Typical reactions included within this group are anaphylaxis and urticaria. Betalactams antibiotics continue to be the most frequent drugs that induce these reactions.16,17 The facility for binding spontaneously to endogenous proteins renders them to some extent immunogenic and able to induce IgE antibodies.9,16 In addition, immediate reactions have been reported by many drugs with an increasing list although at a much lower proportion than betalactams. Relevant emerging drugs are NSAIDs and quinolones.18,19

In the case of betalactams several determinants generated from benzyl penicillin have been proposed (BPO and MDM).8,14 Moreover, other betalactams provide determinants that must be considered in the immunological evaluation.16,20-25 The more relevant for the diagnosis of immediate reactions is amoxicillin and recently clavulanic acid as well as cephalosporins.20,22,25 Second relevant drugs are NSAIDs that although in most instances induce non immunological mediated reactions, up to 30% of subjects can develop urticaria/angioedema or anaphylaxis with a selective response mediated by an IgE mechanism.18 Pyrazolones are the most relevant drugs although propionic acid derivatives, particularly ibuprofen, followed by the aryl acetic acid derivative diclofenac are gaining in importance in this type of reactions.12,18,26 Weather ibuprofen itself or some of its metabolites are the responsible for IgE mediated reactions is not known at present.

Skin testing is the most sensitive tests for the diagnosis of immediate reactions to betalactams.27 General principles for skin testing with these and with the rest of the drugs are provided by the ENDA group.20 Maximal recommended concentrations are for BPO 5×10^-5 M for MDM 2×10^-2 M and for amoxicillin and clavulanic acid 20 mg/mL. With soluble cephalosporins concentrations of 20 mg/mL can produce false positive results in some of them due to an irritant effect and 2 mg/mL is recommended.20 Working concentrations for the rest of drugs involved in immediate reactions are also provided.28

Sensitivity for skin testing with betalactams and with the rest of the drugs is time dependent after allergic episode occurrence with subjects converting from positive to negative as time elapses.29-31 In vivo sensitivity is not optimal what implies that in the event of a clear positive history, if skin test negative, subject can still be allergic.16,17 Only in a minor proportion of cases in vitro tests can be positive with skin test negative but up to 20%-30% of patients may need a control administration of the drug for confirming the diagnosis.32,33

Most assays developed for drug specific IgE detection including the commercial ones consisted on the quantitation of IgE using radiolabelled anti IgE antibodies (RAST) or more recently enzyme (ELISA) or fluoroenzyme (FEIA) assays. The principle consists in a solid phase to which the hapten conjugated to a carrier protein is bound covalently. The carrier proteins can be HSA or other molecules like polylisines or aliphatic spacers.34-36 Although HSA has been used for many years37 this has not shown to be the most suitable carrier being preferable in many instances others with a high capacity for hapten fixation and exposition to IgE antibodies.34-36,38,39 Since last years the radiolabelled method has been substituted for the ELISA and later with the FEIA although no sufficient comparatives studies has been made so far. In general, it is accepted FEIA sensitivity for BPO is reasonable compared to skin testing,40 however, for other betalactams, including amoxicillin, important differences in sensitivity exist.39 This is particularly relevant in the cases of cephalosporins where sensitivity non higher than 20% has been reported.41 This can be due to the not inclusion of the culprit cephalosporin. For many years only cefaclor has been available for in vitro testing and it is well known that for this betalactam the side chain at R1 position is very important in the specific IgE recognition.25,42-44 Similar assays have been developed for other drugs, being in most cases experimental prototypes that need further validations in a sufficient number of positives controls.45-52

An alternative solid phase used for many drugs has been epoxy activated sepharose to which drugs bind covalently. Classical drugs used have been cephalosporins,28,29 quinolones,30,31 and muscle relaxants32 with different sensitivity and specificity results.

