cells. Hematopoiesis starts with bone marrow producing hematopoietic stem cells which form into both myeloid and lymphoid progenitors. These naive blood cells differentiate and mature in various other organs. Specifically B cells mature in the bone marrow. They first rearrange their genes to express immunoglobulins: IgM and IgD. The B cells then leave to circulate the blood, but stays naive until it encounters its specific antigen. 


 

Thymus: 

 

The thymus is located behind the sternum and is necessary for the development of T cells, a key component of immune response. Naive T cells migrate from the bone marrow to the thymus to be primed. They are trained to differentiate between cells of self and non self. This is important to prevent our immune cells from attacking our own tissue. T cells are eliminated through positive and negative selections. Those T cells that bind to a major histocompatibility molecule (MHC) can continue as the MHC is critical for presenting foreign peptides to a cell. Negative selection involves removing those that bind too tightly to self-antigens (body’s own proteins). The thymus is divided into the cortex and the inner medulla. This is where positive and negative selection occurs respectively. Thymic stromal cells are a wide variety of epithelial, dendritic, and other cells which present antigens and assist in the maturation of T cells. 


 

Lymph Nodes and Lymphatic system: 

 

Lymph nodes are bean shaped organs that filter the body fluids. They carry lymph, a fluid with immune cells and waste, from tissues to the bloodstream. They are where different immune cells are activated as they will be presented foreign antigens. Immune cells can check on lymph fluid early before it reaches other parts of the body. In addition, immune cells can trap pathogens within the lymph node. Each lymph node contains a dense capsule for its shape. The cortex under the capsule contains dense clusters of B cells and the medulla in the inner region has a mix of B, T, and plasma cells. Lymph flows through sinuses which contain macrophages and other filtering agents. Lymph nodes enlarge when fighting an infection due to proliferation of immune cells. The lymphatic system is responsible for maintaining fluid balance and facilitating nutrient absorption in the human body. Lymphatic vessels, similar to blood vessels, begin with capillaries allowing interstitial fluid around cells to enter. Lymph is constantly taken through lymph nodes to remove the foreign pathogens from which it can then circulate around the body. 

 

MALT

 

The immune sites in various mucosal surfaces in our bodies such as respiratory and gastrointestinal are known as mucosal-associated lymphoid tissue. Pathogens that enter through the mouth, nose, or other openings first encounter this surface. Tonsils and Peyer’s patch are well known structures in the MALT. Specialized immune cells are found in mucosal surfaces because this is the first line of defense in the human body. 


 

Spleen: 

 

The spleen is involved in filtering blood for blood-borne pathogens. It is in the upper left side of the abdomen and beneath the diaphragm. The two regions of the spleen are the white and red pulp. The white pulp contains a dense cluster of immune cells waiting to fight pathogens found in blood. The red pulp is filtered for old red blood cells, recycling hemoglobin, and presenting pathogens to the white pulp. The spleen can also store blood components such as iron. Although it is not essential to live, a damaged or removed spleen would result in a fragile immune system specifically with blood-borne pathogens. 

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Immune Cells
 

Immune Cells:

 

All immune cells originate from hematopoietic stem cells. HSC are multipotent cells which differentiate into the myeloid and lymphoid lineages making up the variety of immune cells that survey our bodies. 

 

Dendritic Cells

Dendritic cells are all antigen presenting cells. They are found in lymphoid tissues and migrate to lymph nodes. These are the only immune cells that can originate from both the myeloid and lymphoid progenitors. They sample antigens and present them to naive T cells through the major histocompatibility complex

The two types of dendritic cells are plasmacytoid and conventional dendritic cells: 

  1. Conventional dendritic cells sample pathogens and present pathogen associated molecular patterns to pattern recognition receptors in naive CD4+ and CD8+ T cells. This primes those cells to recognize those patterns. 

  2. Plasmacytoid dendritic cells recognize viral DNA/RNA and secrete type 1 interferons (signaling molecules) which activate natural killer cells and other adaptive responses. 

 

Myeloid Lineage: 

 

1. Monocytes and Macrophages

Monocytes circulate the blood and differentiate into specific macrophages once they enter the tissue (osteoclasts for bone, alveolar macrophage for lungs, etc)  

  • They perform phagocytosis (engulfing pathogens to break them down) as well as present antigens and secrete signaling molecules to recruit other immune cells

2. Neutrophils

They are attracted through different cytokines like IL-8 to different tissues. Neutrophils are very short lived only living for 6-12 hours in tissue before committing apoptosis. Macrophages clean up the residual after their deaths. 

  • They release granules like lysozyme which kill pathogens and can perform phagocytosis as well. 

3. Eosinophils, Basophils, and Mast cells

All three of these cells are important in combating allergies and triggering allergic reactions. They commonly use granules to kill pathogens. 

  • Eosinophils are specifically used to combat parasites

  • Basophils release histamine to kill pathogens

  • Mast cells are responsible for the allergic reactions such as anaphylaxis and heat or cold sensations


 

Lymphoid Lineage: 

 

1. B Lymphocytes

B cells mature in the bone marrow and contain a B cell receptor (BCR) which is for a specific antigen. With the help of T helper cells the B cell binds to an antigen and can proliferate into either a memory or plasma cell. 

  • Memory cells are long lived and can stay alive for months to even years. They sit in lymph nodes waiting for the antigen to reappear. Once it reappears they respond faster and stronger. 

Plasma cells pump thousands of antibodies to circulate the blood for certain pathogens. 

  • Antibodies neutralize pathogens from binding to other cells. They coat these cells for macrophages to engulf them. And they can lyse holes in these foreign cells. 

