Immunocompromised means your immune system is weaker than normal and less able to fight off infections, viruses, and other threats. Someone who is immunocompromised gets sick more easily, stays sick longer, and faces a higher risk of serious complications from infections that a healthy immune system would handle without trouble. Roughly 3% of U.S. adults are considered moderately or severely immunocompromised at any given time, whether from a medical condition, a medication, or both.
How the Immune System Normally Works
Your immune system is a network of cells, proteins, and organs that detect and destroy foreign invaders like bacteria, viruses, fungi, and parasites. White blood cells are the core players. Some types (called neutrophils) swarm to infection sites and destroy pathogens directly. Others (T cells) coordinate the broader immune response or kill infected cells. B cells produce antibodies, proteins that tag specific invaders so the rest of the immune system can find and eliminate them. Natural killer cells patrol for abnormal cells, including early cancer cells.
A complementary system of proteins circulates in your blood and punches holes in bacterial membranes. Your lymph nodes, spleen, thymus, and bone marrow all serve as staging grounds where immune cells develop, mature, and mobilize. When any part of this system is damaged or missing, gaps appear in your defenses.
Primary vs. Secondary Immunodeficiency
There are two broad categories. Primary immunodeficiency means you were born with a genetic defect that prevents part of your immune system from developing or functioning correctly. More than 400 primary immunodeficiency disorders have been identified. Some are mild enough to go undiagnosed until adulthood, while others, like severe combined immunodeficiency (SCID), are life-threatening in infancy. Newborn screening programs now test for SCID by measuring markers of new immune cell production in a baby’s blood.
Secondary immunodeficiency is far more common. It develops later in life because of a disease, a medication, or a medical procedure. Most people who are described as immunocompromised fall into this second group.
Diseases That Weaken Immunity
HIV is the most well-known cause. The virus specifically targets and destroys a type of white blood cell (CD4+ T cells) that coordinates much of the immune response. As these cells decline, the body loses its ability to respond to infections, fight off certain cancers, and produce effective antibodies. Without treatment, this progressive destruction leads to AIDS.
Blood cancers are another major cause. Leukemia and lymphoma can crowd out healthy immune cells in the bone marrow and lymph nodes. In chronic lymphocytic leukemia, the cancerous cells replace normal white blood cells, and the body’s ability to produce protective antibodies deteriorates. Multiple myeloma disrupts immunity in a different way: the malignant cells release substances that suppress the inflammatory response of other immune cells and interfere with the function of cells responsible for recognizing new threats.
Autoimmune diseases like lupus can also compromise immunity. In lupus, defects have been found in nearly every branch of the immune system, from the complement proteins that help destroy bacteria to the white blood cells that engulf and digest pathogens. Infections are a leading cause of serious illness and death in lupus patients, partly from the disease itself and partly from the medications used to control it.
Other conditions that impair immune function include diabetes (which reduces blood flow and weakens the skin’s barrier defenses), chronic kidney disease, liver cirrhosis, and malnutrition.
Medications That Suppress the Immune System
Many people become immunocompromised not because of an illness but because of the drugs used to treat one. These medications intentionally dial down immune activity, which is necessary in some situations but leaves you more vulnerable to infection.
Chemotherapy is the most obvious example. It kills rapidly dividing cells, including the white blood cells your body needs to fight infection. Corticosteroids (like prednisone), often prescribed for inflammatory and autoimmune conditions, broadly suppress immune signaling when taken at higher doses or for extended periods.
Organ transplant recipients take immunosuppressive drugs for life to prevent their body from rejecting the new organ. Newer biologic therapies used for conditions like rheumatoid arthritis, psoriasis, Crohn’s disease, and multiple sclerosis target specific parts of the immune system. Some block the signaling molecules that coordinate inflammation. Others deplete B cells, the white blood cells responsible for producing antibodies. People on B-cell-depleting therapies can have significantly reduced antibody responses to vaccines, which is why timing of vaccinations around treatment cycles matters.