It is relevant to note that drug allergy IgE mediated can be an occupational disease affecting workers involved in health care, pharmacy and industries producing or manufacturing pencillins, cephallosporins53-55 as well as other antibiotics.56 In this sense the group of Park has published several reports using the methodology outlined above for diagnosing these cases.53-56

These reactions are produced of sensitised T cells that recognise drugs as xenobiotics and induce an immunological effector response.7 Although T cells reactions can occur virtually with any drug in any organ,71 the skin is the most frequent target involved.10,72,73 Clinical entities are shown in Fig. 2. They ranks from maculopapular exanthema to non-immediate urticaria and other less common but more severe entities such as AGEP, drug rash with eosinophilia and systemic symptoms, drug hypersensitivity syndrome (DHS), Stevens-Johnson syndrome (SJS), and Toxic epidermal necrolysis (TEN), fixed drug eruption and contact dermatitis as well as organ specific reactions.72,73 Although maculopapular exanthema is the most common reported reaction, it can sometimes be intense, accompanied with subcutaneous angioedema, and persists for several weeks despite discontinuation of treatment.73 DHS and bullous reactions with mucosal involvement are considered severe diseases.10,72 Erythema multiform, which is less severe, is usually induced by virus and characterized by the presence of typical target lesions.

There is growing evidence that SJS and TEN are a single disease with common causes and mechanisms; the main difference appears are the extent of detachment, which is limited (<10%) in the case of SJS and more widespread (>30%) in that of TEN.74 While rare (2 cases/million population/year), SJS and TEN have high mortality (20%-25%).75

Although all of them are considered T cell mediated drug hypersensitivity reactions, there are important differences in the effector cells involved.73,76 The clinical manifestations described result from the interplay of the immunological system and the cutaneous tissue. In maculopapular exanthema a mononuclear cell infiltrate can be found in the perivascular dermis, with T lymphocytes, mainly CD4 T cells,77-79 with the presence of neutrophils and occasionally eosinophils.80 Recent skin patch test studies have shown CD8 T cells in the dermoepidermal junction81-83 with a cytotoxic capacity.84-87 If this occurs at the onset of the acute response needs to be confirmed. Controversy exists concerning whether non-immediate urticaria or urticarial-like exanthematic rash is a T-cell reaction, although there is evidence that in some cases drug administration induced an urticaria-like reaction with the presence of angioedema and T-cell involvement.78-87 In this process an intradermal infiltration with activated CD4 and CD8 T cells producing granulocyte-macrophage colony-stimulating factors, with IFN-γ and TNF-α with the release of perforin/granzyme B and Fas L expression, which are responsible for keratinocyte death.88,89 These T lymphocytes also produce IL8 (CXCL-8), which is responsible for neutrophil chemoattraction and activation.88-92 Fixed drug eruptions are characterized by intraepidermal resident CD8 T cells with an effector memory phenotype, responsible for keratinocyte necrosis.93-95 CD4 T cells are also found and are thought to act as regulatory cells.84 TEN is characterized by the keratinocyte production of CCL27, which recruits CD4 and CD8 T lymphocytes expressing cutaneous lymphocyte antigen and CCR10 that produce IFN-γ, TNF-α, perforin/granzyme B/granulysin and FasL, which develop the apoptotic mechanisms followed by a massive necrosis.96 In DHS, in addition to the skin exanthema, there are also hematological abnormalities, with eosinophilia and the presence of atypical lymphocytes CD4 Th2 cells producing IL-5 which are involved in the eosinophil activation and trafficking. CD8 T cells also participate, which is the cause of organ damage in this syndrome.95 A general outline of the mechanistic process that occurs in non-immediate reactions is outlined in Fig. 2. Although simplifying drugs as haptens or protohaptens interact with immature dendritic cells Langerhans cells in the skin that become mature or partially mature and migrate to the regional lymph-node interacting with naïve T cells that become memory T cell. To this follows a complex process of homing and skin recruitment that will induce the different pathological entities described.

