I am often getting sick. I believe that it is because of smoking, since I get my throat inflamed a lot. Beside losing that habit is there any diet, that could make my immune system stronger?
Eat a healthy, well balanced diet. If you're already doing that then add a daily multi vitamin. Still getting sick? Seek the advice of a doctor.
For the past 15 years or so of my life I've gotten sinus infections and strep throat every year like clockwork around the later part of February. Last year I decided to go to a doctor due to asthma-like symptoms. He refereed me to an asthma and allergy specialist that told me I had asthma and pretty sever allergies. I started following his medical advice and, so far, haven't been sick this year.
My sister, like you, was always getting strep throat, bronchitis, and other upper respiratory infections. A couple years ago she and her doctor had had enough and looked into it. Turns out she needed to have a tonsillectomy. Afterwards she's been healthy and happy.
My point is, diet usually isn't enough to fix being ill. It will always help you stay healthy, but often times getting sick is out of your control. I'll say it again, discuss this with your doctor.
Getting proper nutrients is essential to maintaining a healthy immune system. Human bodies are built on a system of threes. We start with three macronutrients: carbohydrates, proteins, and fats. Each one of those is broken down by a system of three micronutrients. The micronutrients' balances are maintained through a system of threes as well. The same is true of plants, which is why we give them three part fertilizers, such as nitrogen, potassium, and phosphorus.
If you don't have enough of a balance in these things, your body will become weakened and ill. You should start by maintaining a healthy lifestyle of proper exercise and nutrition. Exercise plays almost as large of a role in staying away from the doctor as proper diet (Source). Taking a vitamin will help to ensure that you are getting most of your necessary vitamins and nutrients, and will help largely with staying away from the doctor.
Lastly, as Sparafusile pointed out, there are some conditions that will require medication or surgical intervention to prevent you from being sick, such as hormonal imbalances. Depending on the type of illness you keep encountering, it would be wise to seek a specialist if you are certain you are maintaining a healthy diet and exercise routine but continue to fall ill.
If your throat is getting inflamed from smoking, boosting your immune system won't help, because the inflammation is your immune system fighting back. Now as far as your immune system being stressed goes, I've found Oil of Oregano to be very good (in the 2 years I've had it in my house, and have taken it when I feel slightly sick, I've never come down with anything serious). Also very important is getting a good amount of sleep - particularly at least a few hours before midnight.
It is the immune system that is responsible for inflammatory responses. Inflammation is generally meant to be a way to help fight infection, but unfortunately sometimes the body's inflammatory response is exaggerated, doing more harm than good. If your "sickness" is because of inflammation, then you want foods which inhibit inflammation, not promote it. Such foods include those with omega-3 fatty acids such as sardines, salmon, herring and walnuts. Other foods which can naturally reduce inflammation include onion, garlic, spinach and ginger.
You should also stay away from omega-6 fatty acids, which promote inflammation. So try to avoid vegetable oils (consider olive oil instead) and deep fried foods.
Check out this website I found on the topic as well: http://theconsciouslife.com/anti-inflammatory-diet-how-to-balance-omega-3-omega-6-fats.htm