In the world of nutrients, you've heard a lot about the good ones and the bad ones. Protein is good, fat is bad. Sugar is bad, except the sugar you need. Starches are bad too. But what, exactly, are starches? Do they impact your immune system, or are they bad for other reasons, or are they actually just fine in moderation?
What Even Is Starch?
In the science of nutrients, there are a lot of different biomolecules. These are divided into broad categories, including lipids, fatty acids, vitamins, and so forth.
One type of biomolecule is a carbohydrate. A carbohydrate is, generally, defined as a molecule made up of carbon, oxygen, and hydrogen in a complex configuration. There are simple and complex carbs, there are acids and alcohols and monosaccharides and a whole lot more. If you're interested in biochemistry, there's a lot here to dig into, but it's not strictly necessary to know from a layman's point of view.
Carbohydrates can, among other things, be divided into three broad categories. These are fibers, sugars, and starches. You've probably heard that fiber is good for you, sugar is bad for you, and starches are probably bad, though there are exceptions to all three. 
Your body needs carbohydrates to live. Carbs are present in just about everything, and they're part of a balanced diet. They're very energy-dense compared to fats and proteins, so they're what your body preferentially uses to burn for energy. That's why sugar gives you energy.
Sugars, of course, can be natural or artificial. Basic, simple natural sugars are things like the fructose in fruit, and the lactose in milk. Refined sugars are things like corn syrup and sucralose, added to foods to give them more sweetness.
Sugars are simple carbohydrates. Complex carbohydrates are also made up of sugars, but the molecules are long and harder to break down. This means the energy you get from them is better for sustained energy levels throughout a day, rather than a brief high and crash cycle. There is, again, a lot of fascinating science here about the glycemic index and so forth that isn't quite relevant to this post.
Starches are a very common kind of carbohydrate. They're a complex carbohydrate, which means they're generally considered better for you than simple sugars, but that's generally referring to natural, unrefined starches. The starches you get from refined and processed foods, like white flour, are stripped of most of their nutrients and are mostly just complex sugars.
Starch itself isn't bad for you, per se, but foods high in starch tend to be low in other nutrients, simply because of how modern food processes refine them. Examples of high-starch foods include cornmeal, Rice Krispies, pretzels, refined flours, oats, and bread.
There's also a kind of starch known as "resistant starch", which is a more complex molecule and takes even longer to digest. This is often touted as having health benefits compared to normal starch and is commonly found in foods like plantains, beans, lentils, whole grains, and rice.
The Health Benefits of Starch
Starch can be good for your health in some cases. Resistant starch is usually the starch chosen for its health benefits, but all starch is better for you than just normal sugar. Starches provide you with energy on a slow-release, fueling your body without giving you the peaks-and-valleys high and crash associated with more bioavailable sugars.
Starch is more balanced for blood sugar uses. Diabetics, in particular, have to watch out for sugar highs and lows, which can trigger extreme health issues. Starch is a more moderate way to get blood sugar your body needs without spikes or dips. However, starch has been linked with an increased rate of Type 2 Diabetes, so it's definitely not curative.
Starch is a dietary fiber and prebiotic. Starch makes its way through your digestive system slowly, and it isn't broken down until late in the large intestine. This means it bypasses the high-risk area in the small intestine, where sugars often break down into fuel for bad gut bacteria. Instead, feed the more beneficial bacteria found later in your digestive system. This effect has been observed in pigs, as well as in some human tests, though how much it helps the body is still unknown.
Starch is an ancestral nutrient. There are a lot of cultures around the world that eat large amounts of starches in their daily diets and are actually healthier on average than the typical American. Now, whether or not this means starches are good for you, or that their lifestyles have other benefits and we'd need to adopt the whole package and not just the diet, remains to be studied.
All of that said, starches are a complex subject. There are tons of them with different chemical compositions. The starch you get from a hand-milled ancient grain is different from the starches you get from a bad of all-purpose flour.
Is Starch Bad For You?
There are a few arguments that starch may be bad for you. Indeed, starch can be both good and bad. It provides long-term energy for the body, which is good, but it's still a sugar and can feed bad bacteria and increase insulin resistance, which is bad.
Part of this confusion comes from the simple fact that there are so many different kinds of starches, but modern food labeling doesn't differentiate between them. Starch is kind of a divisive nutrient, in fact. That's why "resistant starch" is a separate category; people like to promote the benefits of one type of starch while acknowledging that other kinds are bad.
