UNC13D and agammaglobulinemia: In comparison to sporadic HLH with hypogammaglobulinemia, UNC13D deficient patients with hypogammaglobulinemia presented with an earlier age of onset and were more likely to exhibit hepatosplenomegaly (100% vs. 44.5%, and 100% vs. 77.8%), respiratory infections (100% vs. 44.5%), neurological manifestations (75% vs. 33.3%), and a higher mortality rate (80% vs. 66.7%) (Figure 1B).