Natural killer cell

Chronic fatigue syndrome
Numerous studies of chronic fatigue syndrome have found evidence of reduced natural killer cell function. Some studies have showed natural killer cell function correlates with illness severity. Inconsistency in laboratory preparation and analysis have made it difficult to compare results between laboratories or use NK function as a consistent biomarker.

Probiotics
Some probiotics have been shown to increase NK function, including Lactobacillus rhamnosus HN001, Bifidobacterium lactis HN019 and Lactobacillus casei Shirota

AHCC
In animal models, Active Hexose Correlated Compound (AHCC) has been show to increase NK activity. . Other studies have found no significant increase in NK function.

Stress
There is evidence in humans and animal models that psychological stress and physical stress, for example surgery,   decreases NK function and promotes tumor development and metastasis. Mindfulness based meditation or stress reduction may increase natural killer cell function.

Smoking
Smoking decreases natural killer cell function.