Difficulties in the diagnosis of these types of reactions reside in the lack of sensitivity of the available tests both in vivo and in vitro. For the assays it is important to detect the specific cell subpopulation involved and the adequate drug or drug metabolites eliciting the immunological response.15,76 Other difficulties are the lack of knowledge of the cofactors that may have been present at the time of the reaction, and the possibility that symptoms attributed to the drug were not immunologically mediated.73

Non-allergic hypersensitivity reactions refer to adverse drug reactions that are not mediated by specific immunological mechanisms, this is IgE or T cell dependent responses. This group of reactions is commonly described as intolerant, pseudoallergic, or idiosyncratic reactions although the term non-allergic hypersensitivity is actually used for integrating all these terms.13 The most representative group of drugs is made by NSAIDs.12,114 These are medicaments of variable chemical composition that antagonize inflammation by interfering with the function of cyclooxygenases. Cyclooxygenases are enzymes that participate in the conversion of arachidonic acid into prostaglandins and thromboxanes, which generate strong mediators of the inflammatory process. This inhibition results in a shunting of arachidonic acid metabolism toward the 5-lipoxygenase pathway, resulting in the increased release of cysteinyl leukotrienes and a decreased production of prostaglandin E2 mediators that could be involved in the pathogenesis of these reactions.11,115,116

NSAIDs are responsible for 21%-25% of reported adverse drug events which include immunologic and non-immunologic hypersensitivity reactions.11 A recent study indicates that these figures are even higher.117 Depending on the timing, symptomatology and putative mechanism of the reactions there are several subtypes of hypersensitivity to NSAIDs.115,118,119 The following categories are actually recognised:

Because it is estimated that in the 70% of cases with NSAID hypersensitivity non-specific immunological mechanism are involved,12 only in 1/3 of the cases will be theoretically possible to apply in vivo or in vitro specific immunological tests based on the capacity of IgE or T cells. For this, the in vitro test has focused on the mediator release determination.121-123 The capacity for histamine release or other mediators such as leukotrienes or ECP from eosinphils has been used for the development of in vitro assays. Although extensive research has been made these tests cannot be recommended for routine diagnosis.124 These are outlined below.

Biological agents are new medicaments with increasing applications that are progressively implicated in hypersensitivity reactions.6 They are not synthetic organic chemicals (xenobiotics) being structurally similar to autologous proteins. Different mechanisms have been proposed to be involved in the reactions induced by these agents that have been reviewed in detail elsewhere.134 Because many of them affect inflammatory processes, the immune effect can be induced by their activities; however, they can produce true allergic reactions mediated by specific immunological mechanisms.6,134 Amongst the biological agents considered within this group are cytokines, antibodies against cytokines, receptors, cell surface markers and fusion proteins as cytokine receptors or cellular ligands.6

Reactions induced by these agents belong to all the different immunological mechanisms described in the classic reactions by Gell and Coombs.7 Anaphylactic reactions,135-138 cytotoxic reactions like immune haemolytic anaemia and thrombocytopenia,88,89 immune complex like diseases139-143 and T cell responses144-146 have been reported. We will refer in more detail to Type I and Type IV reactions.

The ability of these reagents for inducing true allergic reactions is related with their ability of being recognised as different by the immunological system. Thus chimeric antibodies (ended by -ximab, are more immunogenic than those partially (-zumab) and whole humanises (-mumab).84 The best studied model is cetuximab, an antibody used for treating different neoplasias.147 It has been shown that many of these patients are primarily sensitised to a carbohydrate, α-Gal present in different natural sources as meat or induced after tick bites.148,149 In the case of non-immediate reactions from mild to severe responses to these agents have been reported indicating that these have been recognised by T cells which are involved in the response.146

Common reactions induced by these drugs are fever, chills, nausea, vomiting, headaches and diarrhea that sometimes may mimic an allergy reaction.149 But these usually may appear after the first administration and are often controlled by symptomatic medications. In the case of anaphylactic reactions these usually appear after several therapeutic courses and can be often severe and potentially fatal.

Because these medicaments are proteins it is not difficult to work with them both in vivo and in vitro for the diagnostic evaluation. Different studies have shown positive skin tests136,147-149 and in vitro specific IgE antibodies for immediate reactions136 and also T cell responses.144 The general principles outlined in this section are valid for approaching to these patients if specific immunological mechanisms are the responsible of the reactions.6,7