TEXTBOOK OF IMMUNOLOGY - Mercury Podillia 2013

CONGENITAL IMMUNE DEFICIENCY

Laboratory Diagnostic Methods

Complete Blood count (CBC) helps detect anemia, leukopenia, or thrombocytopenia. The normal absolute neutrophil count should be at least 1800 µl-1, lymphocytes - 1000 µl-1, and in children under 2 years of age, the lymphocyte count is 2800 µl-1. Since T lymphocytes account for approximately 75% of all blood lymphocytes, lymphopenia almost always indicates a decrease in T-lymphocyte count. Neutropenia and lymphopenia can be secondary, for instance, in infections, autoimmune diseases, and The Use of certain medications, particularly immunosuppressants. If neutropenia or lymphopenia is detected, the complete blood count is repeated. Eosinophilia is frequently observed in patients with cellular Immunity deficiency. Impaired leukocyte adhesion is accompanied by persistent leukocytosis. Wiskott-Aldrich syndrome is characterized by a decrease in both the number and size of platelets. In certain immunodeficiencies, such as hyper-IgM syndrome and severe combined immunodeficiency, autoimmune thrombocytopenia occurs.

Quantitative determination of IgG, IgM, and IgA. Immunoglobulin levels falling within 2 standard deviations of the mean for a given age are considered normal (Table 48). A Diagnosis of hypogammaglobulinemia is established when immunoglobulin levels drop by more than 2 standard deviations below the age-adjusted normal range.

Class="center">Table 48. Normal Serum Levels of IgG, IgM, and IgA by E. R. Stiehm, H. H. Fudenberg, 1966

Determination of total serum IgE levels helps differentiate allergic conditions from immunodeficiencies. However, IgE levels can also be elevated in immunodeficiencies, particularly cellular immunity defects. A marked elevation in IgE is characteristic of helminthiases and allergic bronchopulmonary aspergillosis. When evaluating the obtained results, both the method used to measure total IgE and the patient's age must be taken into account.

Determination of isohemagglutinins makes it possible to assess serum IgM levels. Normally, in the majority of children older than 6 months, the antibody titer against erythrocyte antigen A exceeds 1:8, and against antigen B - 1:4 (individuals with AB blood group are an exception). In children over 18 months of age, the antibody titer against erythrocyte antigen A typically exceeds 1:16, and against antigen B - 1:8. Interpreting test results is challenging if IMMUNOGLOBULINS were administered within the month preceding the test. In children under 6 months of age, maternal IgG Antibodies against erythrocyte Antigens are usually present in the serum, which also complicates result interpretation.

Children must be tested for sweat chloride levels and evaluated for exocrine pancreatic function. This is essential in cases of recurrent respiratory infections, malabsorption syndrome, and developmental delay. Normally, sweat chloride concentration does not exceed 60 mmol/L. Because obtaining duodenal contents in children is technically difficult, exocrine pancreatic function is indirectly assessed via serum carotene levels, which are decreased in exocrine pancreatic insufficiency. In ambiguous cases, DNA analysis is performed to detect genetic defects found in 70–75% of patients with cystic fibrosis.

In chronic infections, ESR is determined, and Cell/15.html">Microscopy along with cultures of sputum, nasal secretions, tonsillar swabs, stool, and urine are performed to identify pathogens.

Typical associations between the type of immunodeficiency, infectious agents, and clinical manifestations of the disease:

I. Humoral (B-cell) immunity deficiency.

1. IgG and IgM deficiency

Pathogens: pyogenic extracellular Bacteria (streptococci, staphylococci, Haemophilus); Viruses (enteroviruses, Herpes zoster); Protozoa (Pneumocystis, etc.).

Clinical and laboratory features: impaired opsonization and microbial killing; recurrent infections of the Lungs, Central Nervous system, Stomach, and intestines.

2. Secretory IgA deficiency

Pathogens: pyogenic extracellular bacteria (streptococci, staphylococci), Haemophilus influenzae, Gram-negative bacteria, Fungi, Giardia.

Clinical and laboratory features: recurrent infections of the mucous membranes, respiratory tract, stomach, and intestines.

