There is no other physiological system more important to staying healthy than the immune system. It protects us against infectious organisms and invaders that cause bodily harm. The vast majority of ailments and health problems we experience can be attributed to lapses in immune function and to oxidative stress and related inflammation.
In order for our immune system to operate at peak capacity, it's essential that we provide it with key nutrients required to keep all the parts of the immune system healthy and fully functioning.
What Is the Immune System?
The immune system is a complex network of cells, cell products, tissues, and organs that collectively launch the immune response by utilizing a varied arsenal made up of macrophages, lymphocytes including the B cells (B meaning derived from bone marrow) and T cells (T meaning derived from the thymus), and antibodies.
An antibody is a blood protein produced in response to a particular class of foreign substances known as antigens. Antigens, short for antibody generators, include invaders like bacteria, viruses, and toxins. The defining characteristic of an antigen is that it binds to specific immune receptors, eliciting a certain immune response. Antibodies protect us by combining chemically with antigens, which renders them inactive and removes them from the body.
A healthy, fully-functioning immune system has the capacity to detect a wide variety of antigens and other microorganisms as well as to differentiate between potential threats and the body's own healthy tissue.
White Blood Cells
White blood cells are like the scouts of the immune system's army, constantly surveying the landscape for looming attacks. The human body produces and stores white blood cells in four different locations, which are collectively known as the lymphoid organs.
- Bone marrow: This spongy tissue is found at the center of some bones, such as the hip and thigh bones, and contains immature cells known as stem cells that can develop into any kind of cell found in the human body.
- Thymus: This small organ is situated beneath the breast bone where T cells mature.
- Spleen: The largest lymphoid organ, it not only stores white blood cells but also helps to regulate the volume of blood in the body as well as to dispose of old or defective blood cells.
- Lymph nodes: These small, bean-shaped structures produce and store infection and disease-fighting cells as well as the lymph fluid that transports them throughout the body.
Additionally, the liver is now recognized as an important contributor of factors involved in immune function.
Because the immune system is so complex, it's often divided into subsystems in order to better conceptualize its operations. Two of the most important distinctions drawn are: 1) the innate immune system versus the adaptive immune system and 2) cellular immunity versus humoral immunity.
The Innate Immune System vs. the Adaptive Immune System
One way to conceive of the immune system is as a series of layered defenses designed to respond with increasing precision to threatening pathogens.
Physical barriers, like the skin, form the first line of defense against microorganisms like bacteria and viruses by simply preventing them from entering the body. If a pathogen successfully breaches the physical barriers, that activates the innate immune system. When a pathogen manages to evade the innate immune response, the adaptive immune system kicks into gear.
The adaptive immune system alters its response based on its recognition of the specific pathogen and its understanding of the most effective way to combat that pathogen. It retains this targeted response in the form of immunological memory, allowing it to become more and more effective at eliminating microorganisms it has encountered previously.
The ability of the immune system to distinguish between healthy tissues and foreign invaders such as antigens is a prerequisite for both innate immunity and adaptive immunity.
The Innate Immune System
Once a microorganism has made it past the physical barriers, the innate immune system works to neutralize that substance. This can happen either because pattern recognition receptors perceive components that belong to broad groups of microorganisms known to be foreign to the body, or because injured or stressed cells have sent out distress signals.
The innate immune response is non-specific, meaning it reacts to all foreign substances in the same way. That does not mean its response is simplistic, however. In fact, the innate immune system itself has many subcomponents, such as:
- Surface barriers: This category includes mechanical, chemical, and biological barriers. The skin is the most obvious example of a mechanical barrier, and as previously mentioned, it forms the first line of defense against microorganisms. However, since the human body cannot thrive while completely sealed off from its environment, other, more porous surface barriers are needed too. For example, the respiratory tract secretes antimicrobial peptides that form a chemical barrier, while commensal flora in the gastrointestinal tract serves as a biological barrier.
- Inflammation: This is one of the first ways the immune system responds to infection or injury. Increased blood flow to the affected tissue results in redness, swelling, heat, and pain. Injured or infected cells release eicosanoids, which produce fever and the dilation of blood vessels, and leukotrienes, which attract a kind of white blood cell called leukocytes.
- Complement system: This biochemical cascade attacks the surfaces of foreign cells. It's an important and rapid element of the immune response. Its name refers to how it complements the process by which antibodies kill pathogens.
- Cellular barriers: Leukocytes, first mentioned in relation to inflammatory responses, play a vital role in the innate immune system's response to threats. Innate leukocytes include phagocytes (a class that's subdivided into macrophages, neutrophils, and dendritic cells), innate lymphoid cells, mast cells, eosinophils, basophils, and natural killer (NK) cells. These cells eliminate pathogens via multiple mechanisms, such as attacking through contact or engulfing and then killing the pathogens.
