Common Variable Immunodeficiency: Heterogeneous group of immunodeficiency syndromes characterized by hypogammaglobulinemia of most isotypes, variable B-cell defects, and the presence of recurrent bacterial infections.Agammaglobulinemia: An immunologic deficiency state characterized by an extremely low level of generally all classes of gamma-globulin in the blood.IgA Deficiency: A dysgammaglobulinemia characterized by a deficiency of IMMUNOGLOBULIN A.IgG Deficiency: A dysgammaglobulinemia characterized by a deficiency of IMMUNOGLOBULIN G.Immunologic Deficiency Syndromes: Syndromes in which there is a deficiency or defect in the mechanisms of immunity, either cellular or humoral.Immunoglobulins, Intravenous: Immunoglobulin preparations used in intravenous infusion, containing primarily IMMUNOGLOBULIN G. They are used to treat a variety of diseases associated with decreased or abnormal immunoglobulin levels including pediatric AIDS; primary HYPERGAMMAGLOBULINEMIA; SCID; CYTOMEGALOVIRUS infections in transplant recipients, LYMPHOCYTIC LEUKEMIA, CHRONIC; Kawasaki syndrome, infection in neonates, and IDIOPATHIC THROMBOCYTOPENIC PURPURA.Dysgammaglobulinemia: An immunologic deficiency state characterized by selective deficiencies of one or more, but not all, classes of immunoglobulins.Transmembrane Activator and CAML Interactor Protein: A tumor necrosis factor receptor superfamily member found expressed on peripheral B-LYMPHOCYTES. It has specificity for B-CELL MATURATION ANTIGEN and TUMOR NECROSIS FACTOR LIGAND SUPERFAMILY MEMBER 13.Bronchiectasis: Persistent abnormal dilatation of the bronchi.B-Lymphocytes: Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation.Immunoglobulins: Multi-subunit proteins which function in IMMUNITY. They are produced by B LYMPHOCYTES from the IMMUNOGLOBULIN GENES. They are comprised of two heavy (IMMUNOGLOBULIN HEAVY CHAINS) and two light chains (IMMUNOGLOBULIN LIGHT CHAINS) with additional ancillary polypeptide chains depending on their isoforms. The variety of isoforms include monomeric or polymeric forms, and transmembrane forms (B-CELL ANTIGEN RECEPTORS) or secreted forms (ANTIBODIES). They are divided by the amino acid sequence of their heavy chains into five classes (IMMUNOGLOBULIN A; IMMUNOGLOBULIN D; IMMUNOGLOBULIN E; IMMUNOGLOBULIN G; IMMUNOGLOBULIN M) and various subclasses.B-Lymphocyte Subsets: A classification of B-lymphocytes based on structurally or functionally different populations of cells.Splenomegaly: Enlargement of the spleen.Aortic Arch Syndromes: Conditions resulting from abnormalities in the arteries branching from the ASCENDING AORTA, the curved portion of the aorta. These syndromes are results of occlusion or abnormal blood flow to the head-neck or arm region leading to neurological defects and weakness in an arm. These syndromes are associated with vascular malformations; ATHEROSCLEROSIS; TRAUMA; and blood clots.Immunoglobulin M: A class of immunoglobulin bearing mu chains (IMMUNOGLOBULIN MU-CHAINS). IgM can fix COMPLEMENT. The name comes from its high molecular weight and originally being called a macroglobulin.Pseudolymphoma: A group of disorders having a benign course but exhibiting clinical and histological features suggestive of malignant lymphoma. Pseudolymphoma is characterized by a benign infiltration of lymphoid cells or histiocytes which microscopically resembles a malignant lymphoma. (From Dorland, 28th ed & Stedman, 26th ed)T-Lymphocytopenia, Idiopathic CD4-Positive: Reproducible depletion of CD4+ lymphocytes below 300 per cubic millimeter in the absence of HIV infection or other known causes of immunodeficiency. This is a rare, heterogeneous syndrome and does not appear to be caused by a transmissible agent.Lymphocyte Activation: Morphologic alteration of small B LYMPHOCYTES or T LYMPHOCYTES in culture into large blast-like cells able to synthesize DNA and RNA and to divide mitotically. It