The body has a variety of free-roaming cell populations that function as immunogenic agents. Most immunogenic cells fall into the category of white blood cells, but some remain in tissues and are not found in the blood.

Small molecules that must combine with large proteins to become immunogenic are called haptens. Haptens alone are not immunogenic, but when they bind to proteins in the body, they can elicit an immune response.

Because the molecule is too small to cross-link with immune-cells

Hygienic foods that are neither antigenic or immunogenic to the human body includes vegetable juices made from carrots, tomatoes, cucumbers, and celery. Leafy green vegetables, and nuts. All foods should be consumed in moderation.

Since immunogenic means 'Capable of inducing an immune response; antigenic.' I would suggest that nonimmunogenic would mean that an immune response would not be induced.