Advances in donor selection, donor screening, and infectious disease testing have dramatically decreased the incidence of viral transmission by blood products. However on rare occasions when the donor is acutely infected, there can still be transfusion-related transmission of viruses. Rates of transmission of HIV, hepatitis C, and hepatitis B are estimated to be 1 in 2 million, 1 in 1 million, and 1 in 500,000, respectively. There also remains a low risk of “exotic” infectious agents such as West Nile virus, trypanosomiasis, and babesiosis.
Disorders of the Spleen and Thymus
The spleen is frequently involved in a wide variety of systemic diseases. In virtually all instances the spleen responds by enlarging (splenomegaly), an alteration that produces a set of stereotypical signs and symptoms. Evaluation of splenic enlargement is aided by recognition of the usual limits of splenomegaly produced by specific disorders. It would be erroneous to attribute an enlarged spleen pushing into the pelvis to vitamin B12 deficiency, or to entertain a diagnosis of CML in the absence of splenomegaly. Disorders may be grouped according to the degree of splenomegaly that they characteristically produce:
Myeloproliferative neoplasms (CML, primary myelofibrosis); certain indolent leukemias (CLL and hairy cell leukemia); many lymphomas; infectious diseases (e.g., malaria); Gaucher disease
Chronic congestive splenomegaly (portal hypertension or splenic vein obstruction); acute leukemias; disorders with extravascular hemolysis (hereditary spherocytosis, thalassemia major, autoimmune hemolytic anemia; amyloidosis; Niemann-Pick disease; many infections, including infective endocarditis, tuberculosis, and typhoid; sarcoidosis; metastatic carcinoma or sarcoma
Acute splenitis; acute splenic congestion; infectious mononucleosis; miscellaneous disorders, including septicemia, systemic lupus erythematosus, and intraabdominal infections
The microscopic changes associated with these diseases are discussed in the relevant sections of this and other chapters.
A chronically enlarged spleen often removes excessive numbers of one or more of the formed elements of blood, resulting in anemia, leukopenia, or thrombocytopenia. This is referred to as hypersplenism, a state that can be associated with many of the diseases listed previously. In addition, platelets are particularly susceptible to sequestration in the interstices of the red pulp; as a result, thrombocytopenia is more prevalent and severe in persons with splenomegaly than is anemia or neutropenia.
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As is well known, the thymus has a crucial role in T cell differentiation. It is not surprising, therefore, that the thymus can be involved by lymphomas, particularly those of T cell lineage (discussed earlier in this chapter). The focus here is on the two most frequent (albeit still uncommon) disorders of the thymus: thymic hyperplasia and thymoma.
Thymic enlargement often is associated with the presence of lymphoid follicles, or germinal centers, within the medulla. These germinal centers contain reactive B cells, which are only present in small numbers in normal thymuses. Thymic follicular hyperplasia is found in most patients with myasthenia gravis and sometimes also occurs in other autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis. The relationship between the thymus and myasthenia gravis is discussed in Chapter 22. Of significance, removal of the hyperplastic thymus is often beneficial early in the disease.