The Adaptive Immune System
Early vertebrates developed an adaptive immune response that can more effectively deal with threats thanks to immunological memory. The antigen-specific adaptive immune response occurs when the immune system recognizes an antigen and then launches a response informed by its knowledge of that specific antigen.
The adaptive immune system, like the innate immune system, has a number of subcomponents, including a number of different cell types.
- Lymphocytes: This special form of leukocytes is unique to the adaptive immune system. Two of the major types of lymphocytes, B cells and T cells, come from hematopoietic stem cells in the bone marrow. B cells play a part in the humoral immune response while T cells contribute to the cellular immune response (more on that later).
- Killer T cells: This subgroup of T cells targets microorganisms carrying infectious diseases, other pathogens, and cells that have become damaged or dysfunctional. Killer T cells get activated when their T-cell receptor (TCR) binds to a specific antigen. T cells are particularly crucial when it comes to preventing the replication of viruses.
- Helper T cells: When this group of T cells identifies a possibly dangerous microorganism, they send out stimulatory signals that activate microphages, killer T cells, and B cells. They contribute to both the adaptive and the innate immune response and help the body determine which immune response to use to combat a pathogen. They don't attack pathogens themselves, but control the immune response by directing the activity of other cells.
- Gamma delta T cells (γδ T cells): This class of T cells shares characteristics of helper T cells, cytotoxic T cells, and NK cells. Experts consider them an unconventional T cell subset and have yet to identify exactly what stimulates them into action. They blur the line between innate and adaptive immunity by participating in certain elements of both types of immune responses.
- B lymphocytes and antibodies: Activated B cells divide to create plasma cell offspring, which then secrete millions of copies of an antibody formulated in response to the presence of a specific antigen. The antibodies then circulate in the bloodstream and lymphatic system, binding to the antigens and marking them for elimination or neutralizing them immediately.
Cellular Immunity vs. Humoral Immunity
The other important division related to the immune system has to do with cellular immunity and humoral immunity.
Cellular immunity involves mostly T cells and carries out immune responses to cells that are altered in a specific way, including cancer cells, transplanted cells, and cells invaded by pathogens.
The humoral response, sometimes referred to as the antibody‐mediated response, involves B cells that recognize antigens or pathogens that circulate in the lymph or blood.
This is not an ongoing operational state of the immune system, but it's nonetheless worth addressing. Passive immunity refers to an immune system response that is, in essence, borrowed from another source rather than generated innately.
For example, during the gestational period, a fetus receives antibodies from the placenta. Breast milk also contains antibodies that bolster infants' immune systems after birth. This type of passive immunity can protect babies and toddlers from bacterial infections and other threats during their first years of life.
Understanding Disorders of the Immune System
Given the complexity of the immune system, it's hardly surprising that it can malfunction in a multitude of ways. Immune system disorders arise because the immune system responds too aggressively, which is known as hypersensitivity; fails to respond aggressively enough, which is known as immunodeficiency, or responds in the wrong way, which is known as autoimmunity.
Immune system hypersensitivity describes any instance in which the immune system overreacts, thereby causing collateral damage to healthy cells.
Allergic reactions are a classic example of immune system hypersensitivity. With anaphylactic shock, the immune system response to the presence of an allergen is so intense, it can be fatal.
Immunodeficiency is the flip side of hypersensitivity. A number of underlying conditions and factors can lead to immune deficiency, such as:
It's also possible to develop acquired immunodeficiency as the result of another disease, as can happen in patients with HIV/AIDS or cancer. In some cases, immunodeficiency has genetic causes, such as severe combined immunodeficiency (SCID) or chronic granulomatous disease.
The uniting characteristic of autoimmune disorders is that the body's ability to differentiate between healthy tissue and pathogens or faulty cells becomes compromised. This causes the immune system to begin to target healthy cells.
There are a variety of autoimmune diseases, all of which affect the body in different ways. Some common ones include:
- Celiac disease
- Type 1 diabetes
- Rheumatoid arthritis
- Graves' disease
- Hashimoto's thyroiditis
- Systemic lupus erythematosus
How Nutrition Impacts Immune System Function
Nutrition is a critical determinant of immune function capability, and malnutrition is one of—if not the—most common cause of immunodeficiency. According to an article published in Trends in Immunology, "Malnutrition, which encompasses under- and over-nutrition, is responsible for an enormous morbidity and mortality burden globally."
Low-protein intake as well as deficiency or suboptimal intakes of single nutrients results in altered immune responses. The levels of zinc, selenium, iron, copper, vitamins A, C, E, and B-6, and folic acid in a person's diet all strongly influence immune system function.
Additionally, certain elements of a person's diet and lifestyle may degrade immune function. Two major culprits are excessive alcohol consumption and drug use, which harm the body’s immune system in two ways. First, they deplete the body of many of the key nutrients required for immunocompetence, such as zinc, B vitamins, and folic acid. Second, they directly impair cells and molecules that shape the immune response. The liver plays an important role in producing specialized proteins as part of the immune response, and direct damage to liver cells by ethanol or other drugs can also compromise immune function.
