A strong immune system is our most important shield. But it is a double-edged sword: if it reacts too weakly, we get sick. If it reacts too strongly or wrongly, it attacks the own body (autoimmunity). This fine-tuning is deeply anchored in our DNA.
The genes of the HLA complex (Human Leukocyte Antigen) are like the ID checker of the body. They present fragments of viruses (or gluten) to the immune cells. Certain variants like HLA-DQ2/DQ8 are notorious for triggering celiac disease.
Other genes like IL6 or TNF control the intensity of inflammation. Some people are genetically "pro-inflammatory". They tend to have permanent, subtle inflammation ("Silent Inflammation") which can damage vessels in the long run.
Genetics is not fate. Knowing your predisposition allows you to get the best out of your biology through a targeted lifestyle.