A Guide to the Human Immune System

The human immune system is a complex interconnected network of bodily structures, organs, and processes that work together to protect the body from foreign invaders, or pathogens. From Day 1 of life, the immune system protects the body from multiple pathogenic invaders, continually adapting to new threats throughout the developmental years of life. By adulthood, the immune system is capable of detecting a wide variety of viruses, bacteria, and even parasites, and not only distinguishing them from the body’s own healthy systems as pathogenic invaders, but handily attacking and defeating them. As the body ages, biological changes occur due to natural aging and environmental factors that weaken the immune system, making elderly people particularly susceptible to immune disorders. In this article, we provide an overview of the immune system, including how it works and how it evolves throughout our lives.


The human immune system protects the body from foreign invaders by producing a defensive response that is generated by the thymus, spleen, lymph tissues, lymph nodes, and the lymphatic system, and through the production of leukocytes and antibodies.1

Leukocytes/white blood cells. The most important cells involved in the immune system are white blood cells, also known as leukocytes. These cells seek out and destroy disease-causing organisms or substances. Leukocytes are produced and stored in many locations in the body, including the thymus, spleen, and bone marrow, as well as in lymphoid tissue and lymph nodes, which are found throughout the body. The lymphatic system is a network of blood vessels that circulate the leukocytes throughout the body carrying them to the organs, tissues, and nodes. This allows the leukocytes to continually monitor the body for germs or other pathogens. When the leukocytes find a target, they begin to multiply and send signals out to other cell types to do the same. There are two different types of leukocytes: phagocytes and lymphocytes.1,2

Phagocytes. Phagocytes “eat” invading organisms and include 1) the neutrophil, which is the most common type—it usually attacks bacteria; 2) the monocyte, which is the largest type—it plays several roles in immunity; 3) the macrophage, which patrols for pathogens and removes dead and dying cells; and 4) the mast cell, which helps heal wounds and defends against pathogens.

Lymphocytes. Lymphocytes allow the body to remember and recognize previous invaders and help the body destroy them. These include 1) the B-lymphocyte, which stimulates B-cells to produce antibodies and helps alert T-lymphocytes; and 2) the T-lymphocyte, which stimulates T-cells to destroy compromised cells in the body and helps alert other leukocytes to start attacking. Helper T-cells coordinate the immune response by communicating with other cells, which stimulates B-cells to produce more antibodies and attracts more T-cells and cell- eating phagocytes.  Killer T-cells (cytotoxic T-lymphocytes) attack pathogenic cells and are particularly useful for fighting viruses. They work by recognizing small parts of the virus on the outside of infected cells and then destroying the infected cells.1–5

Antibodies. Antibodies are cells that lock onto an antigen when it has been detected. Antigens are proteins that are found on the surface of a . The B-lymphocytes make billions of different types of antibody receptors (B-cells) capable of binding specific antigens and marking them for death. It is the job of the helper T-cells to give a signal to the immune system that allows the release of the particular B-cell that has been programmed to bind to the specific antigen being detected. Once the binding occurs, cells
like T-cells and phagocytes step in and kill the antigen. This process produces memory cells and plasma cells that continue to secrete the antibody necessary for binding that particular antigen and initiating the kill process. That way, if that particular antigen, or foreign invader, ever appears in the body again,
the memory cells are ready to produce a more rapid and much stronger immune response.1–5

The lymphatic system. The lymphatic system carries a substance called “lymph,” which is excess cell- free tissue fluid that has been drained from the body compartments. The lymph is passed through the lymph nodes, picking up the activated B- and T-cells and carrying them out into the bloodstream.1–5

Lymph nodes. The lymph nodes are small oval structures that filter the lymph before it re-enters the bloodstream. Lymph nodes are able to remove 99 percent of all the foreign substances that pass through them. Lymph nodes are located throughout the body, such as at the base of the neck, the armpit, and the groin. Swelling or inflammation of a node is usually in response to an infection in one of the areas that is drained by that particular lymph node.1–5

Lymphoid tissue. Lymphoid tissue is scattered throughout the body and is home to the lymphocytes, which are usually present in parts of the body most often exposed to foreign invaders. These sites include the gastrointestinal system as well as the tonsils. Tonsils play a role in protecting the body from air-borne infections.1–5

Thymus. The thymus is located in the lower part of the neck and front of the chest and contains lymphoid stem cells, which are immature cells that divide rapidly and mature into T-cells. As we age, the thymus becomes smaller and loses most of its active immune cells.1–5

Spleen. The spleen is the largest of the lymphoid organs and is located in the upper left of the abdomen behind the stomach. The spleen filters the blood, getting rid of blood cells that are old or defective. It also stores T- and B-cells. 1–5

Bone marrow. Bone marrow is. the spongy tissue inside the bones. Lymphocytes are produced in bone marrow. Some stay there and develop into B-lymphocytes, while others head to the thymus and become T-lymphocytes.1–5


Innate immunity.

