The antibodies, phagocytes (neutrophils, monocytes, macrophages and dendritic cells) and complement are the major components of the immune system which provide defence against extracellular bacteria. Toll like receptors (TLRs) are also involved, they are present on the surface of the phagocytic cells and recognize specific patterns on the pathogens and are thus, able to phagocytosize them. When an infecting bacteria gains entry into the body, it is usually coated by the antibody and complement, this helps the neutrophil to recognize it as foreign. Subsequently, it is engulfed and destroyed by the neutrophil. This results in elimination of the bacteria. When there are defects in the antibody, complement or neutrophil, recurrent sinopulmonary bacterial infections and abscesses are observed. These infections are recurrent because the bacteria are not eliminated due to deficiency in the components of the immune system and so they continue to persist.

The respiratory tract (RT) is the organ system which is commonly involved in immunodeficiency disorders, hence an immune defect must be considered in case of any child who is suffering from RT infections which are recurrent, chronic and which require surgical or postural drainage.

As complement proteins and phagocytes together collaborate with antibodies to clear infections, deficiencies of either of these components need to be considered while evaluating cause for recurrent bacterial infections.

Complement deficiencies
Deficiencies of 'C3' are very rare; they result in severe problems of recurrent bacterial infections because of the central position of C3 in the complement pathways, besides this there are deficiencies in other components of the complement pathways.

Defects in phagocytes
Phagocyte defects are observed in patients suffering from lung abscess, recurrent chest infections with bacteria or fungi. Chronic granulomatous disease (CGD) is a major disorder related to phagocyte defect. Phagocytes can ingest bacteria but cannot produce substances like hydrogen peroxide and superoxide that kill certain bacteria and fungi. Pockets of pus (abscesses) can develop in the lungs, liver, granulomas of the skin are also observed.
Sometimes one may have defective phagocytes, but more data is required to confirm the diagnosis. Since antibodies and phagocytes are the primary defence against recurrent bacterial infections, measurement of serum immunoglobulins (Igs) and analysis of neutrophil count and function needs to be considered.

Primary antibody deficiency
The hallmark of primary antibody deficiency is recurrent bacterial infection mostly in the RT which tends to be chronic rather than acute.

Diagnostic tests
Early diagnosis of the deficiency is crucial so that treatment can be started. It is very important to study the family history in case of these disorders.
Screening tests

Serum Ig levels:
These tests are necessary to measure the serum Ig concentration. They are performed using single radial diffusion, radioimmunoassay, ELISA, or automated laser nephelometry.

Serum specific antibody titers:
Isohemagglutinins are naturally occurring IgM antibodies to ABO blood group substances. Depending on the blood group 70% of the infants have isohemagglutinin titers by 1 year of age.

Advanced tests

Antibody response to booster immunization
In patients with low specific antibody levels, response to booster immunization can be evaluated. A known antigen (tetanus vaccine) is taken for this purpose. If there is no response to boosting or if normal initial response is observed but it cannot be sustained this indicates presence of antibody deficiency syndrome.

Tests to enumerate B and T lymphocytes
This is done using flow cytometry or by immunofluorescence with monoclonal antibodies.
Pulmonary testing should be performed at least once every 6 months, also sputum cultures should be obtained routinely. HRCT (High Resolution Chest Computed Tomography) is useful. Besides this full blood count and IgA, IgG, IgM electrophoresis should be carried out.

The treatments are tailored to suit individuals and some of the common treatments are:

Immunoglobulin replacement
Intravenous immunoglobulin therapy (IVIG) is used to treat patients with antibody deficiencies, subcutaneous immunoglobulin therapy (SCIG) is also used.
In case of IVIG, serum IgG trough levels greater than 500 mg/dL are achieved. When one is suffering from chronic sinopulmonary infections which requires occasional surgical drainage a slightly higher dose may be necessary.

Antimicrobial therapy
Antibiotics are required in bacterial infections observed in antibody deficiencies. Prophyalactic cotrimoxazole is particularly useful in case of IgA deficiency.

Inactivated or subcomponent vaccines are also used. Live vaccines may be used in case of less severe antibody deficiencies (IgA) but not for severe deficiencies (CVID).

Surgical drainage
Sometimes there is a requirement of periodic surgical drainage due to lung abscesses. This can be done by use of one way valve to drain out the fluid from the pleural cavity.

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