A granuloma is a small cluster of immune cells that forms when your body tries to wall off something it can’t eliminate. Think of it as your immune system building a tiny barricade around a foreign substance, an infection, or an irritant. Granulomas can form in almost any organ, from the lungs and liver to the skin, bones, and eyes. Most are benign, and many are discovered incidentally on imaging done for unrelated reasons.
How Granulomas Form
Your immune system’s first responders to infection or foreign material are macrophages, a type of white blood cell that engulfs and destroys threats. When macrophages encounter something they can’t break down on their own, they don’t just give up. Instead, they transform into specialized cells called epithelioid cells, which pack tightly together. Sometimes these cells fuse into larger structures called multinucleated giant cells. Other immune cells, particularly a type of white blood cell involved in long-term immune memory, form a ring around this core.
The result is a tiny, organized ball of immune tissue, typically a few millimeters across, though clusters can merge into nodules measuring 1 to 4 centimeters. A key signaling molecule produced by macrophages acts as the glue holding this structure together. It triggers inflammation, attracts more immune cells to the site, and helps maintain the granuloma’s shape over time. If that signaling molecule is blocked (which can happen with certain medications used for autoimmune conditions), the granuloma can break apart, potentially allowing an underlying infection to reactivate.
Two Main Types
Pathologists divide granulomas into two broad categories based on what they look like under a microscope, and the distinction matters because it points toward different causes.
Caseating (necrotizing) granulomas contain dead tissue at their center. The term “caseating” comes from the fact that this dead material has a soft, cheese-like consistency. Under a microscope, the normal tissue architecture in that central zone is completely destroyed. Caseating granulomas are the hallmark of tuberculosis and other mycobacterial infections.
Non-caseating granulomas lack that central zone of dead tissue. They’re made up of tightly packed immune cells without significant destruction. Non-caseating granulomas are more commonly linked to immune-mediated conditions like sarcoidosis and Crohn’s disease rather than infections, though the distinction isn’t absolute.
There’s also a separate category for foreign body granulomas. These form when your body reacts to inert material like surgical sutures, splinters, tattoo ink, or inhaled dust. In these cases, immune cells simply surround the material they can’t digest, walling it off without triggering the broader immune cascade seen in infectious granulomas.
Common Causes
The list of things that can trigger granuloma formation is long, but causes fall into two main groups: infectious and non-infectious.
- Infectious causes include tuberculosis (the most well-known), fungal infections like histoplasmosis and coccidioidomycosis, parasitic infections like schistosomiasis, and bacterial infections including cat scratch disease and syphilis.
- Non-infectious causes include sarcoidosis (a condition where granulomas form without any known infection), Crohn’s disease, foreign body reactions, certain vasculitis conditions, and occasionally lymphomas.
In many parts of the United States, granulomas found in the lungs turn out to be the remnants of a past fungal infection, particularly histoplasmosis, which is common in the Ohio and Mississippi River valleys. The person may never have realized they were infected. Their immune system handled it, left a granuloma behind as evidence, and moved on.
Where They Show Up
The lungs are the most common location, partly because airborne infections and inhaled particles land there first, and partly because lung imaging is so routine that granulomas are frequently spotted incidentally. Pulmonary granulomas are the most common type of benign lung nodule.
But granulomas can form in virtually any tissue. Sarcoidosis alone can produce them in the lungs, lymph nodes, eyes, skin, heart, liver, spleen, muscles, bones, joints, kidneys, and nervous system. When granulomas affect the skin, they may appear as raised bumps or rashes. In the eyes, they can cause blurred vision or light sensitivity. Granulomas in the nervous system can lead to numbness, tingling, nerve pain, or in rare cases, seizures. Joint involvement tends to show up as pain and swelling, often in the ankles.
Symptoms depend entirely on location and size. A single small granuloma in the lung typically causes no symptoms at all. Multiple granulomas in the liver or spleen may cause fatigue, weight loss, or a low-grade fever. Many people with granulomas never experience any symptoms.
How Granulomas Are Found
Most lung granulomas are discovered on chest CT scans or X-rays done for something else entirely: a pre-surgical workup, a screening scan, or evaluation of an unrelated complaint. On imaging, they typically appear as small, well-defined round spots. Older granulomas often calcify over time, giving them a bright, dense appearance on CT that helps distinguish them from more concerning nodules. Some develop a characteristic “eggshell” pattern of calcification around the edges.
When a granuloma is found, the key question is whether it’s benign or something more serious. Small, calcified nodules in people without risk factors for lung cancer are almost always noncancerous. For nodules that look less clearly benign, doctors may recommend follow-up imaging over several months to check whether the spot is growing, or a biopsy to examine the tissue directly.
Under a microscope, a pathologist looks for the characteristic architecture: a core of epithelioid cells, possibly with giant cells, surrounded by a ring of immune cells. They check for the presence or absence of central necrosis (the caseating versus non-caseating distinction), look for foreign material under polarized light, and may run special stains to check for bacteria or fungi. Various microscopic inclusions can appear within granulomas, but these are generally non-specific and don’t point to a single diagnosis on their own.
What Determines Whether Treatment Is Needed
Many granulomas require no treatment at all. A calcified lung granuloma from a long-resolved infection is essentially a scar. It won’t grow, won’t cause symptoms, and doesn’t need to be removed.
Treatment becomes necessary when granulomas are actively causing problems, either because an underlying infection needs to be addressed or because widespread granulomatous inflammation is threatening organ function or quality of life. For sarcoidosis, the decision to treat depends on two factors: whether the disease poses a risk of organ failure, and whether it’s significantly affecting daily life. Not everyone with sarcoidosis needs medication. Those who do typically start with anti-inflammatory drugs that reduce the immune overreaction driving granuloma formation. Long-term use of these medications carries its own side effects, so doctors often add other agents to reduce the dose needed.
For infectious causes, the treatment targets the underlying organism. Tuberculosis requires a multi-month course of specific antibiotics. Fungal infections may need antifungal therapy, though many mild cases resolve on their own. Foreign body granulomas sometimes resolve once the material is removed, but others persist indefinitely as harmless lumps of scar tissue.
Why Your Body Does This
Granuloma formation is not a disease in itself. It’s a containment strategy. When your immune system encounters something it can’t destroy outright, walling it off is the next best option. In tuberculosis, granulomas restrict the bacteria’s ability to grow and spread through the body. The bacteria may survive inside the granuloma in a dormant state for decades, held in check by the immune cells surrounding them. This is why tuberculosis can reactivate years later if the immune system weakens.
In conditions like sarcoidosis, the immune system appears to mount this same containment response even when no clear infectious trigger can be identified. The granulomas themselves then become the problem, accumulating in organs and interfering with normal function. Researchers are still working to understand exactly what triggers the immune system to form granulomas in these cases, though genetic susceptibility and environmental exposures both play a role.