Innate immunity is the immunity with which we are born and includes the external barriers of the body, such as the skin and mucous membranes of the throat and gut. The innate immune response is more general and nonspecific. If a pathogen is able to get through the innate immune system, adaptive immunity kicks in.3,4

Adaptive (or acquired) immunity.

Adaptive immunity is highly specific to particular pathogens and can be either temporarily borrowed from an artificial source (passive immunity) or can be permanently acquired through the body’s own immunological memory (active immunity).3,4

Passive immunity.

This type of immunity is “borrowed” from another source and is only effective temporarily. For example, a baby receives antibodies from the mother through the placenta before birth and in breast milk following birth. This passive immunity protects the baby from some infections during the early years of its life, but the baby will need to develop its own immune response for long-term protection from pathogens. Passive immunity can also come from artificial sources. For example, antibodies can be administered to someone intravenously or intramuscularly in the form of donated blood plasma or serum, pooled human immunoglobulin, immunoglobulin that is donated by an immunized person or from a person recovering from a specific disease, and as monoclonal antibodies. These treatments can be used as preventative measures in cases of immunodeficiency diseases, as treatment for several types of acute infections, and to treat poisoning. Passive immunity will provide immediate protection to the body, but the body will not develop immunological memory from these sources, and thus the person will be at risk of being infected by the same pathogen later unless he or she acquires his or her own active immunity or is vaccinated.3,4

Active immunity.

This type of immunity is permanently stored in the body. After the body is exposed to a pathogen or vaccination and the immune system response kicks in, the body builds up a store of specific antibodies that are produced to react to the antigens of that specific pathogen. This is sometimes referred to as immunological memory or cell memory because the immune system will “remember” the antigens associated with that particular pathogen to which it was previously exposed and thus can initiate the immune response and eliminate the threat much more quickly the next time it is exposed to that pathogen. Vaccinations result in active immunity.3,4


Immunodeficiency disorders.

Primary immunodeficiency disorders.

Some people are born with an immunodeficiency, and this is known as a primary immunodeficiency,
even if the symptoms don’t appear until later in life. For example, some people are unable to produce enough of an immunoglobulin antibody that guards the entrances of the body (IgA deficiency), resulting in more frequent colds and other respiratory infections. Another example would be thymic dysplasia in which a person is born without a thymus gland, which would impact their ability to produce T-lymphocytes.2,5

Acquired (or secondary) immunodeficiencies. Secondary immunodeficiencies usually develop as the result of a disease, malnutrition, injury, or other medical problem. HIV (human immunodeficiency virus) infection/ AIDS (acquired immunodeficiency syndrome) is an example of a secondary immunodeficiency disease that slowly and steadily destroys the immune system.

Some medicines can also suppress the immune system. For example, chemotherapy not only attacks cancer cells, but other fast- growing, healthy cells, including those found in the bone marrow and other parts of the immune system.2,5

Autoimmune disorders. In auto immune disorders, the immune system mistakenly attacks the
body’s healthy organs and tissues as though they were foreign invaders. Lupus, which is a chronic disease marked by muscle and joint pain and inflammation (the abnormal immune response also might involve attacks on the kidneys and other organs) is an example of an autoimmune disorder. Multiple sclerosis (MS) is another example of autoimmune disorder in which dysfunctional T-cells attack myelin (the fatty substance that coats and protects nerve fibers in the brain and spinal cord).2,5

Allergic disorders. An allergic disorder is a more common type of autoimmune disorder that happens when the immune system over reacts to exposure to allergens in the environment, such as pollen or pollution. The immune response can cause body parts that come into contact with the allergen to swell, eyes to water, sneezing, and
in severe cases, anaphylaxis, which is an allergic reaction that can result in death if not immediately treated. Asthma is an example of an allergic disorder in which the lungs are over sensitive to certain allergens (like pollen), causing breathing problems. Another example would be a bee sting allergy.2,5

Cancers of the immune system.

Examples of cancers of the immune system are leukemia, which is caused by an overgrowth of leukocytes, and lymphoma, which involves the lymphoid tissues.2,5


In the later years of life, the organs and processes involved in the immune system decline significantly, making us more susceptible to infection, cancer, and auto immune diseases. This collective change in the aging immune system is called immunosenescence, and it affects both our innate and adaptive  immunities.For example, functioning of the thymus gland declines significantly as we age; in fact, this decline is probably the most dramatic change that occurs in the immune system during the later years of life. And because the thymus gland produces the T-cells—aka the pathogen killers—a decline in its functioning means less T-cells and more opportunity for a pathogen to flourish in the body. Another example is the skin, one of the body’s most important barriers to infection. As we age, the skin becomes thinner, drier, and weaker, thus allowing pathogens to more easily enter the body’s tissues. A third example is a decline in the healthy bacteria that live in our gut and help balance our immune function. While science isn’t clear why immunosenescence occurs, research has shown ways to boost the immune system as we age.3,6–9

Get vaccinated. Vaccines help protect infants, children, and teens from serious diseases by aiding in the development of immunity against the diseases before they come into contact with them. Because young children are at increased risk for infectious diseases due to their underdeveloped immune systems, diseases like whooping cough or pneumococcal disease can be very serious—even fatal—for infants
and young children. This is why it is important to start vaccinations early in life to protect children before they are exposed to these diseases.

