Yoga can greatly help in strengthening the lymphatic and, by extension, the immune system by inhalation through the chest and lifting the legs, opening the arms as well as with inversions dynamic or restorative like the posture of viparita karani.
In Ayurveda for a strong immunity the proper functioning of all vayus is needed and the opposite appears to a weak vyana vayu. We need a strong agni, a good digestion and elimination that can provide the essential ojas for health and a strong immune system. This creates the essential rasa plasma for alkaline blood, strong muscles, strong bones and nerves and a good reproductive system.
Good food and positive thoughts nourish the immune & protect us from internal and external factors!