CD40LG and agammaglobulinemia: Biallelic mutations in CD19 (n = 10; Kanegane et al., 2007; Skendros et al., 2014; van Zelm et al., 2006; Vince et al., 2011; Wentink et al., 2018) or CD81 (n = 2; van Zelm et al., 2010; Yang et al., 2022) cause severe recurrent respiratory tract infections, hypogammaglobulinemia (reduced IgG in all; low IgM and/or IgA in ∼50%), impaired vaccine responses, and reduced memory B cells; however, B cell development is intact (Fig. 2).