In classical homeopathy, it is assumed that there is a force in the body that regulates the processes in the organism and thereby keeps the organism healthy and in balance. It is subtle, and therefore only conditionally equated with the immune system. Samuel Hahnemann called this force Vital force.
Although there are so many different words for it, in the end all speak of the same concept:
A healthy, strong life force means a state of harmony and balance in the human being. Disease, on the other hand, is the expression of a Vital force that is in imbalance. A well-chosen homeopathic remedy will be able to re-harmonize this imbalance.