Infection during biologic and immunosuppressive treatment

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Targeted treatments create recognisable immune gaps. TNF is required for macrophage activation and granuloma integrity; CD20-directed treatment removes B cells and weakens new antibody responses; IL-6 blockade masks acute-phase signalling; JAK inhibition interferes with multiple cytokine pathways; terminal-complement inhibition impairs serum killing of Neisseria. Conventional agents and corticosteroids add dose-dependent, broad cellular, humoral and phagocyte dysfunction. Combination therapy, age, diabetes, lung disease and previous infection compound risk.

Timing extends beyond the dosing interval. Rituximab effects may persist until B-cell and immunoglobulin recovery, corticosteroid risk accumulates with dose and duration, and opportunistic infection can appear after a biologic is stopped. Conversely, an untreated inflammatory-disease flare can itself require hospitalisation and higher steroid exposure. Decisions to pause and restart therapy therefore balance infection control against organ-threatening relapse and should involve the prescribing specialty.

A mechanism-informed assessment asks two questions in parallel: what common source would explain this syndrome, and which unusual pathogens are enabled by the immune defect? A patient on anti-TNF therapy still gets ordinary pneumonia and urinary infection; a patient on tocilizumab still has non-infectious abdominal disease. Cultures, imaging and tissue remain more reliable than an exhaustive opportunistic screen without a clinical target.

Key points

Name every immune-modifying medicine, last dose, cumulative corticosteroid exposure and combination therapy; risk follows mechanism and intensity rather than the word biologic alone.
TNF inhibition impairs granuloma maintenance and raises tuberculosis and other intracellular-pathogen risk; monoclonal anti-TNF antibodies generally carry greater TB-reactivation concern than etanercept.
B-cell depletion can cause hepatitis B reactivation, hypogammaglobulinaemia, impaired vaccine responses and prolonged viral susceptibility months after the last infusion.
JAK inhibitors increase serious infection and herpes zoster risk; MHRA restrictions also address cardiovascular, malignant and thrombotic harms when choosing treatment.
IL-6 blockade can suppress fever and CRP, so examination, physiology, imaging and microbiology must outweigh reassuring inflammatory markers.
Terminal-complement blockade creates exceptional meningococcal susceptibility that vaccination reduces but cannot abolish; suspected disease receives immediate emergency antibiotics.
Serious infection usually requires temporary interruption of a biologic or targeted immunosuppressant, while corticosteroid continuation and stress dosing prevent adrenal crisis.
Empirical antimicrobials follow the anatomical syndrome and local resistance data; the drug mechanism refines pathogen coverage and sampling rather than dictating one universal regimen.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Common community and healthcare pathogens

Respiratory, urinary, skin, gastrointestinal and device-associated organisms remain the most frequent causes and may behave more aggressively during immune suppression.

Latent infection reactivation

Tuberculosis, hepatitis B and herpesviruses persist within the host and reactivate when granulomatous, B-cell or cytokine-mediated control is removed.

Mechanism-specific opportunists

Pneumocystis, fungi, mycobacteria, nocardia, listeria, salmonella and Neisseria exploit particular cellular, phagocyte, antibody or complement defects.

Treatment combination effects

Concurrent corticosteroids, conventional immunosuppressants, biologics, age and comorbidity produce greater infection risk than a single drug mechanism predicts.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Granuloma destabilisation
TNF blockade disrupts macrophage activation and organised granulomas that contain mycobacteria and some fungi, allowing latent organisms to disseminate.
2
Humoral immune depletion
CD20-directed B-cell removal impairs new antibody production, vaccine responses and viral control and may cause clinically important hypogammaglobulinaemia.
3
Cytokine signal interruption
JAK and IL-6-pathway inhibition weakens multiple inflammatory circuits and may obscure fever, neutrophil recruitment and hepatic acute-phase responses.
4
Complement killing failure
C5 inhibition prevents membrane-attack-complex formation, markedly reducing serum bactericidal activity against meningococci and other Neisseria species.
5
Broad steroid suppression
Glucocorticoids impair leukocyte trafficking, macrophage activation and T-cell function while masking inflammation, with risk rising alongside dose, duration and co-therapy.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Masked bacterial sepsisRed flag

Fatigue, confusion, tachypnoea, hypotension or focal pain can precede fever and CRP elevation during corticosteroid or IL-6-pathway suppression.

Granulomatous infection

Subacute respiratory, nodal, neurological, skeletal or constitutional disease during TNF inhibition may be tuberculosis, atypical mycobacteria or an endemic fungal infection.

Antibody-deficiency pattern

Recurrent sinusitis, pneumonia, prolonged viral infection or poor vaccine response after B-cell depletion suggests hypogammaglobulinaemia and impaired humoral recovery.

