Residency · Residency · Allergy Immunology

Secondary Immunodeficiency

Overview

Secondary, or acquired, immunodeficiency is far more common than primary immunodeficiency in clinical practice and represents a heterogeneous collection of conditions in which immune function is compromised as a consequence of extrinsic factors rather than intrinsic genetic defects of the immune system. The causes of secondary immunodeficiency are diverse and include medications (the most common etiology encountered in modern clinical practice), infections, malignancy, metabolic conditions, and physiologic states. The clinical presentation varies depending on which arm of the immune system is predominantly impaired -- whether humoral, cellular, phagocytic, or a combination thereof. The key guiding principle in management is to identify and address the underlying cause whenever possible while providing appropriate antimicrobial prophylaxis and, in select cases, immunoglobulin replacement to mitigate the infectious consequences of the immune defect.

Medication-Induced Immunodeficiency

Corticosteroids

Corticosteroids remain the most commonly used immunosuppressive agents worldwide, and their effects on the immune system are broad, dose-dependent, and duration-dependent. At the cellular level, corticosteroids induce lymphopenia through redistribution of lymphocytes from the peripheral blood to lymphoid tissues, with CD4-positive T cells being most profoundly affected. They suppress T cell proliferation and the production of key cytokines including interleukin-2, interferon-gamma, and tumor necrosis factor-alpha. Paradoxically, corticosteroids cause a neutrophilic leukocytosis through demargination of neutrophils from the vascular endothelium, yet they simultaneously impair neutrophil migration into tissues, effectively reducing the functional neutrophil response at sites of infection. At the molecular level, corticosteroids inhibit nuclear factor-kappa B (NF-kB) activation and increase the expression of its inhibitor, IkB.

The risk of infection increases significantly when patients receive doses equivalent to 20 mg or more of prednisone per day for two weeks or longer. At these doses and durations, patients become susceptible to opportunistic infections including Pneumocystis jirovecii pneumonia (PJP), cytomegalovirus reactivation, invasive fungal infections, and Strongyloides hyperinfection syndrome. Reactivation of latent tuberculosis and hepatitis B are additional concerns that should be addressed prior to initiating prolonged corticosteroid therapy. Prophylaxis with trimethoprim-sulfamethoxazole for PJP should be considered when patients receive 20 mg or more of prednisone daily for at least one month, particularly when corticosteroids are combined with additional immunosuppressive agents. Patients on prolonged corticosteroids are also at risk of adrenal suppression, and stress-dose steroids should be considered during acute illness or surgical procedures.

Rituximab (Anti-CD20)

Rituximab depletes CD20-positive B cells through antibody-dependent cellular cytotoxicity and complement-mediated lysis, producing B cell depletion that typically persists for 6 to 12 months, though some patients experience prolonged depletion exceeding 18 months. The immunologic consequences of rituximab extend beyond B cell depletion itself. Hypogammaglobulinemia develops in 10 to 50% of patients receiving repeated courses and may persist long after B cell numbers have recovered, likely reflecting depletion of memory B cell populations and impaired reconstitution of plasma cells. Late-onset neutropenia is a paradoxical complication that occurs 4 to 12 weeks after infusion and is usually self-limited. Vaccine responses are predictably impaired during the period of B cell depletion.

The infectious consequences of rituximab-induced immunosuppression include bacterial sinopulmonary infections due to hypogammaglobulinemia, hepatitis B reactivation (mandating screening with hepatitis B surface antigen and anti-hepatitis B core antibody before initiating therapy, with antiviral prophylaxis for positive patients), and, rarely but fatally, progressive multifocal leukoencephalopathy (PML) caused by JC virus reactivation. Monitoring of immunoglobulin levels every six months is recommended, with consideration of IgG replacement therapy for patients experiencing recurrent infections with IgG levels below 400 mg/dL.

