Drug hypersensitivity reactions (DHRs) are adverse drug reactions that may be divided into several categories; namely pharmacologic intolerance, idiosyncratic reactions, pseudo-allergic reactions and allergic reactions. Drug allergic reactions are those DHRs that are mediated by either antibodies or drug-specific T cells. They vary in terms of severity, time-to-onset of clinical manifestations and target organ. Skin is most commonly implicated in drug hypersensitivity reactions; however, it is now apparent that reactions targeting internal organs fall under the definition of drug hypersensitivity. Multiple hypotheses have been proposed to explain the diverse immune mechanisms involved and the heterogeneous clinical presentation. The discovery of human leukocyte antigen (HLA) risk alleles for some DHRs has provided insights in the pathogenesis of these reactions. In this review we summarize immune cells involved in DHRs, discuss the possible immunological mechanisms of DHRs, with an emphasis on the IgE-mediated immediate reactions and T cell-dependent delayed type reactions.