Herpesvirus diseaseRed flag

Dermatomal pain, grouped vesicles, disseminated rash, ocular symptoms or neurological change during JAK inhibition or combined therapy requires rapid HSV or VZV assessment.

Complement-associated sepsisRed flag

Abrupt fever, severe myalgia, headache, vomiting, purpura or shock with eculizumab or ravulizumab exposure warrants immediate meningococcal treatment despite previous vaccination.

Red flags requiring action

Hypotension, confusion, new oxygen need, oliguria or rising lactate is serious infection regardless of a normal temperature or inflammatory marker.
Persistent cough, weight loss, night sweats, lymphadenopathy or a compatible exposure during anti-TNF therapy requires urgent active-tuberculosis assessment and respiratory isolation when pulmonary disease is possible.
Severe abdominal pain or peritonism during IL-6 blockade may represent bowel perforation or sepsis despite a muted CRP response.
Headache, photophobia, purpura or rapidly evolving sepsis during terminal-complement inhibition is meningococcal disease until treated, even after vaccination.
Widespread vesicles, ophthalmic involvement, neurological symptoms or visceral features during JAK inhibition or corticosteroid therapy suggests disseminated VZV.
Jaundice or major ALT rise during rituximab, intensive chemotherapy or prolonged immunosuppression may indicate hepatitis B reactivation and requires urgent HBV DNA testing and hepatology input.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Treatment and immune-defect inventoryFirst step
Why
Identify the mechanistic infection pattern, duration of residual effect and medicines that require temporary interruption or interaction review.
Interpretation and limitations
Record all biologic, JAK, conventional and corticosteroid therapy, dose dates, prophylaxis, baseline screening, vaccination, previous infections, immunoglobulins and neutrophil or lymphocyte trends.
02
First-line syndrome samplingFirst line
Why
Diagnose common bacterial, viral or fungal infection and secure susceptibility data before empirical treatment reduces yield.
Interpretation and limitations
Take blood cultures and FBC, renal, liver, CRP and lactate tests in systemic illness, then obtain urine, respiratory, stool, lesion, joint or deep-tissue samples according to the focus.
03
Early cross-sectional imaging
Why
Find occult pulmonary, abdominal, neurological or deep soft-tissue disease when examination and inflammatory markers are attenuated.
Interpretation and limitations
Lower the CT threshold for focal symptoms, progressive physiology or anti-TNF opportunistic risk. Use urgent contrast imaging and surgical review for severe abdominal pain during IL-6 blockade.
04
Tuberculosis diagnostics
Why
Distinguish active tuberculosis from latent infection or another mimic during or after TNF-pathway suppression.
Interpretation and limitations
Use chest imaging and obtain sputum or site-specific material for rapid molecular testing, microscopy and culture. IGRA does not diagnose active disease and may be falsely negative during immunosuppression.
05
Viral reactivation tests
Why
Identify HBV, VZV, HSV, CMV or another mechanism-linked viral cause and determine organ involvement.
Interpretation and limitations
For possible HBV reactivation obtain HBsAg, liver tests and quantitative HBV DNA urgently; use lesion or compartment PCR for herpesvirus disease rather than relying only on serology.
06
Immunoglobulins and lymphocyte subsets
Why
Characterise prolonged humoral or cellular immune failure after recurrent, severe or unusual infection.
Interpretation and limitations
Measure IgG, IgA and IgM after B-cell-depleting therapy and selected lymphocyte subsets with immunology input. Results inform replacement and prophylaxis but do not replace infection-specific tests.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Underlying disease flare

Inflammatory arthritis, vasculitis, bowel disease and other immune disorders can cause fever and organ symptoms but may coexist with infection and should not prompt reflex escalation.

Drug toxicity

Pneumonitis, cytopenia, hepatitis, colitis, rash and neurological toxicity can mimic pathogen disease and often require imaging, microbiology or tissue to separate.

Malignancy

Lymphoma and other cancers can produce fever, weight loss, nodes, cytopenia and focal lesions, particularly after prolonged immune modification.

Thromboembolic disease

Pulmonary embolism, deep-vein thrombosis and arterial events cause fever, tachycardia, pain or hypoxaemia and are especially relevant with JAK-inhibitor risk factors.

Immune-reconstitution inflammation

Recovery after reducing immune suppression can intensify inflammation around treated tuberculosis or fungal disease, but microbiological failure must be excluded first.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01ACUTE ILLNESSTreat the clinical syndromeFirst stepA patient receiving an immune-modifying medicine develops possible serious infection or sepsis physiology.

