The immune system is a large network of organs, white blood cells, proteins, antibodies and chemicals. These systems function together to protect you from foreign invaders (bacteria, viruses, parasites, and fungi) that cause infection and disease. To maintain immune function, you should focus on eating a balanced diet rich in nutrient-dense whole foods, especially plant foods such as vegetables, fruits, legumes, nuts and seeds. You should avoid or limit refined grains and ultra-processed foods that are high in sugar.