Anti-TNF Agents (Infliximab, Adalimumab, Etanercept)

Anti-tumor necrosis factor agents neutralize TNF-alpha, a cytokine that plays a critical role in maintaining granuloma integrity and orchestrating the immune response to intracellular pathogens. The most significant infection risk associated with anti-TNF therapy is reactivation of latent tuberculosis, which can present with atypical disseminated or extrapulmonary disease. Screening with an interferon-gamma release assay (QuantiFERON-TB Gold or T-SPOT.TB) and chest radiograph is mandatory before initiating therapy, and patients with latent TB infection should complete appropriate treatment. Invasive fungal infections, particularly histoplasmosis and coccidioidomycosis in endemic areas, represent another important concern. The overall risk of serious bacterial infections is increased by an odds ratio of approximately 1.5 to 2.0. Hepatitis B reactivation can occur, and Listeria infections have been reported, prompting recommendations to avoid high-risk foods. Comprehensive screening before starting anti-TNF therapy should include tuberculosis testing (IGRA), hepatitis B and C serologies, HIV testing, and chest radiography.

JAK Inhibitors (Tofacitinib, Baricitinib, Upadacitinib, Ruxolitinib)

Janus kinase inhibitors block JAK-STAT signaling, thereby interfering with the downstream effects of numerous cytokines that signal through type I and type II cytokine receptors. This broad mechanism of action confers a correspondingly broad range of immunosuppressive effects. Herpes zoster reactivation is a particularly notable complication, with a dose-dependent two- to four-fold increase in risk compared to conventional disease-modifying agents, underscoring the importance of vaccination with Shingrix (recombinant zoster vaccine) before initiating JAK inhibitor therapy. Serious infections, including opportunistic infections, have been reported at rates comparable to biologic therapies.

The ORAL Surveillance trial, which evaluated tofacitinib in rheumatoid arthritis patients aged 50 years and older with cardiovascular risk factors, demonstrated increased rates of major adverse cardiovascular events (MACE), venous thromboembolism (VTE), and malignancy compared to TNF inhibitors. These findings led to an FDA boxed warning applied to the entire JAK inhibitor class. Monitoring during JAK inhibitor therapy should include regular complete blood counts (watching for lymphopenia and neutropenia), hepatitis B and C screening, and tuberculosis testing before initiation.

Immune Checkpoint Inhibitors (Anti-PD-1, Anti-CTLA-4)

Immune checkpoint inhibitors represent a unique category in the immunosuppression landscape because they paradoxically cause immune dysregulation through hyperactivation rather than immunodeficiency. By releasing the brakes on T cell-mediated immune responses, these agents generate immune-related adverse events (irAEs) that can affect virtually any organ system. Common irAEs include autoimmune thyroiditis, colitis, pneumonitis, hepatitis, dermatitis, and hypophysitis. Secondary hypogammaglobulinemia may develop, particularly with anti-CTLA-4 agents such as ipilimumab. Rarely, these agents may exacerbate underlying primary immunodeficiency or pre-existing autoimmune disease.

Immunosuppressive Agents: Key Infection Risks and Screening Requirements

Other Immunosuppressants

Mycophenolate mofetil inhibits inosine monophosphate dehydrogenase, a key enzyme in the de novo synthesis of guanine nucleotides, thereby selectively impairing the proliferation of T and B lymphocytes that are uniquely dependent on this pathway. It can cause hypogammaglobulinemia and increases susceptibility to CMV and BK virus infections. Calcineurin inhibitors, including cyclosporine and tacrolimus, block T cell activation by inhibiting the calcineurin-NFAT signaling pathway, predisposing patients to viral infections and EBV-driven post-transplant lymphoproliferative disorder (PTLD). Azathioprine, a purine analog, causes dose-dependent bone marrow suppression and requires assessment of thiopurine methyltransferase (TPMT) activity before initiation to avoid severe myelosuppression in patients with reduced enzyme activity. Methotrexate, an anti-folate agent, produces relatively mild immunosuppression at the low doses used for rheumatologic conditions but can cause hepatotoxicity and myelosuppression at higher oncologic doses. Cyclophosphamide, an alkylating agent, causes profound lymphopenia and neutropenia and carries additional risks of hemorrhagic cystitis and secondary malignancy.

