c5 Ampicilloyl

Summary

Ampicillin is a semi-synthetic, broad-spectrum penicillinase-sensitive aminopenicillin with a beta-lactam ring and side chain as sites for immunological recognition. Ampicilloyl is the major allergenic determinant of ampicillin, formed when the β-lactam ring opens and covalently binds to proteins, creating a stable hapten-protein complex. Most common manifestations of allergy to the penicillin family are benign immediate and delayed cutaneous reactions, however, penicillin allergy is also the leading cause of fatal drug anaphylaxis. Clinical history alone is not reliable for the diagnosis of penicillin allergy, hence one or more among skin and blood tests for penicillin sensitization and penicillin provocation tests are required. Penicillin skin testing and penicillin-specific IgE determination are highly specific but lack diagnostic sensitivity for immediate allergic reactions. A clearly positive IgE test to penicillin has been proposed by international guidelines as a criterion for sparing drug provocation tests for penicillin allergy diagnosis.

Despite a reported global prevalence of 6%-25% for penicillin allergy, fewer than 10% of suspected cases are truly allergic to penicillin. The burden of penicillin allergy overdiagnosis has personal and public health implications. Factors predictive of genuine allergy to penicillins include frequent antibiotic use, female gender, older age, a family history of allergies, and atopic conditions. Cross-reactivity between aminopenicillins and benzylpenicillins and between aminopenicillins and other beta-lactams, such as cephalosporins, is variable, depending on the similarity in chemical structures.

Allergen         

Nature

Penicillins are a family of natural and semi-synthetic antibiotics composed of a beta-lactam ring fused with a thiazolidine ring, collectively denoted as a penam ring, and an acyl side chain (Wright 1999). Aminopenicillins including amoxicillin and ampicillin are semisynthetic penicillins obtained from the parent drug of the penicillin family, the natural antibiotic penicillin G (benzylpenicillin) synthesized by Penicillium notatum (Lobanovska et al. 2017). Ampicillin was the first semi-synthetic aminopenicillin, obtained from benzylpenicillin through the addition of an amino group to the benzyl-penicillin side chain (Wright 1999). Ampicillin is currently available in oral formulation (solid and liquid) and parenteral formulation as general intravenous (IV) injections (NIH-Pubchem 2023) (Bush et al. 2016).

Like other small molecules, penicillins are not immunogenic by themselves, instead requiring binding to a carrier protein, such as serum albumin, through a process called haptenization.

Several mechanisms, namely IgE (immediate reactions) and T cells (nonimmediate reactions), can trigger allergic reactions to penicillins. IgE to penicillins can target the side chain, the thiazolidine ring, or to the conjugate formed by the beta-lactam ring and the carrier protein (Torres et al. 2016). The side chain of ampicillin is identical to those present in cefaclor and in cephalexine, and differs from the side chain of amoxicillin only by the lack of a hydroxyl group at position 4 of the side chain (Macy et al. 2022, Wright 1999).

Epidemiology

Worldwide distribution

Penicillin allergy, with an estimated prevalence of 6%-25%, is the most reported drug allergy globally. However, it is estimated that allergological work-up could delabel beta-lactam allergy in 90% of patients (Brockow et al. 2025).

Indeed, <10% of people labeled as penicillin allergic have a true immunoglobulin (Ig)E-mediated hypersensitivity to penicillin, while most of these could safely tolerate penicillin (Castells et al. 2019, Iuliano et al. 2022). This is possibly due to two reasons: inaccurate diagnosis and the gradual loss of sensitization with time (Siew et al. 2019).

A study conducted in Belgium on 1,009,598 patients (2010-2018), of whom 28,147 (3%) were labeled with antibiotic allergy, found that penicillin allergy labels had the highest prevalence, at 2% (22,824/1,009,598 patients). Aminopenicillins (ampicillin, amoxicillin and combinations) accounted for 0.12% (1208 patients). Among 1233 aminopenicillin labels, 42 were attributed to ampicillin (Gilissen et al. 2021).

A US-California-based retrospective chart review was conducted on 80 pediatric patients (mean age 7.3 years, mean time interval since the initial reaction 3.3 years) with an alleged penicillin drug allergy. Intradermal skin tests were performed in 73 patients, revealing sensitization to  penicillin G, the major determinant of penicillin (benzylpenicilloyl-polylysine), and ampicillin in 19.2%, 8.3%, and 13.7% of patients, while 4.1% reacted to ampicillin only of  (Anterasian et al. 2018).

