The two main reasons people choose to fast are for religious purposes and for cleansing and weight loss purposes. However, any time you're making a large-scale change to your lifestyle, your body is impacted in uncountable ways. Besides just maintaining a caloric deficit, fasting may impact your immune system. The question is, how?
How The Immune System Works
Your immune system is a complex combination of cells and organs in your body that is working 24/7 to prevent external infection, keep internal bacteria in check, and keep you healthy. There's no one single "immune organ", though the closest you might get is the white blood cells.
White blood cells, also known as leukocytes, are the primary method the immune system uses to defend your body. These are divided up into several categories.
- Phagocytes: These are the cells that actively fight off things like bacteria and viruses when they get into your system.
- Lymphocytes: These are the cells that learn about invading particles, bacteria, viruses, and other contaminants, figure out how to destroy them, and maintain a record of how it worked to more successfully fight off future invasions.
You're constantly surrounded by bacteria, viruses, allergens, and other "antigens", foreign particles that can cause problems when they get into your system. Your body is always fighting these off, to some degree or another.
If your immune system isn't operating at full capacity, you will have decreased resistance to disease. This means you will get sick more easily, you will stay sick for longer, and you may need outside assistance from an antibiotic, antiviral, or other treatment to help your body fight off the invaders.
Conversely, if your immune system is operating at over normal capacity, it can misidentify and attack things that aren't invaders. For example, if your body erroneously identifies connective tissue as a foreign invader and attacks it, you'll get inflammation in your joints; rheumatoid arthritis. This is the general cause of various autoimmune diseases.
So how, specifically, does the immune system do what it does?
It starts with your body detecting when an invader has arrived. This could be exposure from an open wound like a cut, from breathing in a particle such as the flu or the coronavirus, or from having that substance enter your body in another way, like food poisoning or a vaccine.
When an invader is detected, a certain subset of the lymphocytes in your blood identify it and essentially check it against their known library of invaders. This library is called your antibodies. Your body can store antibodies for various invaders, though those antibodies aren't always long-lasting or effective. Some examples:
- Polio. If you are infected with polio, your body has a hard time fighting it off. A vaccine, with a non-infectious strain of polio, teaches your body how to make antibodies for it, so it can fight off the real thing if it ever shows up. Polio immunity is life-long in virtually all cases.
- Chickenpox. Almost all of us catch chickenpox once as a child, and never again. There exist vaccines, but often children are simply left to catch the disease unless other factors exist that would make it more dangerous than it normally is.
- The flu. Your body can learn and fight off the flu, but there are actually many strains of the flu, which is why your flu shot doesn't protect you for more than the year you get it. You need a vaccine against every strain of the flu to get more total protection.
- Covid-19. The novel coronavirus is not a virus humans have ever been exposed to before, which is why it's so highly infectious; your body has no natural immune response to it. It remains to be seen if a vaccine can be developed and how long it will last when it does.
In general, immune responses come in three categories. Innate immunity is immunity you're born with and is a sort of library of protection passed down in your genetics. 
Adaptive immunity is the above; your immune system encountering, learning how to fight, and storing the instructions on fighting various diseases. Passive immunity, meanwhile, is temporary immunity obtained from a transient place. The most common example of this is in babies; mother's milk passes on passive immunity to what the mother has encountered. This helps the child stay healthy and grow enough to develop their own immune system responses.
Organs Involved in the Immune System
While white blood cells are the movers and shakers of the immune system, they don't just spring fully-formed from your blood itself. Rather, they are produced, stored, circulated, and otherwise adjusted by other organs in the body.
Your appendix is known to be part of the immune system, though it's not clear exactly how it works or what it does. Some believe that it acts as a storage site for gut bacteria. Others believe it's a remnant of an organ that has since atrophied and it doesn't really do much.
Your bone marrow is a critical component of the immune system. Bone marrow is what produces stem cells, which grow to form other cells, like red blood cells, platelets, and white blood cells.
Your lymph nodes are small glands present in various locations throughout the body. They essentially sample your blood and other systems, look for signs of invaders like bacteria, viruses, and cancer cells, and kill them.
Your skin is technically an immune system organ. It serves primarily as a physical barrier against intrusion but also contains glands and immune cells for immediate response to invasion from an allergen or exposure to infection via a wound.
Your spleen is an organ that filters your blood and is a storage site for white blood cells. You can think of it as a training camp and barracks for the soldiers fighting against invaders.
Your thymus is a gland in your upper chest that stores certain kinds of immune cells, helps them mature, and directs them to fight infection.
