Guideline to work with IgM

Specific IgM antibodies are a result of a primary response to the challenge with antigen. In some cases under production of polyclonal or monoclonal antibodies, certain antigens might elicit antibody response on the level of IgM only.

Pentameric or hexameric IgM has:

Molecular mass [kDa] ~ 160
Isoelectric point ~
Extinction coefficient at 280 nm is 1.18 (Johnstone and Thorpe 1988)

Some important points to consider when working with IgM:

IgM is poorly soluble.
IgM has a tendency to cryoprecipitate.
IgM can "stick" to the walls of a tube.
Purified IgM, stored at 4°C might be stable for around 6-10 months.
IgM might precipitate in the buffers of low ionic strength.
There are possible problems with retaining IgM activity during hydrophobic chromatography.
IgM antibodies often give unexpected cross-reactivities, and it is important to realise that certain problems might occur (Goding 1996)

How to stabilise IgM? Some general considerations.

IgM might require higher salt concentration than 150 mM NaCl; try to increase the NaCl concentration up to 350 mM.
Borate buffer is recommended for storage of IgM, its subunits or its fragments (J. E. Coligan et al. 1991, Current Protocols in Immunology).
Glycine has a stabilizing effect on IgM: adding about 0.01M glycine, should also help. Note: consider a possible influence of a glycine in different assays.
IgM can be also stabilised by addition of BSA or HSA to final 0.05 or 0.5 %.
Addition of glycerol might be of a help as well.

IgM storage conditions:

Recommended is PBS including azide in 0.02 % final concentration. Microbial contamination is a very often cause of a lost activity of antibodies. Use a good preservative and make sure that you prepare things aseptically. Freezing of IgM should be tested for each individual antibody. Some IgM antibodies might be inactivated by freezing.

Valuable information about IgM cab be found in:

"Monoclonal antibodies: principles and practice" by James W. Goding, 1996,
ISBN 0-12-287023-9; Publisher: Academic Press.