Hypogammaglobulinemia is characterized by a decrease in plasma gamma globulins due to a deficiency in B lymphocytes. It can be caused by primary (congenital) disorders including X-linked agammaglobulinemia, autosomal recessive agammaglobulinemia, specific antibody deficiency, and hyper-IgM syndromes. Secondary causes include nephrotic syndrome, protein-losing enteropathy, and immunosuppressive therapy. Treatment involves immunoglobulin replacement therapy through vaccination or bone marrow transplantation for certain syndromes.