Evolutionary immunology: Which immunoglobulin class is considered the phylogenetically oldest (earliest to appear in vertebrate evolution)?

Introduction / Context:
Antibody classes evolved over time, with some isotypes appearing earlier in vertebrate evolution. Knowing which class is phylogenetically ancient helps explain why it plays foundational roles in primary responses across species.

Given Data / Assumptions:

• Comparison across vertebrates (e.g., fish, amphibians, mammals).
• Focus on earliest-emerging class in evolutionary history.
• General immunology consensus on isotype evolution.

Concept / Approach:
IgM is widely recognized as the earliest immunoglobulin class to appear in vertebrates. It exists as a pentamer in serum (in mammals) and serves as the initial secreted antibody in primary responses. Other classes like IgG, IgA, and IgE are evolutionarily later, with specialized roles in systemic, mucosal, and allergic immunity respectively.

Step-by-Step Solution:
Identify isotype present across the broadest range of early vertebrates: IgM. Connect evolutionary antiquity with its role as first-line humoral responder. Rule out IgG, IgA, IgE as later specializations. Select IgM.

Verification / Alternative check:
Comparative immunology texts document IgM-like molecules in cartilaginous and bony fish, preceding the emergence of class-switched isotypes typical of mammals.

Why Other Options Are Wrong:

• IgG / IgA / IgE: Appeared later with specialized functions.
• All are similar: Evolutionary timelines differ; they are not simultaneous.

Common Pitfalls:
Equating “most abundant in serum” (IgG) with “oldest.” Abundance today does not imply earliest evolutionary origin.

Final Answer:
IgM.