Differentiating Between Acute and Chronic Inflammation
It's worth circling back to one element of the immune system response—inflammation—to clarify some misconceptions about this physical process.
When we talk about inflammation, we rarely bother to differentiate between chronic inflammation and acute inflammation, but grasping that distinction is a vital part of learning how to optimize immune system function as well as your overall health and well-being.
When all is going well, inflammation is simply another way in which the immune system acts to heal injured body parts.
Acute inflammation occurs in response to a specific injury, such as an infected cut or an ingrown toenail. It is characterized by redness, swelling, and soreness around the injured tissue as white blood cells infiltrate the area to destroy germs, dead and damaged cells, and other foreign material.
Chronic inflammation, far from being a part of the healing process, can have quite a destructive effect. Many of us today worry about how chronic systemic inflammation might be eroding our health, and for good reason. Researchers have identified a number of lifestyle and environmental factors that can result in chronic inflammation, such as:
- Excess body weight
- Poor diet
- Lack of exercise
- Poor oral health
- Illicit drug use
- Heavy alcohol consumption
There is a specific physiological basis to this type of inflammation related to the release of pro-inflammatory cytokines from immune-related cells. The term cytokine refers to a number of substances, such as small proteins like interferon, interleukin, and growth factors. These immune cells have an effect on other cells which, when released throughout the body, can result in the chronic activation of the immune system.
Control of both types of inflammation may be disrupted by an inadequate supply of either macronutrients (protein in particular) or micronutrients. A strong immune system then, is dependent upon proper nutritional strategies that fortify and optimize function. An immune system replete with solid antioxidant and anti-inflammatory defenses is absolutely necessary for good health and can be assured by a combination of proper nutrition, rest, and an active lifestyle.
The Relationship Between Antioxidants and the Immune System
No discussion of the connection between nutrition and immune function would be complete without at least a nod to antioxidants. While the immune system does not include an antioxidant component per se, the two are tightly intertwined in that antioxidant protection is crucial to maintaining a strong, vital immune system.
Antioxidants and anti-inflammatories are important tools in the repertoire of the immune system. Antioxidants are agents that inhibit the oxidation of other molecules. Oxidation is a chemical reaction that can produce free radicals, leading to chain reactions that may damage cells.
Free radicals or reactive oxygen species (ROS) are highly reactive compounds that adversely alter the structure and integrity of lipids and proteins, including many of the protein components of the immune system. They can attack at several sites, often starting at the level of DNA itself.
If the DNA of a compound is altered, it will ultimately lead to mutations and genomic instability. These changes could result in the development of a variety of cancers or inflammation. It has been proposed that the process of aging is in part due to accumulation of DNA damage.
Oxidative damage that changes the chemical structure of a compound will almost always affect its function, given the precision with which biological compounds are designed. Lipid membranes are very susceptible to peroxidation, the oxidative degradation of lipids in which free radicals steal electrons from the lipids in cell membranes. This damage leads to leaky membranes and loss of cellular integrity and function.
Damage of this nature to cells involved with immune function will compromise the ability of the immune system to ward off bacteria, viruses, and other harmful compounds. The body relies on internal and external sources of antioxidants to assist in coping with this oxidative stress. Not coincidentally, many nutrients shown to boost the immune system have significant antioxidant properties, such as vitamin E, vitamin C, beta-carotene, selenium, copper, iron, and zinc. Dietary antioxidants are important in establishing resistance to infectious pathogens.
Can Nutrients Boost Immune System Function?
Although more research needs to be done to quantify the precise effects of various nutrients on the immune system, as well as the effects of nutrition on the development (as opposed to the treatment) of diseases, the studies that have been conducted so far certainly indicate connections between micronutrient deficiencies and immune response.
As mentioned in the preceding sections, without an adequate supply of certain nutrients, the immune system is unable to operate at peak capacity. Studies done with animals have shown that deficiencies of the following nutrients adversely impact immune function:
- Folic acid
- Vitamin A
- Vitamin B6
- Vitamin C
- Vitamin E
It's important to note that the interrelationship between deficiencies of those nutrients and overall health has yet to be demonstrated, and controlled studies with human subjects have yet to be carried out. However, given that eating a balanced, nutritious diet rich in the nutrients above will certainly not harm your health, there's no reason to hold off on prioritizing your intake of those vital nutrients.
If you're concerned that diet alone will not meet your micronutrient needs, due to ethical restrictions, allergies and food sensitivities, or simply personal preferences, it may be valuable to seek out high-quality supplements to fill in the gaps. For instance, if you're worried your diet may not provide an optimal amount of protein to meet your body's amino acid needs, adding a well-formulated essential amino acid supplement, such as the Amino Co. essential amino acid blends, can make up the difference.