Isopathic remedies are made from the actual substances that cause allergic reactions or illnesses. For example, if you are allergic to cats, you could make a homeopathic remedy out of cat dander; if you're allergic to ragweed, diluted ragweed pollen would be an appropriate isopathic remedy.
This practice is different from standard homeopathic remedies, which are based on unrelated substances that coincidentally produce a similar symptom picture.