Abstract
Remarkable progress has been made in our understanding of the mechanisms by which the immune system responds to challenge by invading pathogens. Driven by urgent clinical problems, including the pandemic caused by human immunodeficiency virus type 1 (HIV-1) (1,2), these basic advances are starting to yield rewards in the treatment of human disease. Despite these gains, many fundamental questions remain unanswered, including many related to the factors that govern reconstitution of the immune system following its destruction in the setting of human disease.