II. Cellular (T-cell) immunity deficiency

Pathogens: intracellular bacteria (Mycobacteria, Listeria, Legionella, Salmonella, Nocardia, Chlamydia); fungi (Candida, Histoplasma, Mucor mycosis), Introduction/6.html">DNA-containing viruses (Herpes simplex virus, varicella-zoster virus, cytomegalovirus, papovavirus), protozoa (Toxoplasma, Cryptosporidium, Pneumocystis).

Clinical and laboratory features: decreased T-lymphocyte counts and impaired intracellular pathogen killing; frequent severe infections of the lungs, central nervous system, gastrointestinal tract, and Skin.

III. Phagocytic system deficiency

Pathogens: Gram-negative enteric and pyogenic bacteria (E. coli, Pseudomonas, Klebsiella, Staphylococcus), fungi (Candida, Aspergillus, Mucor mycosis).

Clinical and laboratory features: impaired chemotaxis, oxygen-dependent METABOLISM, and phagocytosis; septicemia, Pneumonia, bacterial endocarditis, and anorectal abscesses.

X-ray examinations are performed when necessary. Lateral Skull radiography may reveal hypoplasia of the palatine and pharyngeal Tonsils, which is characteristic of hypogammaglobulinemia. The visualization of the Thymus on chest radiographs in newborns casts doubt on the diagnosis of severe cellular immunodeficiency. It should be borne in mind that thymic hypoplasia can occur in severe systemic illnesses and therefore cannot serve as a pathognomonic sign of Primary immunodeficiencies.

Cellular immunity is evaluated using skin tests based on delayed-type hypersensitivity reactions. Antigens for testing are selected on The basis of the patient's medical history. A positive reaction rules out severe cellular immunity deficiency, whereas a negative result is uninformative in the absence of documented prior contact with the test antigens. Approximately 85% of healthy adults show a positive reaction to one or more antigens (with a wheal diameter exceeding 5 mm). In children, positive reactions to the same antigens are observed less frequently than in adults, though the frequency increases with age. For children under 2 years of age, Candida albicans antigens and tetanus toxoid are used for skin testing. A positive reaction to Candida albicans antigens is observed in approximately 30% of healthy infants without immunodeficiency. As noted previously, candidiasis in infants with cellular immunodeficiency occurs more frequently than in healthy peers; however, their skin tests with Candida albicans antigens are typically negative. A positive skin test with tetanus toxoid is observed in 67% of healthy children after the second DT immunization and in 97% after the third. A positive skin test allows for the exclusion of severe cellular immunodeficiency, whereas a negative reaction has no diagnostic significance.

Complement testing is indicated if There is a family history of complement deficiency or autoimmune disorders, or if the Clinical presentation strongly suggests a complement deficiency.

Hemolytic complement activity allows for the functional Assessment of the classical complement pathway components (C1-C9). A normal total hemolytic complement activity does not rule out the deficiency of individual complement components or defects in The alternative pathway. When diagnosing complement deficiencies, the determination of hemolytic complement activity is performed simultaneously with the measurement of C3 and C4 levels.

- A simultaneous decrease in C3, C4 levels and hemolytic complement activity indicates classical pathway complement activation, such as by viruses in acute Viral Hepatitis or by immune complexes.

- A normal C3 level combined with a low C4 level and reduced hemolytic complement activity points to a C4 deficiency. This is observed in hereditary angioedema, malaria, and in certain patients with systemic lupus erythematosus.

- A normal C4 level combined with a low C3 level and reduced hemolytic complement activity is observed in congenital C3 deficiency, C3b inhibitor deficiency, and alternative pathway activation, such as by gram-negative bacterial endotoxins. C3 levels are also decreased in newborns, extensive Burns, and wasting syndromes.

- Normal C3 and C4 levels in the presence of reduced hemolytic complement activity indicate a deficiency of other complement components. In such cases, further laboratory investigation is warranted.

Additional Laboratory tests. If initial laboratory findings fail to establish or confirm a diagnosis, further testing is performed. Because defects in different Components of the immune system frequently coexist, uncovering an abnormality warrants a comprehensive immunological evaluation, which is typically conducted in specialized laboratories. Treatment should not be initiated until a definitive diagnosis is made.