our immune system is constantly engaged in a battle with pathogens. When these harmful microorganisms overpower your immune system, you are infected with the pathogen and you fall ill. However, the germs cannot multiply continuously in your body because your immune system produces killer cells that destroy the microbes and restore your health. A strong immune system can easily inhibit and limit infections and preserves your health. But when the immune system is weak, it cannot perform its functions and you become susceptible to infections.
The immune system is activated by danger signals known as danger-associated molecular patterns (DAMPs). These signals help the immune system to differentiate between the cells that are healthy and unhealthy. Cells are unhealthy when they are infected or damaged by non-infectious agents such as cancer and sunburn. When pathogens invade the body cells, they release signals known as pathogen-associated molecular patterns (PAMPs). The immune system recognizes these signals and acts accordingly. If these signals fail to activate the immune systems, you fall ill. In some cases, the immune system is activated even in the absence of a real threat or the activities of the immune system continue even after the danger has passed away. Such abnormal immune response leads to allergic reactions and autoimmune diseases. 
What is the Immune System?
The immune system is a highly complex system that pervades the entire body. A huge number of immune cells are circulating throughout the body and there are immune cells located at specific areas of the body. There are specialized cells that are recognizing the dangers, communicating with the other cells and fighting the ailments. The immune cells develop from the stem cells present in the bone marrow. The innate immune cells that form the body’s first line of defense to infections such as the macrophages, dendritic cells, basophils, neutrophils, eosinophils, monocytes and mast cells are derived from the common myeloid progenitor stem cells. The adaptive immune cells known as the B cells and T cells that fight specific microbes based on the immunological memory of an earlier infection develop from the common lymphoid progenitor stem cell. This same stem cell is also the precursor to the natural killer (NK) cells. The natural killer cells defend the body like the innate cells and are also known to preserve the immunological memory like the adaptive cells. All these immune cells circulate in the bloodstream and are commonly known as the white blood cells. When the level of any type of immune cell in the blood is below or above normal, it is a sign of a health problem.
The T cells mature in the thymus, a small organ located in the upper chest.
The lymphatic system is an important part of the immune system. It comprises of a network of vessels through which extracellular fluid known as lymph circulates and converges in the lymphoid organs called lymph nodes. The lymph nodes are present throughout the body. They are the sites where the immune cells communicate. When a pathogen is transported through the lymph from a distant body part into a lymph node, the adaptive immune cells present in the lymph node recognize the harmful germ. The immune cells are then activated. They start replicating and leave the lymph node to combat the pathogens.
The spleen is not directly associated with the lymphatic system. Immune cells present in specific sites of the spleen are activated when they recognize pathogens. Immune cells are present on the mucosal surfaces of the respiratory tract and the gastrointestinal tract that are the main entry points of the disease causing microorganisms. The skin, which forms the body’s first line of defense against pathogens present in the environment, contains immune cells in specific layers. 
How to Boost the Immune System
The immune system can weaken and make you vulnerable to infections and chronic diseases due to several reasons. Not getting enough sleep is known to weaken the immune system. Although, the association between sleep and the immune system is currently unknown, nevertheless, studies have shown that adults who usually sleep for 7 to 9 hours have stronger immune system. Lack of exercises is also known to affect the immune system. Daily moderate exercise such as walking for about 30 minutes could help in boosting the infection-fighting capacity of the immune system. Excess sugar intake is believed to reduce the ability of the immune system to fight bacteria. Adding plenty of fruits and vegetables to the daily diet and especially eating foods rich in beta-carotene, Vitamins C and E and zinc help in strengthening the immune system. Stress weakens the immune system. Chronic stress is known to suppress the immune system. Lowering stress through stress management techniques helps in reducing the levels of the stress hormones, thereby helping the immune system to function effectively.  In addition to a healthy lifestyle, certain herbs and supplements are known to boost the immune system naturally.
Garlic is recognized for its therapeutic properties for centuries. Studies have shown that garlic helps in stimulating specific immune cells especially the lymphocytes, natural killer cells, eosinophils, dendritic cells and macrophages, thereby improving the function of the immune system.  Over 100 sulfuric compounds are present in garlic that can help in destroying bacteria and assist in fighting infection. Garlic can boost your immune system only when eaten raw. The sulfur enzymes in garlic are destroyed by heat and water inactivate sulfur enzymes, therefore cooked or dried garlic cannot exhibit the desired effect. 
The German Commission E has approved the use of Echinacea for fighting recurrent infections. The root of Echinacea is especially known for its healing effect. It is best known for its ability to activate the immune system and helps in fighting bacterial and viral infections. It is considered most effective when taken in the form of tincture. Traditionally, Echinacea is taken as an immune system booster to speed up healing of cold and flu. It works best when taken at the early stage of cold or flu. Pregnant women should use Echinacea with caution. This herbal immune booster is not recommended for people allergic to sunflowers and those who are suffering from a severe systemic immune disorder or a collagen disease. 
Because of their high antioxidant content, all types of berries are known to be good for the immune system. However, elderberry appears to be a more powerful immune booster. It obstructs the activities of an enzyme known as neuraminidase present in viruses that enables the pathogens to puncture the cells walls and infect the cells. Studies have shown that elderberry aids faster recovery from influenza. It is known to alleviate the symptoms of influenza virus infections such as cough, fever and muscle pain within 24 hours in 20% patients and it takes 48 hours to improve the symptoms in 73% patients.  In addition to its antiviral effect, elderberry is known to activate the healthy immune system. It helps in promoting production of inflammatory cytokines. A standardized black elderberry extract is believed to possess immunostimulatory as well as immunoprotective properties. The beneficial effect of black elderberry extract may help people suffering from AIDS and cancer. 
