Q40: Given below are two statements: Statement (I): Most IgE is... - Veterinary Microbiology, Immunology, Pathology & Parasitology - ICAR AIEEA PG 2024 - Veterinary Science | Agrigyan

Both Statement (I) and Statement (II) are true: circulating IgE levels are the lowest among all serum immunoglobulins because monomeric IgE has an exceptionally high affinity ($K_a \approx 10^{10} \text{ M}^{-1}$) for $Fc\varepsilon RI$ receptors on tissue mast cells and basophils, leaving almost all IgE cytophilic and cell-bound. Unlike IgG or IgM, IgE cannot simply neutralize or opsonize soluble antigens in fluid phase; it functions by cross-linking cell-bound $Fc\varepsilon RI$, which triggers mast cell degranulation and release of vasoactive mediators.