In a Thailand-based cross-sectional study on 13,959 patients who had received at least one treatment course with penicillins and/or cephalosporins, adverse drug reactions were documented in 99 patients, of whom 6 (6.1%) had received ampicillin (Aiyaka et al. 2018).

Risk factors

Risk and predictive factors contribute to the stratification of patients’ risk of having genuine beta-lactam allergy, and thus to the choice of the diagnostic strategy (Brockow et al. 2025).

A study identified antibiotic overuse to be significantly associated with the risk of allergic diseases in both adults and children (Chen et al. 2019). Penicillin allergy has also been found to be more prevalent in females and older individuals (Yuson et al. 2022). The presence of atopic diseases like asthma, family history of drug-induced allergy, and higher age at reactions may be considered risk factors for true amoxicillin allergy in children (Faitelson et al. 2018).

Pediatric issues

Beta-lactam antibiotics are the most prevalent medications that trigger allergic reactions in children. Approximately 5% of the pediatric population globally is diagnosed with a penicillin allergy but upon careful diagnostic testing, <5% of these cases are confirmed (Kwok et al. 2023). Conversely, delayed rashes occurring during amoxicillin or ampicillin treatment in children with acute Epstein-Barr primoinfection (infectious mononucleosis) may be due to an allergic mechanism more often than previously suspected, since 5 of 10 children investigated in  a case series exhibited T-lymphocyte activation to amoxicillin and ampicillin (Mori et al. 2019).

A study in Turkey with 81 children (median age 9 years, median time since index reaction 6 months) suspected of beta-lactam allergy, with aminopenicillins (amoxicillin, ampicillin and combinations) incriminated in 49 (60.5%) patients, found that upon allergological work-up, only 10 (12.3%) of patients received a confirmed diagnosis of beta-lactam allergy. Ampicillin was suspected in 8 patients prior to the investigations and confirmed in only 1 patient (Suleyman et al. 2021).

Pregnancy

The evaluation of 222 pregnant women labeled as penicillin-allergic with clinical history, appropriate skin tests including ampicillin and if necessary a drug challenge test resulted in the delabeling of 209 (95%) of these patients, confirming the overestimation of penicillin allergy diagnosis (Wolfson et al. 2021). 

Route of Exposure     

Main

Exposure to ampicillin mainly occurs during ampicillin treatment via two routes: orally in the form of a suspension, or tablets, and parenterally via injections of ampicillin (NIH-Pubchem 2023).

Clinical Relevance    

Penicillin allergies are divided into two categories: immediate and delayed, each stemming from different immune responses. Immediate reactions happen within 1 to 6 hours of taking the drug, with symptom onset taking place during the first 15 minutes in 85% of patients and can range from urticaria to life-threatening anaphylaxis. Delayed reactions occur at least 6 hours after dosing, with a majority taking place 1 to 2 weeks after intake, and may present as benign maculopapular exanthema but severe, life-threatening  presentations are also reported, such as DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms) syndrome, Stevens-Johnson syndrome (SJS), and toxic epidermal necrolysis (Castells et al. 2019, Khan et al. 2022, Kopač  et al. 2012).

In particular, aminopenicillins present with a higher risk for benign drug-induced delayed exanthema, typically described during acute Epstein-Barr virus infection (Castells et al. 2019).

Risk stratification is vital for healthcare providers in diagnosing and managing penicillin allergies, affecting decisions regarding beta-lactam therapy or alternatives. High-risk patients may show severe immediate reactions such as anaphylaxis, low blood pressure, laryngeal edema, bronchospasm in association with urticaria, angioedema, and generalized exanthema. Conversely, low-risk patients are reported to have milder immediate reactions like isolated pruritus, gastrointestinal symptoms, contact dermatitis, or mild maculopapular exanthema (Romano et al. 2020).

Diagnostics Relevance

Clinical predictors for true beta-lactam allergies are lacking, hence suspected allergy cases need to be confirmed by specific IgE testing and if necessary, by an in vivo drug challenge test (Siew et al. 2019).

Skin testing for ampicillin is performed with the injectable formulation (Castells et al. 2019).

Serum specific IgE testing is safe, is not influenced by ongoing antihistamine medication and can be performed in primary care settings (Lendal et al. 2025). Both skin tests and IgE tests for penicillin allergy display high specificity but insufficient sensitivity, with a systematic review and meta-analysis finding 96.8% and 97.4%, and 30.7% and 19.4%, respectively (Sousa-Pinto et al. 2021).