JAK inhibitors, a newer class of oral medications used for autoimmune conditions, also carry distinct infection risks because they interfere with the chemical pathways immune cells use to communicate.
What Counts as a Low Immune Cell Count
One of the most practical measures of immune suppression is the absolute neutrophil count (ANC), which tracks the number of neutrophils in your blood. Neutrophils are your first responders to bacterial and fungal infections.
- Normal: above 1,500 cells per cubic millimeter of blood.
- Mild neutropenia (1,000 to 1,500): generally does not increase infection risk on its own, but your doctor may investigate the cause.
- Moderate neutropenia (500 to 1,000): slightly elevated infection risk, especially if other parts of the immune system are also impaired.
- Severe neutropenia (200 to 500): meaningfully increased risk of infection for most people.
- Agranulocytosis (below 200): risk of severe, life-threatening infections, including from organisms that rarely cause illness in healthy people.
Doctors also measure immunoglobulin levels (the antibodies circulating in your blood), count specific types of lymphocytes using a technique called flow cytometry, and check whether your body produces a proper antibody response after vaccination. A very low B cell count, below 2% of total lymphocytes, typically signals a near-complete inability to make antibodies.
How Being Immunocompromised Affects Daily Life
The practical impact depends on how severe the immune suppression is. Someone with a mildly reduced neutrophil count may not need to change much about their routine. Someone with severe neutropenia after chemotherapy or on high-dose immunosuppressants after a transplant faces real, daily considerations.
Respiratory infections are a primary concern. The CDC recommends that moderately or severely immunocompromised people wear a well-fitting, high-quality mask or respirator in crowded or poorly ventilated indoor settings. Improving ventilation when indoors with others, such as opening windows or using air purifiers, also helps reduce exposure. If you’ve been exposed to a respiratory virus like COVID-19, wearing a high-quality mask for 10 full days around others indoors is recommended.
Food safety matters more when your immune system is compromised. Raw or undercooked meat, unpasteurized dairy, and unwashed produce carry bacteria and parasites that a healthy immune system can usually handle but a weakened one cannot. Careful hand hygiene before eating and after being in public spaces is one of the simplest and most effective protective measures.
Vaccines and Immunocompromised People
Vaccines are more important for immunocompromised people because they’re at higher risk of severe illness, but vaccines also tend to work less well when the immune system is suppressed. The body may not mount a strong enough response to build full protection.
To compensate, immunocompromised individuals often follow a modified vaccination schedule. For COVID-19, the CDC recommends a longer initial series (three doses instead of two for most age groups) plus an additional dose six months later. People who received a stem cell transplant or CAR-T-cell therapy should restart their COVID-19 vaccination series from scratch at least three months after treatment, because the treatment effectively resets the immune system.
Live vaccines, which contain a weakened but active version of a virus, are generally avoided in people with significant immune suppression because the weakened virus could potentially cause the disease it’s meant to prevent. This includes the live versions of vaccines for measles, mumps, rubella, chickenpox, and certain flu vaccines. Inactivated or protein-based alternatives are used instead when available.
Degrees of Immunocompromise
Not everyone who is immunocompromised is equally vulnerable. A person taking a low dose of a biologic medication for psoriasis faces a very different risk profile than someone who just received a bone marrow transplant. Doctors think about immune suppression on a spectrum, and the precautions you need to take scale with where you fall on it.
If you’ve been told you’re immunocompromised, the most useful step is understanding which part of your immune system is affected. Someone with low antibody levels is primarily at risk for bacterial infections of the lungs and sinuses. Someone with low T cell counts is more vulnerable to viral infections and certain fungal organisms. Someone with very low neutrophil counts is at greatest risk for bacterial and fungal infections that can escalate quickly. Knowing your specific vulnerability helps you and your doctor make targeted decisions about prevention, monitoring, and when to seek care promptly for symptoms that might otherwise seem minor.