1Perform ABCDE, obtain blood and focus cultures immediately and start the local syndrome-specific empirical regimen when severe infection is possible.
2Identify the immune mechanism and add tests or cover only for credible gaps, such as tuberculosis with anti-TNF, VZV with JAK inhibition or meningococcus with complement blockade.
3Withhold the biologic or targeted agent during serious infection and contact its prescribing team; continue necessary corticosteroid and provide stress cover when indicated.
4Control the anatomical source and narrow antimicrobials from culture, molecular or tissue results rather than continuing broad opportunistic coverage indefinitely.

02ANTI-TNFExclude active granulomatous diseaseConstitutional, respiratory, nodal, neurological or focal symptoms develop during or after TNF inhibition.

1Stop further anti-TNF dosing and assess urgency, travel, birthplace, exposure, previous latent-TB treatment and the timing of biologic therapy.
2Use respiratory isolation when pulmonary TB is possible and obtain molecular and culture specimens from the involved compartment before treatment when safe.
3Treat active tuberculosis through the specialist pathway and manage major rifamycin interactions with immunosuppressants, anticoagulants and other medicines.
4Restart or change immune therapy only after infection and disease-control specialists agree that microbiological response, treatment duration and inflammatory-disease risk permit it.

03B-CELL DEPLETIONPrevent reactivation and replace immunityRituximab or another B-cell-depleting treatment is planned or the patient develops recurrent infection, cytopenia or hepatic dysfunction.

1Check the complete pre-treatment HBV panel and baseline immunoglobulins, then route any HBsAg or anti-HBc positivity to hepatology before dosing.
2Use specialist antiviral prophylaxis or HBV DNA monitoring according to serology and reactivation risk; continue surveillance beyond the final infusion for the prescribed interval.
3During recurrent infection, measure immunoglobulins, recover a pathogen and consider respiratory assessment; immunology may recommend replacement after clinical and laboratory evaluation.
4Delay further dosing during serious active infection and reassess vaccine timing because antibody responses remain impaired until B-cell recovery.

04COMPLEMENT BLOCKADEAct on meningococcal riskA patient receiving a terminal-complement inhibitor develops fever, headache, rash, meningism or rapidly progressive systemic illness.

1Give emergency meningococcal therapy immediately after blood cultures if this causes no delay and follow the bacterial meningitis or sepsis pathway.
2Use droplet precautions, notify the health protection team and arrange contact chemoprophylaxis according to national meningococcal guidance.
3Confirm MenACWY and MenB vaccination history and any prescribed antimicrobial prophylaxis, recognising that neither eliminates breakthrough disease.
4Discuss interruption and safe recommencement of complement therapy with the responsible specialist because uncontrolled complement-mediated disease can also be life threatening.

05RESTARTResume treatment safelyThe acute infection is controlled and the underlying inflammatory, malignant or immune disease requires renewed therapy.

1Confirm clinical resolution, source control and the required antimicrobial course, including any pathogen-specific minimum interval before immune treatment resumes.
2Review whether the event revealed latent infection, hypogammaglobulinaemia, neutropenia, a missing vaccine or a need for secondary prophylaxis.
3Choose timing with the prescribing specialty and infection expert from drug half-life, immune recovery, relapse risk and residual microbial burden.
4Give the patient a revised sick-day, exposure and urgent-contact plan and document which medicine to withhold during future serious infection.

Key medicines and prescribing safety5 treatments · regimens, roles and cautions

Treats common time-critical infection first while mechanism-specific microbiology determines whether opportunistic or additional cover is required.

Syndrome-directed empirical antimicrobials

Use the current local severe-infection regimen for the anatomical source at full loading dose, modified immediately for immune mechanism, allergy, organ function, prior organisms and resistance exposure.

Avoid an unfocused antibacterial, antiviral and antifungal bundle; obtain decisive samples, check interactions with immunosuppressants and set a 24- to 48-hour narrowing point.

Preferred Pneumocystis prevention during selected prolonged high-dose corticosteroid or combination immunosuppression; no single corticosteroid threshold applies to every specialty.

Co-trimoxazole prophylaxis

A commonly used adult regimen is 960 mg orally three times weekly or 480 mg once daily when specialist assessment identifies material Pneumocystis risk.

Check allergy, FBC, creatinine and potassium and interactions with methotrexate, renin-angiotensin drugs and marrow-suppressive therapy; pregnancy and glucose-6-phosphate dehydrogenase status may alter the plan.

First-choice intravenous antiviral for severe herpesvirus disease during significant immunosuppression, followed by oral step-down when clinically appropriate.