is induced by INTERLEUKINS; MITOGENS such as PHYTOHEMAGGLUTININS, and by specific ANTIGENS. It may also occur in vivo as in GRAFT REJECTION.Complement C4a: The smaller fragment formed when complement C4 is cleaved by COMPLEMENT C1S. It is an anaphylatoxin that causes symptoms of immediate hypersensitivity (HYPERSENSITIVITY, IMMEDIATE) but its activity is weaker than that of COMPLEMENT C3A or COMPLEMENT C5A.Immunoglobulin Class Switching: Gene rearrangement of the B-lymphocyte which results in a substitution in the type of heavy-chain constant region that is expressed. This allows the effector response to change while the antigen binding specificity (variable region) remains the same. The majority of class switching occurs by a DNA recombination event but it also can take place at the level of RNA processing.Granuloma: A relatively small nodular inflammatory lesion containing grouped mononuclear phagocytes, caused by infectious and noninfectious agents.Immunoglobulin G: The major immunoglobulin isotype class in normal human serum. There are several isotype subclasses of IgG, for example, IgG1, IgG2A, and IgG2B.Tumor Necrosis Factor Ligand Superfamily Member 13: A member of tumor necrosis factor superfamily found on MACROPHAGES; DENDRITIC CELLS and T-LYMPHOCYTES. It occurs as transmembrane protein that can be cleaved to release a secreted form that specifically binds to TRANSMEMBRANE ACTIVATOR AND CAML INTERACTOR PROTEIN; and B CELL MATURATION ANTIGEN.T-Lymphocytes: Lymphocytes responsible for cell-mediated immunity. Two types have been identified - cytotoxic (T-LYMPHOCYTES, CYTOTOXIC) and helper T-lymphocytes (T-LYMPHOCYTES, HELPER-INDUCER). They are formed when lymphocytes circulate through the THYMUS GLAND and differentiate to thymocytes. When exposed to an antigen, they divide rapidly and produce large numbers of new T cells sensitized to that antigen.Immunophenotyping: Process of classifying cells of the immune system based on structural and functional differences. The process is commonly used to analyze and sort T-lymphocytes into subsets based on CD antigens by the technique of flow cytometry.Immunoglobulin A: Represents 15-20% of the human serum immunoglobulins, mostly as the 4-chain polymer in humans or dimer in other mammals. Secretory IgA (IMMUNOGLOBULIN A, SECRETORY) is the main immunoglobulin in secretions.Opportunistic Infections: An infection caused by an organism which becomes pathogenic under certain conditions, e.g., during immunosuppression.HIV-1: The type species of LENTIVIRUS and the etiologic agent of AIDS. It is characterized by its cytopathic effect and affinity for the T4-lymphocyte.Antigens, CD27: A member of the tumor necrosis factor receptor superfamily found on most T-LYMPHOCYTES. Activation of the receptor by CD70 ANTIGEN results in the increased proliferation of CD4-POSITIVE T-LYMPHOCYTES and CD8-POSITIVE T-LYMPHOCYTES. Signaling by the activated receptor occurs through its association with TNF RECEPTOR-ASSOCIATED FACTORS.Immunologic Memory: The altered state of immunologic responsiveness resulting from initial contact with antigen, which enables the individual to produce antibodies more rapidly and in greater quantity in response to secondary antigenic stimulus.Cardiovascular Abnormalities: Congenital, inherited, or acquired anomalies of the CARDIOVASCULAR SYSTEM, including the HEART and BLOOD VESSELS.Interleukin-2: A soluble substance elaborated by antigen- or mitogen-stimulated T-LYMPHOCYTES which induces DNA synthesis in naive lymphocytes.Lymphocyte Count: The number of LYMPHOCYTES per unit volume of BLOOD.Anemia, Hemolytic, Autoimmune: Acquired hemolytic anemia due to the presence of AUTOANTIBODIES which agglutinate or lyse the patient's own RED BLOOD CELLS.Lung Diseases, Interstitial: A diverse group of lung diseases that affect the lung parenchyma. They are characterized by an initial inflammation of PULMONARY