This study will explore screening for immunodeficiency diseases (diseases that cause problems in fighting infections). There is no method at present to screen all babies at birth for immunodeficiency. However, babies with low numbers of T-cells-an important type of immune system cell-may be found by studying T-cell products called TRECs (T-cell receptor excision circles). This study will: * Collect samples from children with several different immunodeficiencies to find out which disorders can be found by screening dried blood spots for TRECs. * Try to develop screening tests based on other kinds of material derived from dried blood spots. Children with primary immunodeficiency and low numbers of T cells who have not had a bone marrow transplant may be eligible for this study. Participating children donate up to 5 ml (1 teaspoon) of blood. The sample may be collected when the child is having other blood tests. The liquid blood is analyzed to determine the number of T cells, and the rest of the blood is used to make dried blood spots on filter paper. The blood spots are used to develop screening tests for immunodeficiency. The blood spots and data about the child's age, diagnosis, and current medicines will be kept coded by diagnosis and a code number instead of the child's name.
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