BCR and isolated agammaglobulinemia: The BCR signaling pathway is essential for B-cell survival and proliferation; the loss of one of the key intracellular downstream signaling proteins, Bruton tyrosine kinase (BTK) leads to Bruton’s X-linked agammaglobulinemia (known as X-linked agammaglobulinemia (XLA)), which is a severe immunodeficiency characterized by the developmental arrest of B-cell precursors and no immunoglobulin production [68,69].