To learn and understand the path physiology of hypersentivity reaction against antitubercular drug and mechanism of desensitisation. The term hypersensitivity refers to a conditions where in a sensitized host following contact with a specific antigen, an immune response results in exaggerated or inappropriate reactions that are harmful to the host. Subsequent contact with the antigen causes manifestation of hypersensitivity, this is known as the shocking dose. Drug hypersensitivity syndrome also known as drug reaction with eosinophilia and systemic symptoms (DRESS) is a severe, idiosyncratic multi system reaction defined by the clinical triad of fiver, rash and internal organ involvement. Elucidated with clinical case reports.