Immunotoxicology is the dysfunction of immune system because of exposure of organism to extrinsic factors. The immune system is a target following exposure to a diverse group of xenobiotics including chemical pollutants, recreational drugs, ultraviolet radiation and therapeutics. There is a well-established cause and effect relationship between suppression of the immune response and reduced resistance to infections. Xenobiotic exposure may result in stimulation of immune function. Xenobiotic can act as allergens and elicit hypersensitivity responses, or they can modulate hypersensitivity responses to other allergens. The immune dysfunction may take the form of immunosuppression or alternatively, allergy or autoimmunity or any number of inflammatory-based diseases. Review articles are the summary of current state of understanding on a particular research topic. They analyze or discuss research previously published by scientist and academicians rather than reporting novel research results. Review article comes in the form of systematic reviews and literature reviews and are a form of secondary literature. Systematic reviews determine an objective list of criteria, and find all previously published original research papers that meet the criteria. They then compare the results presented in these papers. Literature reviews, by contrast, provide a summary of what the authors believe are the best and most relevant prior publications. The concept of "review article" is separate from the concept of peer-reviewed literature. It is possible for a review to be peer-reviewed, and it is possible for a review to be non-peer-reviewed.