IgM is the largest antibody, and it is the first antibody to appear in the response to initial exposure to an antigen. IgM is constructed of five or six units (i.e. mostly as pentamers but also hexamers occur) which are each comprised of two heavy-chains (mu-chains) and two light chains, bound together by disulfide bonds and a so-called J-chain.
In antigen-based serological SARS-CoV-2 tests secondary antibodies against human IgG or human IgM are used as capture antibodies:
anti-Human IgG Antibodies
SARS-CoV-2 ELISA Kits |
SARS-CoV-2 PCR Kits |
SARS-CoV-2 Antibodies |
ACE2 Antibodies |
TMPRSS2 Antibodies |
SARS-CoV-2 Proteins |
SARS-CoV-2 Protein Interactome |
SARS-CoV-2 Life Cycle Stages & Inhibition Targets |
HLA in Adaptive Immune Response
IgM is present on B cells and its main function apparently is the control of B-cell activation. B-cells create IgM antibodies as a first line of defense. Their large size gives them excellent binding avidity, and can pick up trace amounts of infection to mark for recognition by phagocytes. IgM is primarily found in serum and due to its size, it cannot diffuse well, and is found in the interstitium only in very low quantities.
Interestingly, it is also the primary antibody against A and B antigens on red blood cells and responsible for the blood-clotting reaction during blood-transfusion from wrong donors.
Immunoglobulin M (IgM) is constructed of five or six units (i.e. mostly as pentamers but also hexamers occur) which are each comprised of two heavy-chains (mu-chains) and two light chains, bound together by disulfide bonds and a so-called J-chain. A J-chain is usually, but not always, found in pentameric IgM but not in the hexameric form. The J chain, which is added just before secretion of pentamer helps in the polymerization of the monomers.
It is not yet clear if pentamers without the J-chain fullfill different functions. Each monomer has two antigen binding sites, hence a pentameric IgM has 10 binding domains for different antigens; however, not all of these sites can be occupied at the same time due to limitations in space.
IgM is by far largest antibody in the human circulatory system. In its pentamer form IgM has a molecular mass of approximately 900 kDa.