A number of studies have revealed the beneficial effect of ginseng, especially Panax ginseng, on the immune system. In a study, researchers found that 100 mg of aqueous or standardized ginseng extract taken every twelve hours for eight weeks helped in stimulating the immune system in healthy volunteers. The standardized extract of ginseng was more effective than the aqueous extract. According to another study, regular intake of 100 mg of Panax ginseng extract standardized to 4 percent total ginsensenosides for twelve weeks helped in increasing the effectiveness of influenza vaccine. Panax ginseng helped in boosting the NK cell activity and increased the level of antibodies. Panax ginseng is therefore considered a relevant herbal remedy for supporting the immune system in people prone to infections especially older adults. 
Astragalus is an herb widely used by Chinese herbalists. It is believed to be an effective immune booster. In a study, researchers found that astragalus could help in restoring the normal immune function in cancer patients with weak immune system. It is known to be an effective general immune tonic that can help in lowering the risks of cold, cough, sore throat and ear infections in children.  Traditionally, decoction prepared by simmering the dried herb in water for a long time is used for boosting the immune system. The dried roots are also taken in the form of powder, capsules and tincture. Up to 100 ml of astragalus decoction or one teaspoon of tincture can be taken daily. It is often taken with reishi mushroom, another powerful immune booster. 
6. Shiitake Mushroom
Shiitake mushroom, also known as Lentinula edodes, can help in enhancing human immunity. In a clinical study, healthy young adults were asked to consume 5 or 10 grams of shiitake mushrooms daily for four weeks. At the end of the study period, researchers noted that consuming shiitake mushroom increased proliferation of the gamma-delta T cells and the natural killer T cells. Furthermore, mushroom intake was associated with improved gut immunity. The pattern of cytokine production changed after eating shiitake mushroom. It helped inhibit production of C-reactive protein, indicating lower inflammation. 
7. Aloe Vera
The most potent immune boosting compound present in aloe vera is acemannan. In experimental studies, acemannan has helped in increasing the production of immune cells in mice with compromised immune system. Another study suggests that acemannan can help in boosting the effectiveness of medications used for arresting progression of AIDS. 
8. Vitamin D
A number of studies have found an association between Vitamin D deficiency and prevalence of immune disorders such as multiple sclerosis, Crohn’s disease and type 1 diabetes. Insufficient Vitamin D in the body may also increase risks of infections such as tuberculosis. The immune cells convert the precursor for Vitamin D into the active form of Vitamin D. This helps in increasing the responses of the immune cells to pathogens and supports maturation of the antigen presenting dendritic cells. Research suggests that the innate and adaptive immunity of the body is adversely affected by Vitamin D deficiency.  Half an hour of sun exposure and eating oily fish and eggs help in boosting the vitamin D level in your body. You can also get Vitamin D from multivitamins and fish oil supplements. 
Probiotics supplied by yogurt or fermented milk is known to stimulate the immune system. They are known to boost nonspecific cellular immune response by activating the natural killer cells, macrophages, T-lymphocytes and various cytokines. They could enhance the gut mucosal immune system.  Studies suggest probiotics could help in improving immune response related ailments such as viral infection, eczema and allergy and can enhance the effect of vaccines. 
10. Essential Oils
Certain essential oils are believed to stimulate the immune system. The molecules of the essential oils can easily penetrate the skin through the pores. When absorbed into the skin, they enter the intercellular fluid and are transported into the bloodstream and the lymphatic system. It is believed that specific essential oils are capable of stimulating production of white blood cells and help in strengthening the immune system.  (SNAPSHOT REQUIRED) Frankincense essential oil is known to work as an immune system booster. It helps in increasing production of the signaling molecules known as cytokines and immunoglobulins or antibodies and could boost the activities of the T cells.  Clove essential oil is also known to boost the immune response. Research has shown that it helps in increasing the total white blood cell count. Furthermore, in laboratory studies, clove essential oil helped in restoring the healthy cellular and humoral immune responses in immunosuppressed mice.  The immunomodulatory effects of ginger essential oil have been demonstrated in laboratory studies. The beneficial effect of ginger oil on the cell-mediated immune response is believed to help people suffering from autoimmune diseases or chronic inflammation. 
11. Vitamin C and Zinc
The body’s demand for Vitamin C decreases rapidly during infections and stress. Vitamin C supplementation is known to stimulate proliferation and activities of the immune cells. Furthermore, Vitamin C helps in protecting the cells from the harmful effect of reactive oxygen species. Similarly, deficiency of zinc is known to weaken the immune system and increase oxidative stress. According to clinical trials, up to 30 mg of zinc and up to 1 gram of Vitamin C can be taken. Adequate intake of Vitamin C and zinc helps in reducing duration and symptoms of respiratory tract infections. 
12. Cysteine Supplement
The optimal level of the antioxidant glutathione present in the lymphoid cells helps the immune system to work effectively. Even moderate changes in the glutathione level adversely affects the lymphocyte functions. The amino acid cysteine is the precursor for glutathione. Cysteine supplementation is found to be effective in enhancing the functions of the immune system. Research has shown that the immunological function in HIV infected patients suffering from cysteine and glutathione deficiency improved with cysteine supplementation.  Whey protein is a natural source of cysteine. In glutathione deficient people, ingesting whey protein is known to enhance the plasma glutathione level.