2. T Lymphocytes

T cells mature in the thymus and use T cell receptors to recognize antigens presented from MHC molecules on dendritic cells. Once it recognizes the antigens it can separate into helper, cytotoxic, or regulatory T cells. 

  • T helper cells (CD4+) recognize MHC class 2 molecules and release various cytokines to recruit other immune cells when encountering a pathogen. 

  • T cytotoxic cells (CD8+) are MHC class 1 specific and kill pathogens through various enzymes and triggers of apoptosis. 

  • T regulatory cells prevent autoimmunity and chronic inflammation by suppressing the responses of other T cells. 

3. Natural Killer Cells

Natural killer cells mature in bone marrow as well as other tissues. They can be enhanced by antibodies and activate macrophages themself. 

  • They use perforin to create pores in pathogens and secrete granzymes as well to induce apoptosis. They recognize cells lacking MHC class 1 and target those cells making them important for targeting tumors. Many tumors downregulate MHC class 1 so natural killer cells are important to fight against cancer. 

Immune Diseases

Autoimmunity:

 

Autoimmune diseases involve the immune system mistakenly attacking its own tissues. A few forms of autoimmunity include: 

 

Type 1 Diabetes:

The body cannot produce insulin leading to high blood sugar. This is due to T cells (CD8+) and macrophages attacking the cells that produce insulin. Frequent urination and an increased thirst are common symptoms. Insulin therapy and lifestyle changing are two methods for patients to recover. 

 

Multiple Sclerosis: 

The immune system attacks the myelin sheath that surrounds many nerve cells. This affects the central nervous system and leads to symptoms like muscle weakness and vision problems. Drugs such as immunomodularity drugs and corticosteroids are common treatments. 

 

Rheumatoid arthritis:

The immune system (mostly T cells and macrophages) targets the joints leading to inflammation. This swelling eventually destroys the joints. Common symptoms include stiffness and swelling. RA is usually treated with various drugs like NSAIDs and corticosteroids. 


 

Immunodeficiency:

 

Immunodeficiency itself does not harm the body, but having an underdeveloped or underactive immune system makes one more susceptible to diseases, infections, and cancer. 

 

Human immunodeficiency virus/Acquired immunodeficiency syndrome:

HIV is a virus that specifically attacks and targets CD4+ T cells. It can be spread through fluids or the mother. Over time the helper T cell count decreases until it reaches its final stage of AIDS. This makes the body susceptible to infections due to its inability to mount a proper response. There is no cure to AIDS, but it can be managed through antiretroviral therapy. These drugs prevent HIV from converting its RNA to DNA in the body’s cells. 


 

Hypersensitivity:

 

When the immune system reacts strongly or overreacts to molecules that may be harmless or have little risk it leads to hypersensitivity.

 

Anaphylaxis 

Anaphylaxis occurs when mast cells and basophils release a lot of histamine causing tissue damage and severe inflammation. This can lead to difficulty breathing, swelling, hives, and low blood pressure. The common treatment is a dose of epinephrine. 

 

Asthma

Asthma is due to chronic inflammation in airways from eosinophils. Inhaled corticosteroids and bronchodilators are treatments for wheezing and chest tightness. 

 

Chronic inflammation:

 

The prolonged exposure or activation of the immune system will damage tissues. Chronic inflammation is one of the most common causes of cancer as it leads to DNA damage. The immune system abnormally or continuously stays active resulting in a consistent inflammatory state. The release of reactive oxygen species (used to target pathogens) will damage cells, tissues, and allow for ineffective repair. In addition, fibrosis (a common repair process) results in scarring and the constant remodeling of tissue. The continuous release of cytokines brings in more immune cells only contributing to this inflammation. Chronic stress, obesity, and persistent infections can lead to chronic inflammation. Anti-inflammatory and immunosuppressive drugs are common treatments as well as diet changes. 

Key Nutrients

Probiotics

 

Probiotics support the gut microbiome which can assist in regulating inflammation. 

 

Common foods with probiotics include: 

  • Yogurt

  • Kefir

  • Pickles 

 

Fiber

 

Fiber can reduce chronic inflammation in the body. 

 

Common foods with fiber include: 

  • Whole Grains

  • Fruits

  • Vegetables 

 

Vitamin A

 

Vitamin A is essential for epithelial cells. These structural cells line airways, intestines, and other mucosal surfaces. They maintain structure and the production of immune cells. 

 

Common foods with vitamin A:

  • Carrots

  • Eggs

 

Vitamin D

 

Vitamin D activates T cells and is important for fighting respiratory infections. 

 

Common foods with vitamin D include: 

  • Fatty Fish

  • Mushrooms

  • Dairy

 

Vitamin C

 

Vitamin C has antioxidant properties that protect the body from oxidative stress from immune function. They also are important for the production of white blood cells. 

 

Common foods with vitamin C include: 

  • Citrus fruits

  • Kiwi 

  • Spinach

Sources:

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  • “White Blood Cell.” NCI Dictionary of Cancer Terms, National Cancer Institute,https://www.cancer.gov/publications/dictionaries/cancer-terms/def/white-blood-cell. Accessed 8 Aug. 2025.

  • Kindt, Thomas J., et al. Kuby Immunology. 6th ed., W.H. Freeman, 2006.
    https://muhammad1988adeel.wordpress.com/wp-content/uploads/2011/04/kuby-immunology-6th-edition.pdf. Accessed 8 Aug. 2025. 

  • “What Is the Immune System?” Biointron Biological Inc.,
    https://www.biointron.com/blog/what-is-the-immune-system.html. Accessed 8 Aug. 2025. 

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