Similar to other penicillins, the interpretation of ampicillin-specific IgE should ideally consider the level of total IgE (Garvey et al. 2019). Low levels of penicillin-specific IgE display lower diagnostic specificity in the context of elevated total IgE levels  (Lendal et al. 2025, Vultaggio et al. 2009, Zidarn et al. 2009). Penicillin sensitization decreases over time and may become undetectable with both skin tests and serum specific IgE (Hjortlund et al. 2014, Lendal et al. 2025). The half-life of serum specific IgE was found to be 1 year in most patients (Hjortlund et al. 2014). Accordingly, penicillin allergy work-up is recommended 4-6 weeks after the drug-induced reaction (Demoly et al 2014) and no later than one year after the index reaction (Hjortlund et al. 2014, Lendal et al. 2025). Early investigation can be performed with specific IgE determination in acute samples obtained at the time of an immediate reaction (Garvey et al. 2019). Specific IgE testing is recommended as the first-line investigation in high-risk patients, e.g. having experienced severe immediate reactions to penicillin (Demoly et al. 2014, Dona et al. 2025). A clearly positive IgE test can be considered sufficient for the diagnosis of beta-lactam allergy in patients with an evocative clinical history, especially anaphylaxis (Garvey et al. 2019, Romano et al. 2020)

If specific IgE tests are negative, additional examinations with skin and drug provocation test become necessary (Dona et al. 2025, Lendal et al. 2025).

Conversely, USA practice parameter recommends skin testing over blood IgE testing due to lower diagnostic sensitivity of the latter (Khan et al. 2022).

Prevention and Therapy

Prevention strategies

It is considered that patients with confirmed ampicillin allergy must not be administered the trigger and cross-reactive molecules, based on similar side chains, particularly those belonging to the same subclass, such as other aminopenicillins and aminocephalosporins: amoxicillin, cefaclor, cephalexine (Brockow et al. 2025). However, the rate of real-world cross-reactivity might be lower than inferred from chemical structures (Liang et al. 2020, Macy et al. 2022, Ramsey 2022).

Molecular Aspects

Cross-reactivity

Partial cross-reactivity within the penicillin family is reported, due to shared structural features such as side chains and the beta-lactam ring (Bhattacharya 2010, Brockow et al. 2025). Aminopenicillins (such as amoxicillin and ampicillin) have a side chain identical or similar to that of several first and second generation cephalosporins, explaining their cross-reactivity (Brockow et al. 2025, Romano et al. 2020, Trubiano et al. 2017). A study throughout 2014 – 2018 identified 328 patients with cephalosporin allergy labels. Among them, 4.8% (16/328) had positive skin tests for ampicillin, and cross-reactivities were found between ampicillin and cephalexin, cefuroxime, ceftriaxone, and amoxicillin (Stone et al. 2021). Cross-reactivity between carbapenems and penicillins or cephalosporins is reportedly less than 1% with monobactams (Trubiano et al. 2017).

Explained Results

Allergen information

Ampicillin is a commonly prescribed semi-synthetic, broad-spectrum, penicillinase-sensitive beta-lactam antibiotic of the aminopenicillin group. Its allergenic properties require haptenization with carrier proteins such as serum albumin. IgE to ampicillin can target the side chain, the thiazolidine ring, or the conjugate formed by the beta-lactam ring and the carrier protein.

Clinical information

Ampicillin hypersensitivity reactions with a demonstrated adaptive immune mechanism are termed ampicillin allergy. Immediate ampicillin allergy (1-6 hours after administration) is IgE-dependent, while delayed ampicillin allergy (>6 hours after administration) is T cell-dependent. Urticaria and exanthema are the most common symptoms. Like other penicillins, confirmed ampicillin allergy represents around 10% of patients with a label of ampicillin allergy, prompting recommendations for proactive allergy delabeling after risk stratification.

Cross-reactivity

Clinical reactivity to ampicillin may be selective if IgE is directed to the side chain, or cross-reactive with other penicillins if IgE is directed to the beta-lactam ring.  The side chain of amoxicillin is identical or similar to that of several first- and second-generation cephalosporins, resulting in partial cross-reactivity with potential clinical relevance.

Compiled by Turacoz

Reviewed by Dr. Joana Vitte, September 2025