 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 childs age, diagnosis, and current medicines will be kept coded by diagnosis and a code number instead of the childs name. 