c1 Penicilloyl G

Summary

Penicilloyl G, also called benzyl-penicilloyl, is the major allergenic determinant of penicillin G, formed when this antibiotic binds to lysine residues in proteins during the process of haptenisation. Benzyl penicilloyl is responsible for 90%-95% of allergic reactions associated with penicillin. 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. Risk factors include frequent antibiotic use, female gender, older age, a family history of allergies, and atopic conditions. Cross-reactivity between benzylpenicillins and aminopenicillins, and between penicillins and other beta-lactams, such as cephalosporins, is variable, influenced by 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). Penicillin G or benzylpenicillin is the natural antibiotic synthesized by Penicillium notatum and the parent drug of the penicillin family, which also comprises natural penicillin V (phenoxymethyl penicillin, gastroresistant), semisynthetic aminopenicillins (e.g. amoxicillin, ampicillin), penicillinase-resistant penicillins (e.g., methicillin), carboxypenicillins, and others (Lobanovska et al. 2017) .

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. Covalent binding of penicillin to lysine residues in carrier proteins of the host leads the formation of the major determinant, called benzyl-penicilloyl (BPO), responsible for triggering 90%-95% of allergic reactions associated with penicillin G (Castells et al. 2019). BPO is further processed by dendritic cells, presented by them to naïve T helper cells, and follows subsequent pathways towards several allergy mechanisms, including IgE-dependent (immediate) allergy (Castells et al. 2019).

The BPO reagent for skin tests is obtained by coupling penicillin to poly-L-lysine, resulting in penicilloyl-poly-L-lysine (PPL) (Castells et al. 2019).

In addition to BPO, several minor penicillin allergenic determinants are also produced as a result of penicillin administration (Adkinson et al. 2018, Castells et al. 2019) .

Epidemiology

Worldwide distribution

Penicillin allergy, with an estimated prevalence of 6%-25%, is the most reported drug allergy globally. Nonetheless, <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   (Meng et al. 2016, Kouma et al. 2023). This is possibly due to two reasons: inaccurate diagnosis and the gradual diminishing of hypersensitivity with time (Siew et al. 2019) .

In a US-based study involving 80 pediatric patients (average age 7.3 years), 73 tested positive to penicillin; 19.2% of children had hypersensitivity to penicillin G and 8.3% to benzylpenicilloyl-polylysine (Anterasian et al. 2018).

In a UK-based retrospective study on 84 adult patients (age 49.3 ± 19.4 years) with self-reported allergy to penicillins (including penicillins G and V, amoxicillin, and others), 24 (28.6%) were diagnosed with true allergy, and of them, 16 were allergic to penicillin. However, in vitro specific IgE tests of 16 penicillin-allergic patients revealed that only 3 patients (18.8%) were positive for penicilloyl G, penicilloyl V, and amoxicilloyl (Meng et al. 2016). A study in Slovenia aimed to assess penicillin allergy in 606 patients (average age 42 years) demonstrated that 8% (49/606) of patients had specific IgE to penicillin (Kopač  et al. 2012).

A retrospective study from the UK involving 1092 patients (age range 15-95 years) with suspected beta-lactam allergy were subjected to allergy test via skin testing performed with benzylpenicillin polylysine (PPL), minor determinants (MD) such as sodium benzylpenilloate, and also amoxicillin. About 70% of the patients tested negative for both PPL and MD (Siew et al. 2019) . In an extensive cohort analysis of 1218 individuals with suspected beta-lactam allergies, skin test results affirmed allergies in 21.1% (n=178) of patients; of these, 84.8% (n=151) patients were positive only for penicillin (Bousquet et al. 2008).

Considerable geographical and temporal differences in terms of patient sensitization profiles to beta-lactams have been reported. In European populations, a decreased reactivity rate to PPL was observed in standard penicillin skin tests, which could be due to more prevalent amoxicillin/ampicillin allergies, especially in Southern Europe, leading to sensitization to the R-group side chain. Conversely, in the US, there is a higher tendency for hypersensitivity to the beta-lactam core of penicillin (Sacco et al. 2017). In South Australia, a survey done in 2977 adult patients to evaluate the prevalence of self-reported drug allergy found that 9.3% of patients were allergic to penicillin (Yuson et al. 2022). In Saudi Arabia, the prevalence of penicillin allergy was found to be approximately 9.5% in a nationwide cross-sectional survey to assess self-reported penicillin allergy prevalence in 2022 adults (Alowais et al. 2023).

Risk factors

Frequent use of antibacterial drugs is a recognized risk factor for developing an allergy to penicillin. Drug allergy, particularly penicillin allergy, was found to be more prevalent in females and increased with age (Yuson et al. 2022). Furthermore, the risk of penicillin allergy was increased in patients with previous negative reactions to non-beta-lactam drugs, and those with a family history of penicillin allergies, and existing atopic conditions (Apter et al. 2008). Patients with penicillin allergy were at an increased risk of C. difficile (26%) and methicillin-resistant S. aureus (69%) infections compared to non-penicillin allergic individuals (Vivo et al. 2022).

Pediatric issues

In the pediatric population, approximately 5% are diagnosed with a penicillin allergy, making it a prevalent medication allergy worldwide. Upon rigorous diagnostic testing, less than 5% of these cases are validated as clinically significant immediate (IgE-dependent) or delayed (T cell-dependent) allergy to penicillin. Additionally, the differentiation between an allergic rash and a viral exanthema in children poses a differential diagnostic challenge for penicillin allergy (Kwok et al. 2023). It was shown that about 10% of pediatric drug reaction cases are initially reported positive, but only a few are verified through detailed allergy assessments (Piccorossi et al. 2020). In pediatric patients, skin tests using major and minor determinants, along with amoxicillin, without a follow-up penicillin antibiotic dose challenge, was proposed as a safe and effective method for assessing IgE-mediated penicillin hypersensitivity (D'Netto et al. 2022).

