A person is immunocompromised when one or more parts of their immune system are weakened or missing, leaving them less able to fight off infections. This can happen because of a genetic condition someone is born with, a disease they develop later in life, medications they take, or simply the natural process of aging. In most cases, the cause falls into one of a few well-defined categories.
How Your Immune System Normally Works
Your body defends itself through layers. The first layer is physical: your skin, stomach acid, and the mucus lining your airways all block or trap invaders before they get inside. Behind those barriers sit two branches of the immune system working together.
The innate immune system is your rapid-response team. It includes white blood cells called neutrophils and macrophages that swallow and destroy bacteria, natural killer cells that target virus-infected cells, and a set of proteins called complement that punch holes in invaders. These cells recognize broad patterns on foreign organisms and attack quickly, without needing to have seen the threat before.
The adaptive immune system is slower but far more precise. It relies on B cells, which produce antibodies, and T cells, which coordinate attacks and kill infected cells directly. Each B or T cell carries a unique receptor that matches a specific target. Once they encounter an invader, they multiply and create memory cells so the response is faster next time. When any of these layers is damaged or depleted, the result is some degree of immunocompromise.
Diseases That Weaken Immunity
HIV is the most widely recognized infection-driven cause. The virus specifically infects and destroys CD4+ T cells, which act as the coordinators of adaptive immunity. As CD4 counts drop, the body becomes progressively less able to mount effective responses to infections it would normally handle easily. Without treatment, this progresses to AIDS, where the CD4 count falls below 200 cells per microliter and life-threatening opportunistic infections become common.
Blood cancers are another major cause. Chronic lymphocytic leukemia (CLL) and multiple myeloma directly involve the immune cells themselves. About 25% of CLL patients already have abnormally low antibody levels at the time of diagnosis, and up to 80% develop this problem as the disease progresses. When antibody production drops, the body loses its ability to tag and neutralize bacteria and viruses before they spread.
Chronic kidney disease impairs immunity through a different route. As the kidneys lose function, waste products called uremic toxins build up in the blood. These toxins interfere with neutrophils’ ability to kill bacteria by disrupting their energy production. They also skew T cell behavior, suppressing the cells that fight infections while promoting inflammation that damages healthy tissue. Diabetes similarly affects neutrophil function, making infections harder to clear and slower to heal.
Medications That Suppress the Immune System
Sometimes immunosuppression is deliberate. After an organ transplant, the recipient’s immune system would attack the new organ as foreign tissue if left unchecked. Transplant patients take immunosuppressive drugs for the rest of their lives to prevent rejection. Uninterrupted access to these medications is considered essential for keeping the transplanted organ functioning, which means the immune tradeoff is permanent.
Corticosteroids, commonly prescribed for conditions like asthma, lupus, and inflammatory bowel disease, are immunosuppressive at higher doses. They reduce inflammation by broadly dampening immune cell activity. At doses equivalent to 40 mg or more of prednisolone daily for longer than a week, the immune suppression becomes significant enough that live vaccines are considered unsafe. Patients on corticosteroids are particularly vulnerable to invasive fungal and viral infections.
Cancer chemotherapy works by killing rapidly dividing cells, and immune cells divide rapidly. Most chemotherapy regimens cause temporary but significant drops in white blood cell counts, creating windows of high vulnerability to infection. Newer cancer treatments like CAR T-cell therapy, which reprograms a patient’s own immune cells to attack tumors, can also leave the immune system depleted afterward. Biologic drugs used for autoimmune conditions, such as those that block inflammatory signaling, suppress specific immune pathways and increase infection risk in more targeted ways.
Genetic Immune Disorders
Some people are born immunocompromised. Primary immunodeficiency diseases (PIDDs) are caused by genetic abnormalities that prevent normal immune development. More than 400 of these disorders have been identified, ranging from mild to life-threatening.
Severe combined immunodeficiency (SCID) is the most serious form. Infants with SCID have virtually no functioning T cells or B cells, which means they cannot fight infections at all. Without treatment, typically a bone marrow transplant, SCID is fatal in the first year or two of life. Common variable immunodeficiency (CVID) is more common and less severe. It usually appears in the teens or twenties and causes low antibody levels, leading to frequent sinus, lung, and gut infections. Many people with CVID manage the condition with regular antibody infusions.
How Aging Weakens Immune Defenses
Aging itself is an underappreciated cause of immune decline. The process, called immunosenescence, involves measurable changes across nearly every part of the immune system. One of the most important is the shrinking of the thymus, the organ where T cells mature. The thymus begins shrinking after puberty and continues losing functional tissue throughout life. This means the body produces fewer and fewer new T cells capable of recognizing novel threats, relying instead on a shrinking pool of memory T cells trained against past infections.
At the same time, aging triggers a state of chronic, low-grade inflammation sometimes called “inflammaging.” Inflammatory signaling molecules circulate at higher levels even without infection, which paradoxically impairs the immune system’s ability to mount a focused response when one is actually needed. Neutrophils become less effective at engulfing bacteria, natural killer cells lose some of their ability to destroy infected cells, and B cells produce fewer high-quality antibodies. This is a major reason why older adults are more vulnerable to influenza, pneumonia, and other infections, and why vaccines tend to be less effective in this population.
How Immunocompromise Is Measured
Doctors assess immune function through a combination of blood tests tailored to the suspected problem. For suspected T cell deficiencies, the starting point is a complete blood count with a differential, which reveals the total number of lymphocytes in circulation. In adults, a lymphocyte count below 1,000 per cubic millimeter suggests a problem. In infants, the threshold is higher: below 3,000 per cubic millimeter is considered abnormal because young children normally have much higher counts.
For B cell and antibody problems, doctors measure circulating levels of the major antibody types (IgG, IgA, and IgM) and compare them to age-matched normal ranges. They may also check whether the body can produce antibodies in response to vaccines. A normal response means antibody levels rise at least fourfold after vaccination. If they don’t, it signals the immune system isn’t generating adequate protection even when given a clear target. For people with HIV, the CD4+ T cell count is the primary marker of immune status. Counts between 200 and 300 indicate moderate suppression, counts between 100 and 200 indicate severe suppression, and counts below 100 are considered potentially life-threatening.
Degrees of Immunocompromise
Not all immunocompromised people face the same level of risk. Someone taking a low dose of a corticosteroid for seasonal allergies has a very different immune profile than someone who just received a bone marrow transplant. Clinical guidelines, including those from the CDC, distinguish between moderate and severe immunocompromise when making recommendations about things like vaccine schedules. People who are moderately or severely immunocompromised may need additional vaccine doses or different timing than the general population.
The specific category depends on the underlying cause and its severity. A person with well-controlled HIV on effective treatment may have a near-normal immune system, while someone with uncontrolled HIV and a CD4 count below 100 is profoundly immunosuppressed. Similarly, a patient six months after a kidney transplant on stable medications faces different risks than someone in the first weeks after transplant when immunosuppressive doses are at their highest. Understanding where you fall on this spectrum matters because it shapes which infections pose the greatest threat and what preventive steps are most important.