Although secreted IgM has 10 antigen-binding sites, more than any other antibody, it does have important limitations. Which of the following are limitations of secreted IgM?
The correct answer and explanation is:
Correct Answer: Secreted IgM has limitations including low affinity for antigen and restricted ability to leave the bloodstream.
Explanation:
Secreted Immunoglobulin M (IgM) is the first type of antibody produced during the initial stages of an immune response. It is typically secreted as a pentamer, meaning it is composed of five immunoglobulin units joined together, resulting in a total of 10 antigen-binding sites. While this structure gives it the highest valency of any antibody, allowing it to bind strongly through multiple low-affinity interactions (high avidity), secreted IgM also has notable limitations that affect its function.
One major limitation is its low affinity for individual antigen-binding sites. Although IgM binds antigens effectively through multiple sites at once, each binding site alone interacts weakly with antigens. This low affinity occurs because IgM is produced early in the immune response, before processes like affinity maturation have optimized the antibody’s binding sites for stronger interaction.
Another limitation is IgM’s restricted ability to leave the bloodstream. Due to its large size as a pentamer, IgM does not easily cross the endothelial barrier to enter tissues. As a result, IgM is mostly found in the blood and lymphatic fluid and is less effective in reaching sites of infection within tissues or mucosal surfaces compared to smaller antibodies like IgG or IgA.
Additionally, IgM has limited ability to interact with certain effector cells through Fc receptors. While it can activate the classical pathway of the complement system effectively, which helps in pathogen destruction, it lacks strong binding to many Fc receptors that mediate other immune responses such as phagocytosis or antibody-dependent cellular cytotoxicity.
In summary, despite its high valency and early role in immunity, secreted IgM’s large size, low individual binding strength, and limited tissue penetration reduce its effectiveness in later stages of the immune response.