Globulin does not refer to a single protein but rather to a heterogeneous group of serum proteins other than albumin. Collectively, globulins play essential roles in immune defense, inflammatory regulation, molecular transport, and the maintenance of physiological homeostasis. In routine clinical chemistry, serum globulin concentration is commonly calculated by subtracting albumin from total serum protein. Serum protein electrophoresis further separates globulins according to their electrophoretic mobility into the major α1-, α2-, β-, and γ-globulin fractions. The α1- and α2-globulin fractions contain proteins such as α1-antitrypsin, α1-acid glycoprotein, haptoglobin, ceruloplasmin, and α2-macroglobulin. Many of these proteins behave as acute-phase reactants and undergo substantial alterations during infection, tissue injury, and systemic inflammation. The β-globulin fraction includes transferrin and several complement components and is therefore involved in iron metabolism, complement activation, and innate immune responses.

The γ-globulin fraction consists predominantly of immunoglobulins (Igs). Immunoglobulins are produced by plasma cells differentiated from B lymphocytes and are composed of two identical heavy chains and two identical light chains. The variable regions within the antigen-binding fragments (Fab) confer antigen specificity, whereas the Fc region interacts with Fc receptors and components of the complement system, thereby coupling antigen recognition to downstream effector mechanisms such as phagocytosis, antibody-dependent cellular cytotoxicity, and immune regulation. According to the constant region of the heavy chain, human immunoglobulins are classified into five major isotypes: IgG, IgA, IgM, IgD, and IgE. IgG is the most abundant immunoglobulin in the circulation and mediates neutralization, opsonization, and Fc receptor-dependent immune functions. Its relatively long serum half-life is facilitated by recycling through the neonatal Fc receptor (FcRn). IgA is predominantly associated with mucosal surfaces, particularly those of the respiratory and gastrointestinal tracts, where it constitutes a major component of mucosal immune defense. IgM is generally produced early during primary humoral immune responses and is highly efficient in activating complement, whereas IgE interacts with Fcε receptors on mast cells and basophils and has a central role in allergic inflammation.

Alterations in serum globulin levels therefore provide clinically useful information, although they are not disease-specific. Chronic infections, inflammatory disorders, autoimmune diseases, and certain liver diseases may cause polyclonal hyperglobulinemia. In contrast, clonal expansion of plasma cells or B cells can result in production of a monoclonal immunoglobulin, which may appear as an M protein or monoclonal spike on serum protein electrophoresis and is characteristic of disorders such as monoclonal gammopathy of undetermined significance and multiple myeloma. Reduced globulin concentrations may occur in primary or secondary immunodeficiency, protein-losing nephropathy or enteropathy, and severe impairment of protein synthesis. Consequently, abnormal globulin concentrations should generally be interpreted together with the albumin-to-globulin ratio, serum protein electrophoresis, immunofixation electrophoresis, and serum free light-chain assays to determine their underlying biological and clinical significance.

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References

1. Park S, Park JK. 2024. Back to basics: the coagulation pathway. Blood Research.