Vaccines aren’t just for children, however. Seniors should consider getting flu vaccines each year, as well as being vaccinated for pneumonia and herpes zoster—also known as “shingles,” a latent form of the chicken pox. Consult with your physician to see which vaccinations are right for you.10,11 cardiovascular and metabolic health and boosting immunity. If you suspect your diet is not providing you with all your nutritional needs—for example perhaps you really don’t like eating your vegetables—taking a daily multivitamin might be prudent. Keep in mind, however, that actually consuming the fresh fruits and vegetables, if possible, is a better way to get the nutrients than taking a multivitamin. It’s also important to note that more is not necessarily better, so if you do decide to take vitamin supplements, avoid megadoses of a single vitamin; instead opt for a multivitamin. Check with your doctor for advice on what’s best for you.11,12

Supplement diet with probiotics.

Probiotics contain different types of “good” microorganisms, such Saccharomyces boulardii (a type of yeast) and Lactobacillus and Bifidobacterium (types of bacteria) that, in sufficient amounts and in active states, exert positive health effects inside our intestines. Research has shown that supplementing the diet with probiotics increases T-cell and phagocytic activity, especially among older individuals, who often suffer from an imbalance of gut microorganisms. Probiotics have also been shown to reduce the detrimental effects of malnutrition on immunity in elderly people. Probiotics are available in foods (such as yogurt, milk, juices, soy beverages) and as dietary supplements (capsules, tablets, powders). However, the strain and number of bacteria can vary greatly among products and foods, as well as have different uses, so check with your physician or pharmacist before starting a probiotic supplement regimen.13

Reduce stress.

Research has shown that stress weakens the immune system, so engaging in activities that reduce stress is a healthy choice. Regular physical activity like walking, yoga, or even just stretching; joining a support group if you are dealing with hardships in life; attending a mind-body program that focuses on, for example, pain management or heart disease; and even getting a pet have all been shown to reduce stress.14,15

Get adequate sleep. The production of T-cells (the pathogen killers) declines if we are sleep deprived, which means you are at greater risk of becoming ill if you aren’t sleeping well. If you are having trouble getting the proper amount of Z’s each night, create a bedtime ritual and try to stick to it as many nights a week as you can. For example, go to bed at night and get out of bed in the morning at the same time each day, disengage from the TV or computer at least an hour before bedtime, and spend the last half hour or so before bedtime doing something quiet and relaxing like reading, listening to relaxing music, or meditating. Also, avoid eating a big meal or drinking a lot of alcohol or caffeine right before you go to bed.16

Don’t smoke. Smoking is one of the worst things you can do to your body. And reducing your immunity is just one of its many unhealthy effects. Not only does smoking make it harder for your body to fight infections, it makes it harder for your body to fight cancer! If you need help quitting, start by visiting smokefree.gov for tips and resources.11

Moderate alcohol consumption.

Research has shown that alcohol consumption can disrupt immune functioning, meaning if you already have an infection, binge drinking might make it worse or make you more susceptible to a new infection. The United States National Institute on Alcohol Abuse and Alcoholism defines binge drinking as drinking five or more drinks within two hours for men, and for women, it’s four. So keep your alcohol intake moderate (no more than 2 drinks a day for men and 1 drink a day for women). And if you’re sick, it probably would be best to avoid alcohol altogether until you’re well.17

Wash your hands. According to the United States Centers for Disease Control and Prevention (CDC), washing your hands is the number one way to prevent the spread of infection to yourself and others. The CDC recommends washing your hands in warm, soapy water for at least 15 seconds whenever you do any of the following:18

  • Before, during, and after preparing food
  • Before eating food
  • Before and after caring for someone who is sick
  • Before and after treating a cut or wound
  • After using the toilet
  • After changing diapers or cleaning up a child who has used the toilet
  • After blowing your nose, coughing, or sneezing
  • After touching an animal, animal feed, or animal waste
  • After handling pet food or pet treat
  • After touching garbage.


The human immune system is a complex interconnected network of organs and processes that
seek out and destroy pathogens in the body, protecting us from illnesses ranging from the common cold to certain types of cancer. A functioning immune system relies on the collective cooperation and communication between its multiple components. As babies, our underdeveloped immune systems need to borrow pathogen fighters from our mothers, but as our bodies grow and our minds develop so too does our immune system, constantly adapting and learning how to protect us as we continually encounter new pathogens throughout our lives. By the time we are adults, our immune systems are at their peak functionality, and illnesses are few and far between for most of us.

As we enter our later years of life, the immune system starts a steady course of decline. A healthy lifestyle (including the addition of probiotics) and the recommended vaccinations can help strengthen the immune system, and reduce the chances of disease at all stages of life.

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