Library · Molecule
Immunoglobulin M
Make it clearer
Immunoglobulin M, or IgM, is an antibody, that is, a defense protein that recognizes something foreign. Where it falls in the sequence matters: when a new antigen shows up, the adaptive immune system, the part of the immune system that learns, makes it first. An antigen is the piece of the intruder that gets recognized.
Its shape is what sets it apart. When it is secreted it does not travel alone: five copies join together into what is called a pentamer, a large molecule of more than 900 kilodaltons. That assembly has ten gripping points. None of them grips especially hard, but ten hooks at once simply do not let go. That is what high avidity means.
That shape gives it a concrete advantage. A single pentamer stuck to a microbe is enough to switch on C1q, the piece that starts complement, a system of proteins that attacks the intruder. IgG, which is another antibody, needs two molecules side by side to do the same thing.
It does have two limits. In that first encounter its affinity, the strength of each individual grip, is lower than that of the antibodies that come later. And because it is so big, it has a hard time getting out of the blood and into the tissues.
In brief
IgM is the first antibody the adaptive immune system secretes against a new antigen.
Secreted IgM circulates mostly as a pentamer of more than 900 kDa with ten binding sites, which gives it high avidity.
Ten loose hooks that together never let go.
One pentamer bound to a pathogen is enough to activate C1q and switch on the classical complement pathway; IgG needs two.
Its affinity in the primary response is lower than that of later isotypes, and its size makes it hard to leave the bloodstream.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingBiochemistry, Immunoglobulin M, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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