<image>A comprehensive table-style illustration of medication-induced immunodeficiency. Organized by drug class (columns): Corticosteroids, Rituximab, Anti-TNF, JAK inhibitors, Mycophenolate, Calcineurin inhibitors. For each class, rows showing: (1) Mechanism of immunosuppression (with simple molecular diagram), (2) Immune cell(s) most affected (T cell, B cell, neutrophil icons), (3) Key infection risks (specific organisms listed with risk level: high/moderate/low), (4) Required screening before initiation (TB, Hep B, etc.), (5) Monitoring during therapy (labs and frequency), (6) Specific prophylaxis needed (TMP-SMX, antiviral, etc.). Color-coded risk levels: red for high-risk infections, yellow for moderate, green for low. Highlighted warning boxes for: rituximab-HBV reactivation, anti-TNF-TB reactivation, JAK-herpes zoster.</image>

Infection-Related Immunodeficiency

HIV/AIDS

Human immunodeficiency virus infection remains the most important infectious cause of secondary immunodeficiency worldwide. HIV targets CD4-positive T cells, producing progressive depletion that correlates directly with susceptibility to specific opportunistic infections. When the CD4 count falls below 200 cells per microliter, patients become vulnerable to Pneumocystis jirovecii pneumonia, Candida esophagitis, and Toxoplasma encephalitis. At counts below 100, the risk of Cryptococcus neoformans meningitis, disseminated Mycobacterium avium complex infection, and CMV retinitis increases substantially. At counts below 50, disseminated MAC and CNS lymphoma become predominant threats.

An immunologic paradox characterizes HIV infection: despite profound impairment of specific antibody responses, patients typically demonstrate polyclonal hypergammaglobulinemia resulting from nonspecific polyclonal B cell activation. Antiretroviral therapy (ART) restores immune function through a process termed immune reconstitution, though the early phase of ART may be complicated by immune reconstitution inflammatory syndrome (IRIS), in which the recovering immune system mounts an exuberant response to previously subclinical infections. Vaccination guidelines for HIV patients recommend avoiding live vaccines when the CD4 count is below 200, with the exception of select clinical situations, while inactivated vaccines should be administered regardless of CD4 count.

Measles

Measles virus infection causes a remarkable phenomenon termed "immune amnesia," in which the virus directly destroys pre-existing memory B and T cell populations, effectively erasing years of accumulated immune memory. The resulting immunosuppression persists for two to three years after measles infection and significantly increases susceptibility to other infectious diseases during this window. Landmark work by Mina and colleagues, published in Science in 2019, demonstrated that measles infection reduces the pre-existing antibody repertoire by 11 to 73% in affected individuals. These findings provide a powerful argument for measles vaccination not merely as protection against measles itself, but as a critical measure to preserve overall immune memory and protection against a wide range of other pathogens.

Other Infections

Several other infections produce clinically significant immunomodulatory effects. Epstein-Barr virus (EBV) can trigger X-linked lymphoproliferative-like disease and drives lymphoproliferative disorders in immunosuppressed hosts. Cytomegalovirus exerts immunomodulatory effects even in immunocompetent individuals and is a major pathogen in the post-transplant setting. Human T-lymphotropic virus type 1 (HTLV-1) is associated with adult T cell lymphoma/leukemia and tropical spastic paraparesis. COVID-19 causes transient lymphopenia affecting both CD4 and CD8 T cell populations, and some patients experience prolonged immune dysregulation; severe disease is associated with increased susceptibility to secondary bacterial and fungal infections.

Malignancy-Associated Immunodeficiency

Hematologic Malignancies

Chronic lymphocytic leukemia is the most common cause of secondary hypogammaglobulinemia, with approximately 25% of patients demonstrating low immunoglobulin levels at the time of diagnosis, a proportion that increases with disease progression and treatment. IgG replacement therapy is indicated for CLL patients who experience recurrent infections in the setting of IgG levels below 500 mg/dL. Multiple myeloma produces immunoparesis, the suppression of uninvolved (non-clonal) immunoglobulin classes, resulting in increased infection risk despite the overproduction of monoclonal immunoglobulin. Lymphoma causes immunodeficiency through both direct disease effects and the consequences of chemotherapy. Thymoma is associated with Good syndrome, a rare but important condition in which thymoma coexists with a combined B and T cell immunodeficiency, leading to recurrent infections, hypogammaglobulinemia, and absent B cells in the peripheral blood.