Pregnancy

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

Route of Exposure

Penicillin exposure occurs parenterally via intravenous or intramuscular injections of benzathine (as potassium or sodium salts) (Kado et al. 2020).         

Clinical Relevance

Penicillin allergy is acknowledged as a health challenge for hospitals and communities, with notable negative consequences on patient’s health (Trubiano et al. 2020).

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 administration, with a majority taking place after 1 to 2 weeks, 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, and toxic epidermal necrolysis (Castells et al. 2019, Khan et al. 2022, (Kopač et al. 2012).

The majority of reports on penicillin allergy portrayed skin reactions or ‘unknown’ reactions. Penicillin allergy documentation often omits specific details, with 'unknown' reactions noted in 26% of cases in health records. Documented reactions attributed to penicillin allergy include exanthema, urticaria, angioedema, gastrointestinal symptoms, anaphylaxis, and pruritus. Among these reactions, urticaria and exanthema are the most frequently reported symptoms (Shenoy et al. 2019).

In a study involving 1348 documented penicillin-allergic adult patients, the most common allergic reactions to penicillin were exanthema (37%), urticaria (18.9%), and edema (11.8%). Anaphylaxis occurred in 6.8% of cases; however, in most of these patients (92.4%) details of anaphylactic reactions were not documented (Albin et al. 2014). Another study on 193 adult patients (18->65 years) with penicillin allergy reported anaphylactic reactions in 46.1% (89/193) patients and non-anaphylactic reactions in 35.8% (69/193). Exanthema (n=53) was the most commonly reported non-anaphylactic reaction, followed by pruritus (n=39) while facial edema of lips or eyes was the least reported reaction (n=6) (Alowais et al. 2023).

Similar types of allergic reactions to beta-lactam antibiotics were reported in a pediatric population. The predominant immediate reactions were non-urticarial rash (65%), followed by urticaria (32%) and angioedema (9%). Among these, 38% exhibited symptoms indicative of IgE-mediated reactions, such as urticaria, facial edema, or anaphylactic symptoms. Only one patient (1.3%) experienced a high-risk reaction involving respiratory and gastrointestinal symptoms with hemodynamic instability (Anterasian et al. 2018).

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).

Diagnostic 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 DCT (Siew et al. 2019).

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). Technical improvements in penicillin reagents for in vitro diagnosis, introduced in 2006, resulted in a reported 82% reduction of false positive penicillin-specific IgE results (Zidarn et al. 2009).

The interpretation of penicillin-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  (Vultaggio et al. 2009, Zidarn et al. 2009, Lendal et al. 2025). A study in 171 patients with immediate reactions to penicillins and positive allergy work-up found that a ratio of specific IgE to penicillins (sum of specific IgE to a panel including benzylpenicillins and aminopenicillins) to total IgE greater than 0.002 had a positive predictive value of 92.5% (Vultaggio et al. 2015).

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, Dona et al. 2025)

If specific IgE tests are negative, additional examinations with skin and drug provocation test become necessary (Lendal et al. 2023, Dona 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 Strategies

Avoidance

Patients with confirmed penicillin 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 aminopenicillins (Brockow et al. 2025). For individuals with a confirmed penicillin allergy and no available alternative treatments (such as syphilis), desensitization is necessary (Lteif et al. 2019, CDC 2022).

Molecular Aspects

Cross-reactivity

Different types of penicillin are reported to show partial cross-reactivity due to shared structural features such as side chains and the beta-lactam ring (Bhattacharya 2010, Brockow et al. 2025). Penicillin and cephalosporins share a similar beta-lactam ring and were found to have cross-reactivity in 2% of cases, which is less than the 8% reported in previous studies (Macy et al. 2018, Shenoy et al. 2019). However, cross-reactivity between penicillin and aminocephalosporin was found to be much higher than cephalosporin owing to the shared chemical side chains or R1 groups. Cefazolin shows very low cross-reactivity with penicillin as it contains a unique side chain. The rate of cross-reactivity between penicillin and carbapenems was <1% (Shenoy et al. 2019). Cross-reactivity between penicillins and third-generation cephalosporins affects 2% to 3% of penicillin-allergic patients (D'Errico et al. 2020).

A meta-analysis of 21 studies involving 1269 penicillin-allergic patients to determine the risk of cross-reactivity of penicillin with cephalosporins and carbapenems concluded that cross-reactivity rates between penicillin and cephalosporins/carbapenems were dependent on the similarity of R1 side chains. Cephalosporins having R-side chains similar to penicillin show more cross-reactivity than the cephalosporins with different side chains. Aminocephalosporins showed 16.5% cross-reactivity, while cephalothin, cephaloridine, and cefamandole showed 5.6% cross-reactivity and cefazolin, had a cross-reactivity rate of only 2.1% (Picard et al. 2019).

Explained Results

Allergen Information

Penicillin G or benzylpenicillin is a commonly prescribed beta-lactam antibiotic. Its allergenic properties require haptenization with carrier proteins such as serum albumin, followed by the generation of major and minor allergenic determinants. The major determinant (benzyl penicilloyl) is responsible for 90%-95% of allergic reactions to penicillin.

Clinical information

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

Cross-reactivity

Different types of penicillin and cephalosporins exhibit partial cross-reactivity, mainly due to shared beta-lactam rings and similar side chains. The rate of cross-reactivity varies, being higher with aminocephalosporins due to their similar R1 side chains, while cephalosporins like cefazolin show much lower cross-reactivity due to distinct side chains.

Compiled by Turacoz

Reviewed by Dr. Joana Vitte, June 2025