12. TYPICAL DISORDERS OF THE HOST DEFENCE SYSTEM

Typical disorders of the host defence system are described in fig. 12.

Fig. 12. Typical disorders of the host-defence system

IMMUNOLOGICAL DEFICIENCY SYNDROMES

Immunodeficiency state - is a pathological condition of the body characterized by a decrease of specific immune responses or nonspecific defence reactions against alien entities that result from insufficient amount and dysfunction of leukocytes or deficit of noncellular protective factors.

Based on the predominant damage to the certain subset of the immunocompetent cells, immunodeficiencies can be classified as:

• T-system-dependent;

• B-system-dependent;

• A-system-dependent;

• complement-dependent;

• combined.

Based on the mechanisms of disorder immunodeficiencies can be:

• leucopenic, resulting from disorder of proliferation or increased destruction of immunocompetent cells;

• dysfunctional, resulting from disorders of phagocytosis, processing of Ags, transducing of proliferative signal, or interaction between immuno-competent cells.

Primary immunodeficiencies

Most primary immunodeficiency diseases are genetically determined and affect specific immunity (i.e. humoral and cellular) or nonspecific host defence mechanisms mediated by complement proteins and cells such as phagocytes or NK-cells.

Disorders of the T-system

DiGeorge syndrome (thymic hypoplasia).

DiGeorge syndrome derives from the failure of the development of the third and fourth pharyngeal pouches. The latter give rise to the thymus, the parathyroids, some of the clear cells of the thyroid, and the ultimobranchial body.

Manifestations: patients have a variable loss of T-cell-mediated immunity (owing to hypoplasia or lack of the thymus), tetany (owing to lack of the parathyroids), and congenital defects of the heart and great vessels. Plasma cells are present in normal numbers in lymphoid tissues. Ig levels may be normal or reduced, depending on the severity of the T-cell deficiency.