As you can see, there are a lot of organs throughout your system that each play a role in your immune response. Diet obviously would affect them, but how?
How Diet Impacts the Immune System
Diet can boost or inhibit the immune system, and it's all in what you eat.
Contrary to popular belief, while vitamin C is a beneficial vitamin for your body, it's not a super immune booster. Your body needs enough of it, but taking more than a normal dose isn't going to supercharge your immune system, it will just pass through your body and be expelled.
Your body needs an appropriate amount of various beneficial nutrients, including protein, vitamins, and minerals, in order to function. Lacking certain vitamins leads to deficiencies, and in some cases, you can damage your body by taking too much of certain nutrients. For example, too much iron in your system can make your body worse at fighting off malaria, because the parasites that cause malaria feed on iron.
Probably the single most damaging nutrient you might consume on a daily basis is sugar. Sugar has a wide range of effects on the body, some positive, some negative. Your body needs energy to survive, and sugar is very energy-dense, to the point that your body will use sugar preferentially over other sources of energy, like protein and fiber. This is why sugar energizes you; it's very easily broken down and turned into energy. It's also why you gain weight if you eat too much sugar; your body is storing that excess energy as fat for use when nutrients aren't as readily available. Of course, with the modern availability of food, you're rarely without that fuel, so your body always has the surplus to store.
Sugar primarily affects white blood cells, when it comes to the immune system. In the short term, it inhibits the white blood cells that attack diseases, making your immune system weaker and less able to fight off intruders. This effect lasts for a couple of hours after consuming sugary foods or drinks.
In the longer term, sugar leads to diseases like cardiovascular disease, diabetes, and body-wide inflammation. All of these put stress on the body and the immune system, making it harder for you to fight off diseases. This is why people with diabetes tend to get sick more often.
So how much sugar is too much sugar? Research shows that all it takes is around 100 grams of sugar to lead to that suppressant effect. Keep in mind that you're getting a lot of sugar from a lot of different sources; a few grams here, a few grams there. A sugary beverage, like a 20oz bottle of coke, contains 65 grams of sugars. A grande chai latte from Starbucks has around 42 grams. It doesn't take a lot to reach 100 grams a day.
How Fasting Improves the Immune System
At this point, it should be clear that fasting can improve the immune system by reducing the amount of sugar you're consuming on a daily basis. If the only thing you're eating during a day is a small serving of vegetables, or if you're limiting yourself to a healthy formula smoothie, you're dramatically restricting the amount of sugar you consume.
There's one other benefit to fasting: producing more white blood cells. When you fast, your body enters a "power-saving mode" where it starts to shut down various extraneous systems to conserve energy. Since you're not getting energy from external sources, your body starts to burn stored fat for energy, which is why fasting is so often recommended as part of a weight-loss plan.
One thing the body does when you fast is it starts to suppress the immune system. White blood cells start to die off or be consumed for energy by passing cells, which reduces the effectiveness of your immune system while you're fasting.
When you end your fast, your body starts turning all of those systems back on. It produces more white blood cells to replace what is consumed while you were fasting. 
The benefit here is actually relating to aging, in a sense. White blood cells get old and less effective the longer they circulate throughout your body. There's natural turnover, of course; older cells die and are absorbed, and newer cells are produced to replace them.
When you fast, your body kills off a lot of those old, less effective white blood cells. When you then stop fasting, your body produces new, more effective white blood cells to replace them. Essentially, you're stimulating the healing of the immune system by fasting.
So, fasting has two primary effects; reduction of sugar to cease suppressing the immune system, and strengthening the immune system by producing new, lively white blood cells.
Can Fasting Hurt More Than it Helps?
The keen-eyed among you might have noticed a few lines up there that may be worrying: fasting suppresses the immune system initially, and the benefits primarily come when you end the fast.
What this means is that, when you start your fast, you are in your most vulnerable state. It's only once your fast ends that you get the benefits of it, at least in terms of immune activity. Other benefits, like reduced sugar intake and increased fat burning, happen immediately.
Long-term fasting, then, isn't as effective. What's actually most effective is a shorter term of cyclical fasting, called intermittent fasting. This gives you the short-term suppression of the immune system necessary to stimulate the cycling of white blood cells but doesn't leave you in a vulnerable state for a long time.
Fasting is good for you in small doses, but the best benefits come from using it in conjunction with other beneficial activities, such as regular exercise and a healthier diet. When paired with a healthy lifestyle, occasional fasting can potentially give you a much stronger immune system than you would otherwise have.
