Q.94 The total number of complementary determining regions (CDRs) in the functional form of
an IgM antibody is _____

Antibody Structure Basics

Antibodies contain variable domains with three complementarity-determining regions (CDRs) per light chain (L1, L2, L3) and three per heavy chain (H1, H2, H3), totaling six CDRs per Fab arm. A basic monomer like IgG has two Fab arms, yielding 12 CDRs. IgM differs as it polymerizes into a functional pentameric form with five such monomer units plus a J chain.

IgM Specifics

Functional IgM exists as a pentamer comprising 10 heavy μ chains and 10 light chains, forming 10 Fab arms. Each Fab contributes six CDRs, so the pentamer has 10 × 6 = 60 CDRs. This structure enhances avidity for multivalent antigens despite lower affinity per site.

Question Analysis

The query targets CDRs in “functional form of an IgM antibody,” referring to secreted pentameric IgM, not membrane-bound monomeric form on B cells. Thus, 60 is correct; 12 applies to monomers like IgG or non-functional IgM, while 6 is per antigen-binding site.

The total number of complementary determining regions (CDRs) in the functional form of an IgM antibody is a key concept in immunology for CSIR NET aspirants. CDRs form the antigen-binding sites, and IgM’s unique pentameric structure multiplies them significantly.

CDR Fundamentals

CDRs are hypervariable loops in antibody variable domains: three each in VH (H1-H3) and VL (L1-L3), making six per Fab. These create the paratope for antigen specificity.

IgM Functional Form

Secreted IgM functions as a pentamer with five (μL)₂ monomers linked by disulfides and a J chain, yielding 10 Fab arms. Thus, total CDRs = 10 Fabs × 6 CDRs/Fab = 60.

Exam Relevance

CSIR NET questions emphasize this: options like 12 (monomer) or 6 (single Fab) mislead, but functional IgM means pentamer with 60 CDRs. This boosts avidity for early immune responses.