A member emailed this great list of the Top Immune System Boosters to me.  The one important point I want to add is this:  Optimizing and maintaining a strong Immune System begins with a great diet.  Supplements are no replacement for a great diet, but are available just to help us optimize. Regardless of what the disease may be, PN and or others, doctors and derms would have to agree that it's better to have a strong Immune System rather than a weak Immune System.

Here are the top Immune System Boosters in no particular order. 

Ashwagandha (Withania somnifera)

Astragalus (A. membranaceus)

Echinacea (E. purpurea, E. angustifolia)

Eleuthero or Siberian Ginseng  (Eleutherococcus senticosus)
 
Garlic (Allium sativum)

Schisandra (S. chinensis)

Medicinal Mushrooms
     Reishi  (Ganoderma lucidum)
     Shiitake  (Lentinus edodes)
     AHCC  (Active Hexose Correlated Compound)
     Turkey Tail  (Coriolus versicolor, Trametes vesicolor)

Vitamin C

Colostrum

Methylsulfonylmethane

Bee Pollen

Burdock

Selenium

Spirulina

Spleen Glandulars

Thymus Glandulars