Immunoglobulins
Also known as: antibodies, Ig
Immunoglobulins, or antibodies, are Y-shaped proteins produced by B cells and plasma cells that bind specific antigens to neutralize pathogens and mark them for destruction. The five classes are IgG, IgA, IgM, IgE, and IgD.
Immunoglobulins (Ig), commonly called antibodies, are glycoproteins produced by B lymphocytes and plasma cells as the core weapons of humoral immunity. Each immunoglobulin is a Y-shaped molecule built from two heavy chains and two light chains: the variable regions at the tips of the Y bind a specific antigen, while the constant (Fc) region determines the antibody's class and effector functions, such as activating complement or binding to receptors on immune cells.
There are five classes. IgG is the most abundant in serum, dominates secondary immune responses, and is the only class that crosses the placenta. IgM is the first antibody produced in a primary response and circulates as a large pentamer that fixes complement efficiently. IgA, secreted as a dimer, guards mucosal surfaces and is passed to infants in breast milk. IgE binds mast cells and basophils, driving type I hypersensitivity (allergy) and defense against parasites. IgD sits mainly on naive B cell surfaces as a receptor.
Antibodies protect the body by neutralizing toxins and viruses, opsonizing pathogens for phagocytosis, activating the classical complement pathway, and enabling antibody-dependent cellular cytotoxicity. Clinically, immunoglobulin levels and patterns matter: elevated IgM suggests recent infection, specific IgE rises in allergy, and deficiencies of particular classes produce recurrent infections.
USMLE Step 1 tests immunoglobulin structure, class functions, and class switching, along with immunodeficiencies such as selective IgA deficiency. The CCMA exam covers antibodies in the context of blood analysis and serologic testing, and the MBLEx includes immunoglobulins among the protective components of blood in its anatomy and physiology content.
Key takeaways
- Immunoglobulins are antigen-binding antibodies made by B cells and plasma cells.
- The five classes are IgG, IgA, IgM, IgE, and IgD, each with distinct roles.
- IgG crosses the placenta; IgM leads the primary response; IgA protects mucosal surfaces; IgE drives allergy.
- Antibodies work by neutralization, opsonization, complement activation, and antibody-dependent cytotoxicity.
- Exams test class functions, class switching, and deficiency syndromes like selective IgA deficiency.