Immunoglobulin G

Immunoglobulin G, or IgG (from the Latin immunitas — freedom, release from something — and globulus — a small sphere, a drop; G is the first letter of the Greek gamma, the name of a type of protein chain), is the principal and most abundant class of protective antibodies in human serum (accounting for up to 75–80% of all immunoglobulins).

It is the principal defense mechanism of acquired immunity, providing long-lasting and reliable protection against reinfection.

Etiology and pathophysiology

IgG molecules have several specific properties:

• synthesis by B lymphocytes — occurs during the late stages of primary infection (usually 2–3 weeks after infection) or immediately upon repeated exposure to a previously encountered pathogen;
• classic Y-shaped structure and small size — enable IgG to readily pass from the bloodstream into tissues to destroy viruses and bacteria;
• passage across the placental barrier — a unique property. Special neonatal transport receptors (FcRn) are present on the surface of the placenta; they capture maternal IgG antibodies and transport them into the fetal bloodstream. No other class of antibodies is capable of this.

Clinical significance

Detection of immunoglobulin G in a patient’s blood indicates, in most cases, a disease experienced in the past or successful vaccination. The presence of IgG indicates the presence of immunity.

Maternal IgG antibodies are vital to a newborn. They provide passive immunity, protecting the infant from most serious infections (e.g., measles, influenza, and pertussis) during the first months of life, until the infant is eligible for age-appropriate vaccination and begins producing antibodies of their own.