There are many things around us that weaken our immunity. 
Smoking, in particular, not only makes the entire immune system malfunction, but it also increases the likelihood that normal cells will mutate into cancer cells. 

Actually, periodontal disease is also involved. 

Periodontal disease is a condition in which the immune response is ongoing. Therefore, the immune response is prone to malfunction, and the risk of developing autoimmune diseases such as rheumatoid arthritis is inevitably increased. 

It's also related to stress. 
When stress builds up, the brain secretes steroid hormones and neurotransmitters in response to stress, reducing the activity of lymphocytes and cells in white blood cells. 
How to deal with stress is a well-balanced diet, good sleep, light exercise is said to be good. 

Furthermore, when people laugh, natural killer cells (NK cells), which are a type of immune cells, increase. 
Surprisingly, it has been reported that even a smirk boosts immunity. 
Making a smile is effective even when you are sad or in trouble. 

You may also want to be careful about your diet.
About 70% of immunity is in the intestines.
You can maintain your immunity by adjusting the intestinal environment.

Finally, vaccines are also important. 
Vaccines play a big role in reducing the severity of the disease. 

Exactly! But vaccination doesn't always give you antibodies. 
You need to do an antibody test to see if the antibody is actually attached.