Immunodeficiency is a state in which your immune system is weakened or missing key components, leaving your body less able to fight off infections, viruses, and other threats. It can be something you’re born with (primary immunodeficiency) or something that develops later in life from illness, medication, or malnutrition (secondary immunodeficiency). The result is the same: infections happen more often, last longer, and hit harder than they would in someone with a fully functioning immune system.
How the Immune System Fails
Your immune system relies on a coordinated network of cells and proteins to detect and destroy invaders. Immunodeficiency occurs when one or more of these components is absent, too few in number, or unable to do its job. The three major players that can break down are lymphocytes (the white blood cells that include T cells and B cells), phagocytes (cells that swallow and digest bacteria), and the complement system (a group of proteins that help flag and destroy pathogens).
What goes wrong depends on which part is affected. When B cells don’t work properly, your body can’t produce enough antibodies, the proteins that recognize and neutralize specific germs. When T cells are impaired, your body loses its ability to coordinate immune responses and kill infected cells directly. In some conditions, phagocytes can swallow bacteria normally but lack the internal chemistry to actually kill them. Each type of failure creates a different pattern of vulnerability.
Primary Immunodeficiency: Born With It
Primary immunodeficiencies are genetic. They’re caused by inherited mutations that prevent parts of the immune system from developing or working correctly. These conditions are predetermined at birth, though symptoms may not appear until months or even years later. The International Union of Immunological Societies classifies them into 10 different categories based on which immune components are affected.
Two of the most well-known types illustrate the range of severity:
- Common Variable Immunodeficiency (CVID) is caused by genetic abnormalities that prevent immune cells from producing normal amounts of protective antibodies. People with CVID experience frequent bacterial and viral infections in the sinuses, lungs, and upper airway. It often isn’t diagnosed until adulthood because symptoms build gradually.
- Severe Combined Immunodeficiency (SCID) is a group of rare, life-threatening disorders in which early stem cells fail to develop into functional T and B cells. Infants with SCID appear healthy at birth but quickly become susceptible to severe infections that a healthy baby would shrug off. Without treatment, SCID is fatal, typically within the first year or two of life.
Other primary immunodeficiencies affect more specific parts of the immune system. Some people lack a receptor for a key signaling molecule, making them vulnerable specifically to tuberculosis-like infections. Others have defects in the proteins that help white blood cells stick to blood vessel walls and migrate to infection sites, impairing the body’s first line of defense.
Secondary Immunodeficiency: Acquired Over Time
Secondary immunodeficiencies develop as a consequence of something external: a disease, a medication, or a nutritional deficit. They are far more common than primary immunodeficiencies.
HIV is the most recognized cause. The virus directly attacks and depletes a specific population of T cells (CD4+ cells) that serve as the coordinators of the immune response. As these cells decline, the body loses its ability to control not only everyday infections but also cancer-causing viruses like Epstein-Barr and human papillomavirus. HIV also damages intestinal cells, reduces nutrient absorption, and suppresses appetite, which compounds the immune damage through malnutrition.
Malnutrition, in fact, is considered the most common cause of immunodeficiency worldwide. Protein-energy malnutrition impairs both the coordinated immune response and the ability of phagocytes to kill the organisms they engulf. This creates a vicious cycle: malnutrition weakens immunity, which allows infections to take hold, which worsens nutritional status. Researchers sometimes call this nutritional-acquired immune deficiency syndrome, or NAIDS.
Medications are another major driver. Drugs used to treat cancers, autoimmune diseases, and organ transplant rejection often suppress immune function as a side effect. Therapies that deplete B cells, for example, can cause prolonged drops in antibody levels that persist long after the treatment ends. Blood cancers like chronic lymphocytic leukemia and multiple myeloma can themselves cause secondary immunodeficiency by crowding out or disrupting normal immune cells.
Warning Signs to Recognize
Immunodeficiency doesn’t always announce itself with a single dramatic event. More often, it shows up as a pattern of infections that are unusually frequent, severe, or stubborn. The Jeffrey Modell Foundation developed a widely used screening list of ten warning signs:
- Four or more new ear infections in a single year
- Two or more serious sinus infections in a year
- Two or more pneumonias in a year
- Two or more months on antibiotics with little improvement
- Need for intravenous antibiotics to clear infections
- Two or more deep-seated infections, including bloodstream infections
- Recurrent deep skin or organ abscesses
- Persistent oral thrush or fungal skin infections
- Failure of an infant to gain weight or grow normally
- A family history of primary immunodeficiency
Having one of these doesn’t necessarily mean you have an immune deficiency. But two or more, especially recurring over time, is a signal that further evaluation is warranted.
How Immunodeficiency Is Diagnosed
The first step is usually a blood test measuring the levels of the major antibody classes: IgG, IgA, IgM, and IgE. In adolescents and adults, an IgG value below 300 mg/dL is a clear trigger for further workup. In children, results are compared against age-specific reference ranges because normal antibody levels change significantly as a child grows.
If antibody levels are only mildly low, or even normal despite recurring infections, doctors move to functional testing. This means checking whether your immune system can actually respond to a challenge. The simplest version looks at whether you still have antibodies from past vaccinations or from your blood type. The more definitive test involves giving you a vaccine (such as a tetanus booster or a pneumonia vaccine) and measuring your antibody levels before and three to four weeks after. A normal response is at least a fourfold increase in antibody levels. Falling short of that suggests the immune system isn’t producing antibodies effectively, even if baseline numbers look acceptable.
Treatment Options
For secondary immunodeficiencies, treating the underlying cause is the priority. Antiretroviral therapy for HIV, nutritional rehabilitation for malnutrition, or adjusting immunosuppressive medications can all help restore immune function to varying degrees.
For primary immunodeficiencies, especially those involving antibody deficits, the cornerstone treatment is immunoglobulin replacement therapy. This involves receiving concentrated antibodies collected from thousands of donors, essentially borrowing the immune memory of the general population. The goal is to maintain antibody levels high enough to protect against infection.
There are two main ways to receive this therapy. Intravenous immunoglobulin (IVIG) is given through a vein once a month, typically at a dose that achieves protective trough levels of 500 to 800 mg/dL before the next infusion. Some people notice increased susceptibility to infections near the end of each cycle as levels dip. The alternative is subcutaneous immunoglobulin (SCIG), which is injected under the skin at lower doses every one to two weeks. SCIG produces more stable antibody levels without the peaks and valleys of monthly infusions, and many people prefer it because it can be self-administered at home.
For SCID and certain other severe conditions, a bone marrow or stem cell transplant may offer a more permanent solution by rebuilding the immune system from the ground up. Gene therapy is also becoming an option for select conditions.
Long-Term Risks Beyond Infection
Immunodeficiency doesn’t just mean more colds and pneumonias. A weakened immune system also increases the risk of cancer, particularly lymphomas. Among people with CVID, lymphomas and related growths occur in about 17% of patients. The connection between immune function and cancer risk is dose-dependent: the more severe the B-cell depletion and antibody deficiency, the higher the cancer risk.
People living with HIV face elevated rates of aggressive B-cell non-Hodgkin lymphomas, including rare subtypes almost never seen in people with healthy immune systems. The underlying mechanism is that a weakened immune system can’t keep cancer-causing viruses in check. Viruses like Epstein-Barr, human papillomavirus, and others that a healthy immune system would suppress indefinitely can instead drive the development of tumors.
Autoimmune disease is another paradoxical complication. Conditions like Wiskott-Aldrich syndrome combine immunodeficiency with autoimmunity, eczema, and bleeding problems. The immune system can be simultaneously too weak to fight external threats and too dysregulated to leave the body’s own tissues alone.