B cells are lymphocytes responsible for the production of antibody. The most common type of primary immunodeficiency (>50% of cases) involves deficient antibody production. Primary humoral deficiencies vary from complete absence of B cells, serum immunoglobulin (Ig), or both to lacunar deficits that involve specific antibody responses to polysaccharides. The spectrum of antibody deficiency is broad, ranging from decreased total IgG levels to normal IgG levels and from primary B-cell defects to combined immunodeficiencies with antibody abnormalities associated with other immune and often nonimmune abnormalities.
Although this article discusses agammaglobulinemia and hypogammaglobulinemia, the emphasis is on selective Ig deficiencies, including the decreased production of IgA and the various IgG subclasses and impaired antibody responses to specific antigens such as polysaccharide proteins present on certain bacteria.