What foods boost natural killer cells?
Selenium: A deficiency of this mineral reduces the number of natural killer cells, while supplemental selenium has been shown to increase their activity. You need 55 micrograms per day. Good sources include tuna, halibut, shrimp, and brown rice.

Factors that May Raise Natural Killer Cell Function
Exercise [16]
Strength training [36]
Antioxidants [37]
Massage therapy [17]
White button mushrooms [38]

Foods: Vitamins A, D, and E, water-soluble constituents of mushrooms, polyphenolics in fruits and vegetables, and n-3 fatty acids in fish oil can modulate NK cell activities
Cocaine: Intravenous cocaine increases NK cell activity and numbers
Probiotics: Commensal bacteria that help to reinforce and maintain the permeability of the host intestinal cell wall
Resveratrol: A natural compound able to stimulate immune responses including an increase of the NK cell-mediated killing of infected and malignantly transformed cells
NK cells are part of the body's immune system and help fight infection and cancer.

Chronic stress in adults is consistently associated with decreases in natural killer (NK) cell cytotoxicity, lower antibody titers to the influenza vaccine, and decreased lymphocyte proliferation
Importantly, partial sleep deprivation in humans impacts the immune system, reducing the cytotoxic activity of natural killer (NK) cells (Irwin et al., 1994) and impairing antibody production following hepatitis-A vaccination (Lange et al., 2003).
Vitamin C supplements have been shown to alter many aspects of human immune responses, in particular the activation of NK cells, neutrophils and macrophages (Nieman et al., 2002).
Vitamin D triggers the body's immune response by preparing the t-cells for action, setting them up to help antibodies attack infections. We get vitamin D from a variety of sources, including certain foods that make up our modern diets, but a neat way to get the vitamin is by heading out and catching some sun.
Vitamins B6, C and E are all known for their immune-boosting properties. You can get all of these vitamins from a well-balanced diet, so you don't need supplements. Some foods rich in these vitamins include eggs, bell peppers, spinach and almonds
Vitamin C in high oral dose was capable of enhancing NK activity up to ten-fold in 78% of patients.

No, marijuana can suppress natural killer (NK) cell activity. The main psychoactive component of marijuana, delta-9-tetrahydrocannabinol (THC), can reduce the ability of NK cells to kill targets.
Explanation
THC suppresses NK cell activity
In mice, THC injections decreased NK cell activity in the spleen by 25–50%. THC can also suppress the ability of cytotoxic T-cells (CTLs) to kill infected cells.
Endocannabinoids negatively regulate NK cell activity
Endocannabinoids, like anandamide (AEA) and 2-arachidonoylglycerol (2-AG), can negatively regulate NK cell activity.
Cannabinoid receptors may act as immunosuppressors
Cannabinoid receptors (CB) in the tumor microenvironment (TME) may limit the activity of NK cells, which could promote tumor growth.
Cannabis use may also increase the risk of acquiring or transmitting infections like HIV and HCV.