About IgM
Immunoglobulins (Ig) are the antigen recognition molecules of B cells. An Ig molecule is made up of 2 identical heavy chains and 2 identical light chains (see MIM 147200) joined by disulfide bonds so that each heavy chain is linked to a light chain and the 2 heavy chains are linked together. Each Ig heavy chain has an N-terminal variable (V) region containing the antigen-binding site and a C-terminal constant (C) region, encoded by an individual C region gene, that determines the isotype of the antibody and provides effector or signaling functions. The heavy chain V region is encoded by 1 each of 3 types of genes: V genes (see MIM 147070), joining (J) genes (see MIM 147010), and diversity (D) genes (see MIM 146910). The C region genes are clustered downstream of the V region genes within the heavy chain locus on chromosome 14. The IGHM gene encodes the C region of the mu heavy chain, which defines the IgM isotype. Naive B cells express the transmembrane forms of IgM and IgD (see IGHD; MIM 1471770) on their surface. During an antibody response, activated B cells can switch to the expression of individual downstream heavy chain C region genes by a process of somatic recombination known as isotype switching. In addition, secreted Ig forms that act as antibodies can be produced by alternative RNA processing of the heavy chain C region sequences. Although the membrane forms of all Ig isotypes are monomeric, secreted IgM forms pentamers, and occasionally hexamers, in plasma (summary by Janeway et al., 2005).[supplied by OMIM, Aug 2010]

About APC-eFluor 780
APC-eFluor™ 780 (APC-eF780) is a red-emitting tandem fluorophore of APC, excited by the 640 nm red laser and transfers energy to the acceptor, eF780, which emits light that can be captured with a 780/60 nm bandpass filter. APC-eF780 has an excitation peak at 645 nm and an emission peak at 767 nm, and is spectrally similar to APC-CF®750 (Biotium), APC-Cy® 7 (GE Healthcare) APC-H7 (BD Biosciences), APC-eFluor™ 780 (ThermoFisher Scientific), APC-Vio®770 (Miltenyi Biotec) and APC-Fire™ 750 (BioLegend). This dye is part of the larger eFluor™ dye family, developed by eBioscience which is now part of ThermoFisher Scientific.