ALVEOLI that extends to the interstitium and beyond leading to diffuse PULMONARY FIBROSIS. Interstitial lung diseases are classified by their etiology (known or unknown causes), and radiological-pathological features.Acquired Immunodeficiency Syndrome: An acquired defect of cellular immunity associated with infection by the human immunodeficiency virus (HIV), a CD4-positive T-lymphocyte count under 200 cells/microliter or less than 14% of total lymphocytes, and increased susceptibility to opportunistic infections and malignant neoplasms. Clinical manifestations also include emaciation (wasting) and dementia. These elements reflect criteria for AIDS as defined by the CDC in 1993.Simian immunodeficiency virus: Species of the genus LENTIVIRUS, subgenus primate immunodeficiency viruses (IMMUNODEFICIENCY VIRUSES, PRIMATE), that induces acquired immunodeficiency syndrome in monkeys and apes (SAIDS). The genetic organization of SIV is virtually identical to HIV.CD4-CD8 Ratio: Ratio of T-LYMPHOCYTES that express the CD4 ANTIGEN to those that express the CD8 ANTIGEN. This value is commonly assessed in the diagnosis and staging of diseases affecting the IMMUNE SYSTEM including HIV INFECTIONS.Autoimmunity: Process whereby the immune system reacts against the body's own tissues. Autoimmunity may produce or be caused by AUTOIMMUNE DISEASES.Splenectomy: Surgical procedure involving either partial or entire removal of the spleen.CD4-Positive T-Lymphocytes: A critical subpopulation of T-lymphocytes involved in the induction of most immunological functions. The HIV virus has selective tropism for the T4 cell which expresses the CD4 phenotypic marker, a receptor for HIV. In fact, the key element in the profound immunosuppression seen in HIV infection is the depletion of this subset of T-lymphocytes.Infection: Invasion of the host organism by microorganisms that can cause pathological conditions or diseases.Antibody Formation: The production of ANTIBODIES by proliferating and differentiated B-LYMPHOCYTES under stimulation by ANTIGENS.Antigens, CD: Differentiation antigens residing on mammalian leukocytes. CD stands for cluster of differentiation, which refers to groups of monoclonal antibodies that show similar reactivity with certain subpopulations of antigens of a particular lineage or differentiation stage. The subpopulations of antigens are also known by the same CD designation.T-Lymphocyte Subsets: A classification of T-lymphocytes, especially into helper/inducer, suppressor/effector, and cytotoxic subsets, based on structurally or functionally different populations of cells.Flow Cytometry: Technique using an instrument system for making, processing, and displaying one or more measurements on individual cells obtained from a cell suspension. Cells are usually stained with one or more fluorescent dyes specific to cell components of interest, e.g., DNA, and fluorescence of each cell is measured as it rapidly transverses the excitation beam (laser or mercury arc lamp). Fluorescence provides a quantitative measure of various biochemical and biophysical properties of the cell, as well as a basis for cell sorting. Other measurable optical parameters include light absorption and light scattering, the latter being applicable to the measurement of cell size, shape, density, granularity, and stain uptake.Respiratory Tract Infections: Invasion of the host RESPIRATORY SYSTEM by microorganisms, usually leading to pathological processes or diseases.Lymphoproliferative Disorders: Disorders characterized by proliferation of lymphoid tissue, general or unspecified.Lymphocyte Subsets: A classification of lymphocytes based on structurally or functionally different populations of cells.HIV Infections: Includes the spectrum of human immunodeficiency virus infections that range from asymptomatic seropositivity, thru AIDS-related complex (ARC), to acquired immunodeficiency syndrome (AIDS).