BCR and agammaglobulinemia: It is evident the similarity with the pre-B cell receptor (pre-BCR) of B lymphopoiesis: defects of the pre-BCR (Igμ, λ/VpreB, Igα, Igβ, Bruton tyrosine kinase BTK, B cell linker protein BLNK) cause arrest of the development of B lymphocytes at the stage of large pre-B cell and therefore agammaglobulinemia [59]; defects of the pre-TCR, subdivided into 1) defects of V(D)J recombination and 2) defects of signaling through the pre-T cell receptor, cause arrest of the development of T lymphocytes at the stage of large pre-T cell and therefore SCID (Figure 7).