Non-clinical sciences/Immunology · Immunoglobulins

IgG

What it means

Most abundant serum immunoglobulin (~75%); four subclasses (IgG1–4). The only Ig that crosses the placenta → passive neonatal immunity to ~6 months. Activates classical complement, opsonises (Fc receptors on phagocytes), neutralises toxins. Anti-Rh(D) antibodies are IgG (→ haemolytic disease of the newborn).

Worked example

Specific IgG positivity = past exposure / immunity / memory (e.g. anti-HBs after vaccination; rubella IgG = immune).

In the exam

A neonate is protected against an infection for the first months of life by maternal antibody that crossed the placenta in utero.

What settles it

IgG crosses the placenta and signals immunity/memory; IgM is the acute, pentameric, non-crossing first responder.

Classically confused with

IgM

Source: StatPearls — Immunoglobulin