Antibody (Immunoglobulin)

An antibody is built from two identical heavy chains and two identical light chains, about 150 kDa for IgG. Each Fab arm carries six hypervariable loops that form the binding surface, while the Fc stem engages Fc receptors and complement. The heavy-chain constant region defines the isotype: IgM appears first as a pentamer, then class switching under cytokine direction produces IgG, IgA or IgE from the same variable region. The four IgG subclasses differ in complement fixation and Fc receptor engagement, which is why therapeutic antibodies are engineered onto a chosen subclass.

IgG has a serum half-life of about 21 days because the neonatal Fc receptor binds it in acidified endosomes and recycles it to the surface instead of letting it reach the lysosome. That recycling is why antibody drugs are dosed every two to four weeks, and it is the mechanism borrowed by Fc fusion: dulaglutide is a GLP-1 analogue attached to an IgG4 Fc fragment and is dosed weekly, where the unmodified peptide would last minutes.

For peptide readers the contrast is the point. Antibodies buy months of exposure and very high selectivity but bring cost of goods, cold chain, injection-only administration and essentially no central nervous system access. Choosing between the two formats is mostly a decision about duration and target location rather than about potency.

Two misreadings recur. Titre is not neutralisation: a binding assay measures attachment to antigen, and only a subset of anti-drug antibodies found by a binding screen actually block activity, which is why a separate neutralising assay is required. And small peptides are often called non-immunogenic because of their size, yet the same sequence conjugated to a carrier, aggregated, or carrying adjuvant-like impurities can raise a strong response.