What is the Difference Between Membrane Bound and Secreted IgM

The main difference between membrane bound and secreted IgM is that the monomeric receptor IgM molecule is affixed to the plasma membrane, triggering the later stages of B cell differentiation, whereas the pentameric, secreted IgM molecule is an effector of humoral immunity.

Membrane-bound and secreted IgM molecules are one of the several isotypes of immunoglobulins of vertebrates including humans. IgM is the largest type of antibody and it is the first antibody to appear in response to initial exposure to a particular antigen.  

Key Areas Covered

1. What are Membrane Bound IgM
– Definition, Structure, Function
2. What are Secreted IgM
– Definition, Structure, Function
3. Similarities Between Membrane-Bound and Secreted IgM
– Outline of Common Features
4. Difference Between Membrane Bound and Secreted IgM
– Comparison of Key Differences

Key Terms

Membrane Bound IgM, Secreted IgM

What is Membrane Bound IgM

Membrane-bound IgM is the monomeric receptor IgM molecule, affixed to the plasma membrane of the B lymphocytes. The main function of the membrane-bound IgM is to trigger the later stages of B cell differentiation. The main structural feature of the membrane-bound IgM is the absence of a J chain. Generally, the monomeric, membrane-bound IgM consists of heavy and light chains. Here, the µ denotes heavy chains while the L denotes light chains in the structure of the membrane-bound IgM. Therefore, the membrane-bound IgM is a heterodimer.

Figure 1: IgM Structure

Furthermore, disulfide bonds and non-covalent interactions hold the heavy and the light chains together. Other than the µ and L domains, the monomeric IgM molecule contains a constant domain and a variable domain.

What is Secreted IgM

Secreted IgM is the pentameric form of the IgM molecule. It is the largest form of immunoglobulins in vertebrates and it appears in the blood as the first form of immunoglobulin in response to a particular antigen. The main significant structural feature of the pentameric IgM is the presence of a J or joining chain. In general, the pentameric form of IgM contains five monomeric units of IgM. However, the J chain is a third protein in addition to the heterodimer of the monomeric IgM. Polymerization of the monomeric IgM occurs due to the covalently attached form of the J chain.

Figure 2: Pentameric IgM

Moreover, the main function of the secreted IgM is to serve as an effector molecule in humoral immunity. These IgM are antibodies and they bind to the antigen, neutralizing them or causing lysis of the pathogen.    

Similarities Between Membrane Bound and Secreted IgM

• Membrane-bound and secreted IgM are two ways that the IgM can occur.
• IgM is one of the several immunoglobulin isotypes in vertebrates.
• IgM is the largest and the first appearing immunoglobulin in response to the initial exposure to an antigen.

Difference Between Membrane Bound and Secreted IgM

Definition

Membrane-bound IgM refers to the monomeric receptor IgM molecule affixed to the plasma membrane, triggering the later stages of B cell differentiation, while secreted IgM refers to the pentameric, secreted IgM molecule is an effector of humoral immunity.

Made up of

Generally, membrane-bound IgM is the monomeric receptor IgM while secreted IgM is the pentameric form of the IgM.

Occurrence

Membrane-bound IgM occurs affixed to the plasma membrane while secreted IgM occurs in blood, lymph, and mucosal surfaces.

Presence of J (Joining) Chain

Membrane-bound IgM does not contain a J chain while secreted IgM contains a J chain.

Function

The main function of the membrane-bound IgM is to trigger B cell differentiation while the main function of the secreted IgM is to serve as an effector of humoral immunity.

Conclusion

In brief, membrane bound and secreted IgM are two forms of IgM, which is the largest type of immunoglobulin.  IgM is the first-appearing form of immunoglobulins in response to exposure to a particular antigen. However, membrane-bound IgM is the monomeric form of the IgM and it lacks the J (joining) chain. Therefore, membrane-bound IgM occurs affixed to the plasma membrane of B cells. Membrane-bound IgM triggers the further differentiation of B cells. In contrast, secreted IgM is the pentameric form of the IgM, containing the J chain. Hence, secreted IgM occurs in the blood, lymph, and mucosal surfaces of the body. In addition, secreted IgM involves in humoral immunity as an effector molecule. Generally, the main difference between membrane bound and secreted IgM is their structure and function.

References:

1. Kehry, M., et al. “The Immunoglobulin μ Chains of Membrane-Bound and Secreted IGM Molecules Differ in Their C-Terminal Segments.” Cell, Elsevier, 1 Sept. 1980.
2. Cattaneo A, Neuberger MS. Polymeric immunoglobulin M is secreted by transfectants of non-lymphoid cells in the absence of immunoglobulin J chain. EMBO J. 1987 Sep;6(9):2753-8. doi: 10.1002/j.1460-2075.1987.tb02569.x. PMID: 3119328; PMCID: PMC553699.

Image Courtesy:

1. “Schematic model of IgM” By Marc Shulman – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “IgM scheme” By WarX – Own Work (CC BY-SA 2.5) via Commons Wikimedia