Aciclovir for severe VZV

Give aciclovir 10 mg/kg intravenously every 8 hours for disseminated, ophthalmic with systemic features or visceral VZV in an adult with normal renal function, with specialist review.

Adjust for renal function and obesity according to product guidance, ensure hydration and monitor creatinine and neurological status; obtain viral PCR but do not delay treatment in a severe phenotype.

Prevents hepatitis B reactivation in HBsAg-positive and selected anti-HBc-positive patients receiving high-risk treatment such as B-cell depletion.

HBV antiviral prophylaxis

Use the hepatology protocol for a high-barrier nucleos(t)ide analogue, commonly entecavir or tenofovir, selected and adjusted to HBV status, renal function, pregnancy and the immunosuppressive regimen.

Do not use anti-HBs alone to exclude past infection, and do not stop prophylaxis at the last infusion; planned HBV DNA and ALT surveillance continues for the specialist-defined post-treatment period.

Prevents adrenal crisis while other immunosuppressive medicines are reviewed or withheld during serious infection.

Corticosteroid continuation

Continue the patient's established physiological requirement during acute infection and give the local stress-dose hydrocortisone regimen when adrenal insufficiency or severe physiological stress makes it necessary.

Do not abruptly stop chronic therapy; use the lowest disease-controlling dose after stabilisation because ongoing corticosteroid exposure increases bacterial, viral, fungal and Pneumocystis risk.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Disseminated opportunistic infection

Failure of granulomatous or cellular control permits mycobacterial, fungal, nocardial, viral or parasitic spread across lung, brain, skin, bone and bloodstream.

Fulminant meningococcal sepsis

Complement blockade allows exceptionally rapid meningococcal invasion with purpura, coagulopathy, shock and death despite prior vaccination.

Hepatitis B reactivation

Renewed HBV replication can progress from asymptomatic DNA rise to hepatitis, hepatic failure and interruption of essential immunosuppressive or cancer treatment.

Chronic antibody deficiency

Persistent hypogammaglobulinaemia after B-cell depletion causes recurrent respiratory infection, bronchiectasis, prolonged viral shedding and repeated antimicrobial exposure.

Disease relapse after interruption

Necessary withholding of immune therapy may reactivate vasculitis, inflammatory bowel disease, arthritis or another organ-threatening condition, complicating infection recovery and restart timing.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Track physiology and organ-specific findings rather than relying on fever or CRP, especially during tocilizumab, corticosteroids or profound immune suppression.
Review FBC, renal and liver profiles at a frequency matched to infection and treatment; neutropenia, lymphopenia and hypogammaglobulinaemia change both risk and drug safety.
Reconcile the stop, restart and next-dose plan for every biologic or targeted agent with the prescribing specialty and document continued corticosteroid requirements.
For past or current HBV, follow ALT and quantitative HBV DNA through hepatology at the risk-defined interval during treatment and after immune recovery.
After B-cell depletion, repeat immunoglobulins before later cycles and during recurrent infection; consider vaccine response and replacement only within the specialist clinical context.
Report and investigate tuberculosis or another notifiable infection through the required public-health route and assess exposed household or healthcare contacts.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Mechanism beats brand name

Different products can converge on the same immune defect, while combination therapy creates risks that cannot be inferred from either medicine in isolation.

CRP may be pharmacologically silent

IL-6 receptor inhibition directly suppresses hepatic acute-phase signalling, so a low CRP can coexist with perforation, bacteraemia or deep infection.

Vaccination is not absolute protection

Impaired response, incomplete serotype coverage and waning antibody mean meningococcal or other vaccine-preventable infection still requires emergency recognition.

Rituximab risk outlasts dosing

B-cell recovery, immunoglobulins and vaccine responses may remain impaired for months, so the final infusion date does not mark restored humoral immunity.

Inflammatory rebound needs planning

Stopping immune therapy can unmask or worsen inflammatory disease and occasionally infection-associated inflammation; restart decisions require both disease and infection expertise.

11Common pitfallsFrequent interpretation and management errors.

01
Excluding serious infection because tocilizumab has suppressed CRP or corticosteroids have prevented fever.
02
Calling a positive pre-treatment IGRA active tuberculosis or using a negative IGRA to dismiss compatible active disease during immunosuppression.
03
Stopping chronic prednisolone abruptly when withholding a biologic, thereby adding adrenal crisis to sepsis.
04
Assuming meningococcal vaccination removes the need for emergency treatment during eculizumab or ravulizumab therapy.
05
Checking HBsAg alone before rituximab and missing HBsAg-negative, anti-HBc-positive previous infection with reactivation potential.
06
Restarting the same immune therapy after an opportunistic infection without defining source control, secondary prophylaxis, residual microbial burden and a monitoring owner.