This study aims to systematically evaluate the dynamic alterations in peripheral immune subsets among patients with antibody Immunodeficiency undergoing Immunoglobulin therapy

Antibody Immunodeficiency（AID） represent a heterogeneous group of inborn errors of immunity characterized by defective antibody production, predisposing patients to recurrent infections, chronic inflammation, and paradoxical autoimmune manifestations. While Immunoglobulin replacement therapy serves as the cornerstone of management, patient clinical trajectories and responses to treatment remain highly variable. The underlying immunological defects-particularly the impaired cross-talk between T and B lymphocytes-are critical to understanding the pathogenesis and phenotypic diversity of AID.

This single-center, prospective observational study is designed to systematically characterize the dynamic alterations in peripheral immune subsets among patients with antibody immunodeficiency, with a specific focus on T-B cell interactions. By employing high-resolution multiparametric flow cytometry, we will perform comprehensive profiling of critical lymphocyte subpopulations. B-cell evaluation will quantify switched memory B cells, non-switched memory B cells, plasmablasts, and double-negative B cells. Concurrently, T-cell profiling will assess circulating follicular helper T cells (pTfh), regulatory T cells (Tregs), and overarching CD4+ and CD8+ compartments.

Participants will undergo baseline clinical and immunological evaluation prior to or during steady-state immunoglobulin therapy, followed by prospective longitudinal assessments. We hypothesize that specific baseline immune signatures (e.g., skewed pTfh proportions or expanded DN B cells) may serve as predictive biomarkers for the frequency of breakthrough infections and the development of autoimmune complications. Furthermore, this study will investigate the potential long-term immunomodulatory effects of immunoglobulin on restoring cellular homeostasis.

Ultimately, by correlating deep immunophenotyping data with real-world clinical outcomes-specifically annualized infection rates, specific antibody responses, and IgG trough levels-this study aims to elucidate the mechanisms driving clinical heterogeneity in AID and provide an evidence base for more personalized, targeted therapeutic strategies.

Inclusion Criteria:Confirmed Diagnosis: Diagnosis of primary antibody immunodeficiency based on clinical presentation and laboratory evidence of impaired antibody production. Diagnoses should align with standard criteria (e.g., ESID/IUIS diagnostic criteria).

Laboratory Evidence: Documented quantitative deficiency in serum immunoglobulins, defined as reduced IgG, IgA, and/or IgM levels below the age-adjusted lower limit of normal, and/or demonstrated poor specific antibody response to vaccines.

Age: Patients aged between 5 and 50 years (inclusive) at the time of screening. Treatment Status: Currently receiving or scheduled to initiate regular immunoglobulin replacement therapy.

Consent: Provision of written informed consent by the patient or their legal guardian, along with documented assent from pediatric participants where developmentally appropriate.

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Exclusion Criteria:Secondary Immunodeficiency: Presence of other severe comorbidities that may affect immune function. This specifically includes secondary causes of hypogammaglobulinemia, such as active malignancies, protein-losing enteropathy, nephrotic syndrome, or untreated HIV infection.

Other PIDs: Confirmed diagnosis of primary immunodeficiency disorders other than predominantly antibody deficiency.

Confounding Medications: Use of systemic immunosuppressive or targeted immunomodulatory drugs within 3 months prior to enrollment. Patients on stable immunoglobulin replacement therapy are exempt from this exclusion.

Acute Confounding Illness: Presence of active, severe, uncontrolled infections at the time of baseline blood sampling that, in the investigator's clinical judgment, would transiently distort baseline immune subset profiling.

Pregnancy/Lactation: Current pregnancy or breastfeeding, due to the physiological alterations in maternal immune subsets.

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Patients with primary antibody immunodeficiency, aged 18 years and older, who are receiving regular immunoglobulin replacement therapy at the study site. Exclusion criteria include active infection, recent immunosuppressive treatment, or prior hematopoietic stem cell transplantation.

De-identified individual participant data (IPD) that underlie the results reported in this article will be shared. Data will be available beginning 6 months and ending 36 months following article publication, to researchers who provide a methodologically sound proposal approved by an independent review committee, for the purpose of achieving the aims in the approved proposal. Proposals should be directed to the corresponding author.

Will be available beginning 6 months and ending 36 months following article publication.

De-identified individual participant data (IPD) will be accessible to researchers who submit a methodologically sound research proposal that is approved by an independent review committee. Access is limited to achieving the aims specified in the approved proposal. Proposals should be directed to the corresponding author of the study.