Biology of Disease

Combined T and B deficiencies

Deficiencies that affect both B and T lymphocyte (Chapter 4) numbers and/
or their functions are life threatening. Such deficiencies are termed severe
combined immunodeficiency (SCID). The term SCID represents a group
of disorders associated with more than 20 different mutations and with a
frequency of between one in 50 000 and one in 500 000 births. As some forms
of SCID are inherited in an X-linked fashion (Chapter 15), more boys than girls
are affected, with the male : female ratio of approximately 3 : 1.

Depending on the mutation involved, T and B cells may both be decreased (T–B–
SCID) or B cells numbers may be normal or increased (T–B+SCID). In the latter,
the absence of T cells renders B cells functionally inactive, owing to the need
for the cytokines produced by TH cells for antibody production as described in
Chapter 4. In either case, the disorder becomes apparent in the first few weeks

IMMUNODEFICIENCY DISEASES

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Table Deficiencies in: *Examples

I T and B lymphocytes severe combined immunodeficiency (SCID)

II antibody deficiencies X-linked agammaglobulinemia

III other well-defined immunodeficiency

syndromes

Wiskott Aldridge syndrome

DiGeorge anomaly

IV diseases of immune dysregulation Chediak-Higashi syndrome

V congenital deficiencies of

phagocyte numbers and/or function

X-linked neutropenia/myelodysplasia

leukocyte adhesion deficiencies (LADs) types

I and II

chronic granulomatous disease

VI defects in innate immunity mannose binding lectin (MBL) deficiency

VII autoinflammatory disorders hyper IgD syndrome

VIII complement deficiencies C4 deficiency

C2 deficiency

C1 inhibitor deficiency

• examples only, many more disorders are known

Table 5.3Classification of PIDs

Deficiencies of: Consequences

T lymphocytes viral and fungal infections (for example herpes simplex;Pneumocystis carinii)

opportunistic infections such asCandida albicans

infections with intracellular bacteria, e.g. Mycobacteria

Kaposi’s sarcoma (a tumor derived from blood vessels)

Antibodies bacterial infections causing recurrent chest infections and boils in the skin

Phagocytic cells bacterial infections causing boils and abscesses

Complement recurrent meningococcal meningitis

bacterial pneumonia , septicemia

systemic lupus erythematosus (SLE, an autoimmune disorder)

Table 5.2Diseases associated with PIDs of different origins