Solid Tumors

Solid tumors contribute to immunodeficiency primarily through the effects of cytotoxic chemotherapy, which causes neutropenia and lymphopenia of variable severity and duration. Checkpoint inhibitor therapy, as discussed above, produces paradoxical immune dysfunction through hyperactivation rather than suppression. Post-splenectomy patients, whether following surgical resection for solid tumors or other indications, face a lifelong increased risk of overwhelming infection with encapsulated organisms.

Protein-Loss States

Nephrotic syndrome leads to urinary loss of immunoglobulins, with IgG being preferentially lost due to its relatively lower molecular weight compared to IgM. This results in disproportionately low IgG with relatively preserved IgM levels, and affected patients face an increased risk of infection, particularly with pneumococcus. Protein-losing enteropathy, as seen in intestinal lymphangiectasia and inflammatory bowel disease, results in non-selective loss of all immunoglobulin classes as well as lymphocytes into the gut lumen. Severe burns cause massive protein loss through damaged skin in addition to disruption of the physical barrier, creating a dual mechanism of immunodeficiency. Chylothorax leads to loss of both lymphocytes and immunoglobulins in chylous fluid.

Metabolic and Nutritional

Malnutrition is the most common cause of secondary immunodeficiency worldwide. Protein-energy malnutrition impairs T cell function, reduces complement levels, and compromises mucosal immunity. Specific micronutrient deficiencies contribute additional immune impairment: zinc deficiency causes thymic atrophy and impaired T cell function, while vitamin D deficiency reduces the production of antimicrobial peptides such as cathelicidin and defensins, increasing susceptibility to tuberculosis. Diabetes mellitus impairs neutrophil function and wound healing, and patients in diabetic ketoacidosis face the distinctive risk of mucormycosis (zygomycosis). Uremia impairs both T cell and neutrophil function, contributing to the elevated infection risk observed in dialysis patients. Hepatic failure compromises complement synthesis and opsonization capacity.

Physiologic Immunodeficiency

Several physiologic states are associated with relative immunodeficiency. Neonates have an immature immune system with limited ability to mount robust adaptive immune responses; maternal IgG transferred transplacentally provides passive immunity that wanes by 6 to 9 months of age, creating a window of vulnerability until the infant's own antibody production matures. Pregnancy induces a physiologic shift from Th1 to Th2 immunity to prevent rejection of the semi-allogeneic fetus, which modestly increases susceptibility to certain intracellular pathogens. Aging is accompanied by immunosenescence, a gradual decline in immune function characterized by thymic involution, reduced naive T cell output, impaired vaccine responses, and a state of chronic low-grade inflammation termed "inflammaging." These changes increase susceptibility to infections and malignancy and reduce the efficacy of vaccination, prompting recommendations for higher-dose influenza vaccine (Fluzone High-Dose) and adjuvanted vaccine formulations for elderly individuals.

Splenectomy and Functional Asplenia

Post-Splenectomy Vaccination Schedule

Post-Splenectomy Infection Risk

The spleen plays a critical role in filtering encapsulated bacteria from the bloodstream and mounting rapid antibody responses to polysaccharide antigens. Loss of splenic function, whether from surgical splenectomy or functional asplenia, confers a lifelong risk of overwhelming post-splenectomy infection (OPSI), a fulminant and rapidly progressive sepsis syndrome that carries a mortality rate of 50 to 70% once established. The most common causative organism is Streptococcus pneumoniae, followed by Haemophilus influenzae and Neisseria meningitidis. Less common but important pathogens include Capnocytophaga canimorsus (transmitted through dog bites), Babesia species, and Plasmodium species in malaria-endemic regions. Although the risk of OPSI is highest during the first two years following splenectomy, it persists throughout the patient's lifetime. Functional asplenia, as occurs in sickle cell disease, celiac disease, systemic lupus erythematosus, and amyloidosis, carries the same infectious risks.

