Immune reconstitution inflammatory syndrome, or IRIS, is a condition where your immune system causes harmful inflammation as it recovers from severe suppression. It occurs most commonly in people with HIV who begin antiretroviral therapy (ART), affecting roughly 10% to 25% of patients depending on how depleted their immune system was before treatment started. The core paradox of IRIS is that it’s triggered not by the disease getting worse, but by the treatment working.
How IRIS Develops
To understand IRIS, it helps to know what happens when HIV goes untreated. The virus destroys CD4+ T cells, which are the immune cells responsible for coordinating your body’s defense against infections. Without enough of these cells, the immune system can’t properly fight off bacteria, fungi, and viruses. Many of these infections take hold quietly, sometimes without causing obvious symptoms, because the immune system is too weak to mount a response.
When a person starts ART, HIV replication slows dramatically and CD4+ T cells begin to recover. Normally, that recovery is a good thing. But in IRIS, the newly restored immune cells detect infections that were already present in the body and react with overwhelming force. The recovering CD4+ T cells produce a flood of inflammatory signaling molecules, which in turn activate large numbers of immune cells all at once. This creates a spike of inflammation that damages tissue, sometimes severely. In a healthy immune system, the initial detection of an infection and the full immune response happen in a coordinated sequence. In someone recovering from deep immunosuppression, those two phases are “temporally uncoupled,” meaning the body’s innate defenses had already partially responded to the infection, but the adaptive immune system couldn’t follow through until ART restored it. When it finally does, the response is disproportionate.
Two Types of IRIS
IRIS takes two distinct forms, and the difference matters for understanding what’s happening clinically.
Paradoxical IRIS occurs when a person was already being treated for a known infection before starting ART. Despite that treatment, symptoms worsen or return after ART begins. For example, someone being treated for tuberculosis might develop new fevers, swollen lymph nodes, or worsening lung symptoms once their immune system starts recovering. The infection itself isn’t getting worse; the immune system is now reacting to remnants of it.
Unmasking IRIS occurs when ART reveals an infection that nobody knew was there. The infection had been silently present, kept below the radar by the immune system’s inability to mount a visible response. Once the immune system recovers, it “unmasks” the infection through a sudden inflammatory flare. This can be the first time a person or their doctor realizes the infection existed at all.
Which Infections Trigger It
The most common forms of IRIS are linked to mycobacterial infections, fungi, and herpes viruses. Tuberculosis is the single most frequent trigger, with IRIS occurring in 8% to 43% of patients co-infected with HIV and TB. The wide range reflects differences in how sick patients were before starting treatment and how TB is defined across studies.
Cryptococcal infections are another major trigger, and cryptococcal IRIS can show up as meningitis (infection of the membranes around the brain), swollen lymph nodes, lung inflammation, or localized abscesses. Non-tuberculous mycobacteria, particularly Mycobacterium avium complex (MAC), cause IRIS in about 3.5% of patients whose CD4+ counts were below 100 cells per microliter before treatment. Cytomegalovirus (CMV), a common herpes virus, can trigger eye inflammation as part of IRIS. Kaposi sarcoma lesions, caused by another herpes virus, can also temporarily worsen before improving.
Who Is Most at Risk
Four factors consistently predict a higher chance of developing IRIS. The most important is a very low CD4+ T cell count before starting ART. Patients with counts below 200 cells per microliter are significantly more likely to develop the syndrome, while it rarely occurs in people who start treatment with counts above 350. The WHO estimates that up to 25% of patients starting ART with counts below 50 cells per microliter will experience IRIS, compared to about 10% of all patients initiating treatment.
The second factor is, counterintuitively, a strong response to ART. People whose viral load drops by more than a hundredfold within the first 90 days of treatment are at higher risk. A third factor is having a large burden of an existing infection, whether known or hidden. The more pathogen present in the body, the bigger the target for the recovering immune system. Finally, starting ART very soon after beginning treatment for an opportunistic infection increases risk, because the infection hasn’t been sufficiently controlled before the immune system ramps up.
Symptoms and Timing
IRIS symptoms depend entirely on which infection is involved and where it’s located. TB-associated IRIS often presents as new or worsening fevers, enlarging lymph nodes, or worsening chest symptoms. Cryptococcal IRIS can cause severe headaches and signs of meningitis. MAC-associated IRIS frequently involves swollen lymph nodes and fevers. The common thread is inflammation that seems to appear or intensify despite the patient otherwise responding well to HIV treatment.
The timing varies, but IRIS most commonly develops within the first few weeks to months after starting ART. It can occur even without a measurable rise in CD4+ count in blood tests, because the immune cells driving the inflammation may be concentrated at the site of infection rather than circulating in the bloodstream.
How IRIS Is Diagnosed
There is no single lab test or scan that confirms IRIS. Diagnosis is essentially a process of elimination. Clinicians look for several criteria: the patient is HIV-positive and on ART, blood work shows the treatment is working (falling viral load, rising CD4+ count, or both), symptoms are consistent with inflammation, and those symptoms can’t be better explained by a new infection, the expected course of a known infection getting worse on its own, or a side effect of medication.
This makes IRIS a clinical diagnosis, one that requires judgment rather than a definitive test. It’s a diagnosis of exclusion, meaning other explanations have to be ruled out first.
Treatment and Outcomes
Most cases of IRIS are self-limiting, meaning they resolve on their own as the immune system stabilizes. Mild symptoms can often be managed with anti-inflammatory medications like ibuprofen or similar drugs. ART is generally continued throughout, because stopping it would reverse the immune recovery and leave the person vulnerable to the underlying infections again.
In severe cases, corticosteroids are used to suppress the excessive inflammation. For serious TB-associated IRIS, a course of corticosteroids for one to two weeks, followed by a gradual taper, has been shown to improve symptoms within about three days. The decision to use steroids requires careful weighing of risks, particularly in patients with mycobacterial disease, where suppressing the immune system carries its own dangers.
Overall, IRIS is fatal in fewer than 10% of cases. However, the risk is not evenly distributed. Central nervous system involvement, meaning IRIS affecting the brain or its surrounding membranes, accounts for roughly 75% of IRIS-related deaths. Cryptococcal meningitis and TB meningitis are the most dangerous forms for this reason.
Reducing the Risk
The timing of when ART is started relative to treating an existing infection is the main lever for reducing IRIS risk. Current guidelines balance two competing priorities: starting ART as soon as possible to restore immune function, and giving infection treatment enough of a head start to reduce the amount of pathogen the immune system will react to.
For most opportunistic infections, ART should begin as soon as feasible after starting infection treatment. But for TB meningitis and cryptococcal meningitis specifically, a short delay before initiating ART is recommended because of the risk of severe, potentially fatal IRIS in the brain. For infections where no effective treatment exists, such as progressive multifocal leukoencephalopathy, ART is the only option and should be started immediately since immune recovery itself is the treatment. For non-meningeal TB, starting ART during TB treatment provides a significant survival advantage and is strongly recommended despite the risk of IRIS.