The same study in pigs linked above proves two things. First, that resistant starches fed helpful gut bacteria while starving more harmful gut bacteria. That's good! On the other hand, it had an observable effect in depressing the immune system. It's not exactly an immunosuppressant, but it did make the immune system in pigs observably less effective.
Other experiments found that, among people with lupus (which is an autoimmune disease), starchy foods reduced their autoimmune reactions. Since autoimmune diseases are typically an over-active immune system attacking some part of your body, this means that starches make the immune system less active.
This is why saying that starches are good or bad is difficult. Suppressing the immune system in someone who has an autoimmune disease is a good thing. Suppressing the immune system in someone who already has an under-active immune system might make them even more prone to getting sick, and make them worse off overall.
What Should You Do?
Of course, you didn't come here to read a bunch of paragraphs about us telling you vague summaries of vague science. You want actionable advice. You want to know if starches are good, and thus you should supplement them, or if they're bad, and you should avoid them.
The truth is, there's no one piece of advice that will work for everyone.
Starches are sugars. If you're struggling with high blood sugar or are diabetic, you want to carefully regulate your sugar intake, which means monitoring starches as well as traditional sugar content in the foods you eat. 
That said, starches may be better for diabetics than normal sugars, precisely because they take a long time to digest. It's easier for the body to process a consistent, slow drip of sugar added to the bloodstream than it is to process a brief flood and then a drought of easily digested sugars.
Starches may also have some benefits for weight loss. You get energy in a long, slow drip, which means your body can simply use the energy as you need it and receive it. Contrast this with sugars; your body absorbs them quickly but can't use all of that energy. When the body has more energy than it knows what to do with, it stores that energy, as fat. Since starches aren't overwhelming your body's ability to use the energy they give you, they don't get stored as fat nearly as readily.
On the other hand, there's evidence that shows starches do in fact suppress the immune system. If you're in a position where that's a good thing – like you suffer from lupus, or arthritis, or another autoimmune disease – starches might be a good thing to add to your diet. You could supplement with resistant starch, or you can simply adjust your existing diet to cut back on non-starch sugars and raise the level of starches you eat.
Some studies have shown that starches stimulate the production of certain enzymes and acids in the colon, which helps to suppress the formation of tumors. Colon cancer being a dangerous threat, starches might play some role in suppressing its formation. Of course, you're not going to be able to just eat a bunch of starches to cure cancer. You're more likely to have a .01% lower chance of getting cancer if you have a higher starch diet, is all.
Remember that starch is a carbohydrate. If you're trying to eat a low or no-carb diet – like a Keto diet, an Atkins diet, or a Mediterranean diet – you're going to be eating low in starches as well.
Eating starches, including supplementing with resistant starches, is going to pull your body out of ketosis and make it more difficult to burn fat for energy the way you want when you're going after a low-carb diet.
Starch (in general) isn't really as worth worrying about as other parts of your diet. Cutting back on sugars (in general) is going to be better for you than, say, increasing your level of starch consumption. Adjusting your diet overall to reduce carbohydrate consumption is usually going to be healthier for you than any alternative. 
The moral of the story is that diets are fantastically complicated, with a lot of different moving parts. Adjusting those moving parts in a way that benefits your body without stripping you of needed nutrients is a tricky task. That's why dieticians spend so much time learning about biochemistry, after all; it's not something that is easy for a layperson to pick up.
Remember that there are very few things that are universally good or bad for the body. Sugar is bad for you, but you need it to survive. Protein is good for you, but eating too much protein has risks to kidneys, the digestive tract, and other bodily systems.
After all, if something was universally bad for you, we'd call it a toxin and we'd cut it out of what we eat entirely.
Nothing is universally good for you. Vitamins, taken to excess, can drive your bodily systems wild and lead to hair loss, fatigue, nerve damage, and a whole lot more. Even water, when consumed in excess too quickly, can be fatal.
This is why so much of modern dietary advice is all about balance. 
Every single person is different, with different levels of bodily systems, different nutrient needs, different lifestyles, and different environmental stresses. A diet that works for someone else might not work for you. You need to figure out what works to keep you healthy and adjust your diet over time to keep within that range.
This is a complex, life-long task, and it's not one most people are suited to performing. Monitoring carbohydrate intake, sugar levels, and the proportions of macronutrients you consume is just part of an overall healthy diet, and it's something everyone should be doing.
The bottom line is, starch is basically sugar, and excess sugar (and therefore starches) can lower your immune system. Maintain a balanced diet and try to eat clean and you shouldn't have anything to worry about.