Prevention

A comprehensive prevention strategy is essential for all asplenic patients. Vaccination should be completed at least two weeks before elective splenectomy, or at least two weeks after emergent splenectomy, to allow for immune responses while splenic function is still present or during early reconstitution. The pneumococcal vaccine series should include PCV20 (or PCV15 followed by PPSV23), with revaccination with PPSV23 every five years if this formulation is used. Meningococcal vaccines include two doses of MenACWY eight weeks apart with boosters every five years, as well as a two- or three-dose series of MenB vaccine. A single dose of Haemophilus influenzae type b (Hib) vaccine should be administered if the patient has not been previously vaccinated, and annual influenza vaccination is recommended.

Prophylactic antibiotics with amoxicillin or penicillin VK should be administered daily, particularly during the first two years post-splenectomy, with consideration of lifelong prophylaxis in high-risk patients. Additionally, patients should be provided with a supply of emergency antibiotics (typically amoxicillin-clavulanate) for immediate self-treatment at the onset of febrile illness while seeking medical care, as the rapidity of OPSI progression can be measured in hours. All asplenic patients should wear medical alert identification.

<image>A clinical management checklist for the asplenic patient, designed as a one-page reference card. Top section: "Vaccinations" with a table of required vaccines (PCV20, PPSV23, MenACWY, MenB, Hib, Influenza), doses, timing relative to splenectomy, and booster schedule. Middle section: "Prophylactic Antibiotics" showing daily penicillin/amoxicillin dosing with note on duration (minimum 2 years, consider lifelong). Emergency section: "Fever Action Plan" - temperature >38.3C triggers: take emergency antibiotics, seek immediate medical attention, inform providers of asplenic status. Bottom section: "Special Precautions" - tick avoidance (babesiosis risk), dog bite prophylaxis (Capnocytophaga), malaria prophylaxis for travel, medical alert bracelet. Icons for each section. Color-coded urgency: red for emergency (fever), yellow for ongoing prevention, green for lifestyle modifications.</image>

Key Clinical Pearls

• Rituximab-induced hypogammaglobulinemia may persist for years after B cell recovery; monitor immunoglobulins every 6 months and provide IgG replacement if recurrent infections
• Screen for latent TB (IGRA) and hepatitis B before starting anti-TNF therapy; TB reactivation can be fatal
• JAK inhibitors significantly increase herpes zoster risk; administer Shingrix (recombinant zoster vaccine) before initiating therapy
• CLL is the most common cause of secondary hypogammaglobulinemia; IgG replacement reduces infections in patients with IgG <500 and recurrent infections
• Post-splenectomy patients have lifelong risk of OPSI; the highest risk is in the first 2 years; vaccinate and consider prophylactic antibiotics
• Measles causes "immune amnesia" by destroying pre-existing memory lymphocytes; this effect persists for 2-3 years and increases susceptibility to other infections
• Malnutrition is the most common cause of secondary immunodeficiency worldwide; even moderate protein-energy malnutrition impairs T cell function
• Prednisone >=20 mg/day for >=1 month warrants PJP prophylaxis, especially when combined with other immunosuppressants

References

1. Duraisingham SS, et al. Primary vs secondary immunodeficiency. Clin Exp Immunol. 2014;175(2):165-175.
2. Mina MJ, et al. Measles virus infection diminishes preexisting antibodies that offer protection from other pathogens. Science. 2019;366(6465):599-606.
3. Barmettler S, et al. Association of immunoglobulin levels, infectious risk, and mortality with rituximab and hypogammaglobulinemia. JAMA Netw Open. 2018;1(7):e184169.
4. Rubin LG, et al. 2013 IDSA clinical practice guideline for vaccination of the immunocompromised host. Clin Infect Dis. 2014;58(3):e44-e100.
5. Di Sabatino A, et al. Post-splenectomy and hyposplenic states. Lancet. 2011;378(9785):86-97.

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