7 min|Dr. Maya Kuczma
Understanding the Immune SystemWellness, Education, Immune Health
The ‘immune system’ refers to an incredibly complex network of multiple systems, organs, cells, and proteins that work together to mount an immune response. Its main goal is to defend the body against invaders, such as microorganisms (including certain bacteria, viruses, fungi, and parasites), cancer cells, and foreign tissue (such as transplanted organs). It has to distinguish between what is self, or symbiotic with our self (including specific microorganisms), and what is non-self, or foreign.
During a healthy immune response, the immune system:
- Recognizes the threat
- Mobilizes the response, in order to ‘fight’ the threat
- Regulates the immune response, to prevent extensive damage to the body
- Resolves the response, ideally retaining ‘memory’ of the threat to ensure an appropriate response in the future
A healthy immune response requires communication and coordination between many moving parts, all of which can be considered part of the immune system:
The respiratory system includes the mouth, pharynx, larynx, trachea, and lungs that allow oxygen to enter the body, and carbon dioxide to exit the body. Our airways are covered in mucus and hair-like projections, known as cilia, which work together to trap, weaken, and expel pathogens. The respiratory system also has a microbiome, an ecosystem of fungi, bacteria, and viruses, mostly residing in the upper airway, which acts as a gatekeeper to the respiratory tract. (1)
The skin acts as physical barrier between our body and its external environment. The skin is colonized by a diverse milieu of microorganisms, including fungi, bacteria, and viruses, known as the skin microbiome. These organisms are considered symbiotic, aiding in the defense of the skin against harmful or pathogenic organisms. Additionally, dendritic cells, a specialized immune cell found in the epidermis, ‘presents’ invading pathogens to a specific type of white blood cells, known as lymphocytes, in order to coordinate an immune response. (2)
The digestive system includes all organs that aid in digestion, and is located from the mouth to anus. Trillions of bacteria, fungi, and viruses, reside throughout the digestive tract, resulting in the two largest microbiomes in the body: the gut microbiome,(3) and the oral microbiome. (4) Both play a complex role in maintaining systemic health, including acting as a defense system against pathogenic microorganisms. Saliva, digestive enzymes, and hydrochloric acid (stomach acid) can help destroy pathogens that try to enter the digestive system.
The mucous layer of the intestinal tract prevents binding of pathogens to the gut lining, inhibiting infection and inflammation. (5) Mast cells, located on the gut lining, coordinate an immune response to toxins, infections, and allergens. (6)
The lymphatic system is a network of vessels, lymph fluid, lymph organs, lymphoid tissue, and the recently discovered lymph system of the brain, known as the glymphatic system. Our white blood cells, including lymphocytes, neutrophils, monocytes, basophils, and eosinophils, are produced in the bone marrow. White blood cells move throughout the lymph system within lymph fluid via the network of lymph vessels.
Lymph nodes are located along the lymphatic vessel network, filtering the fluid that passes through them. If a pathogen is detected, an immune system response, and swelling, will occur. Mucosa-associated lymphatic tissue (MALT) initiates immune responses to pathogens located on mucosal surfaces. (7) There are many types of MALT, including the nasopharynx-associated lymphoid tissue (NALT), and gut-associated lymphoid tissue (GALT), including the appendix, and Peyer’s patches.
Recognition and Defense
Our first line of defense against invaders are the physical barriers of our body, such as the skin, the cornea, and the linings of the respiratory, digestive, urinary, and reproductive tracts. Fluids, such as tears, the mucous layer of the digestive and respiratory tracts, sweat, and vaginal secretions, defend these barriers.
Our next line of defense involves white blood cells, which act via two distinct, yet complementary, response systems: the innate immune system (non-specific) and the adaptive (specific) immune system. These subsystems work together whenever our immune system is triggered by a germ or harmful substance.
The innate immune system consists of:
- Phagocytes, a type of white blood cell that ingests invaders. Examples of phagocytes include macrophages, neutrophils, monocytes, and dendritic cells
- Natural killer cells, a type of white blood cell that recognizes and kills cancer cells and cells that are infected with certain viruses
- Basophils and eosinophils, white blood cells that release cytokines, a substance involved in inflammation, and histamine, a substance involved in an allergic response
When our physical barriers encounter harmful substances and germs, such as when we eat a food that is contaminated, the innate immune system is triggered, providing a quick and nonspecific response.
The adaptive immune system (also known as the acquired immune system) can mount a targeted attack against an invader, such as a virus or microbial toxin, also known as an antigen. The adaptive immune system consists of:
- B lymphocytes, which produce antibodies that bind the antigen, blocking its ability to bind to host cell receptors, as well as ’signaling’ to the rest of the immune system that the antigen needs to be destroyed
- Multiple types of T lymphocytes that have a variety of actions, including killing virus-infected host cells (to prevent the virus from replicating), producing signaling molecules to activate macrophages (cells that engulf pathogens), and shutting down immune reactions so it doesn’t become rampant
- Memory cells, a type of lymphocyte that will recognize the pathogen in the future, enabling a faster response to subsequent infection
Regulation and Resolution
Many of the symptoms we experience following an injury or infection are signs that the immune system has recognized a threat, and has begun to activate and mobilize. Swelling, redness, and heat at site of an injury indicate an inflammatory response, a natural and normal immune response to damaged tissue; depending on the severity of damage to the tissue, our immune response may be sufficient, and over time, inflammation decreases and normal function of the tissue is restored.
Fever, lymph node swelling, body aches, and an increase in mucous production are by-products of the immune system activating and mobilizing in response to a foreign pathogen, such as a virus, bacteria, or fungus; if the pathogen is sufficiently neutralized, the immune response resolves, and we feel better.
However, when the body fails to mount an adequate response or cannot regulate the immune response, significant damage and disease can occur. Specifically, autoimmune conditions and cancer are examples of dysregulation within the immune system. At times, pathogens can overwhelm the immune system in such a way that the immune system response significantly damages the body, as is the case with the cytokine storm observed in A/H5N1 (8) and Sars-Cov-2 infections. (9-10)
Additionally, chronic exposure to components that activate the immune system can lead to a continual, long-term immune response, as seen in cases of chronic inflammation. Many conditions have been linked to chronic inflammation, including cardiovascular disease, (11) diabetes, (12) and Alzheimer’s disease. (13)
It appears that our immune system has hand in everything, from the way we respond to a cut or a cold, to our likelihood of developing a chronic health condition. For this reason, immunology is a hot topic, particularly in the wake of the COVID-19 pandemic. Our understanding of the immune system is constantly evolving, and in response, therapeutic targets and health recommendations shift.
We are scratching the surface of our symbiotic relationship with the microbes that live within us, and with that, recognizing it is a relationship that begins at birth, which can strengthen - or suffer - over a lifetime. Keep an eye on the blog as we continue to deep dive into the immune system, including how it develops, what it needs, and how it goes awry.