The antibodies that our adaptive immune system produces are part of a class of proteins that we call 'immunoglobulines'. These have 5 different classes:
IgG: small immunoglobulin, in a way the basic antibody group, that comprises the largest group of immunoglobulins. The IgG has a constant part and 2 arms with highly variable composition. The variable part is produced in such a way that it can exactly fit (bind) to the epitope of an antigen. Due to the small size, this class of antibodies can pass through the placenta, and hence transfer antibody protection to the fetus, which will protect the newborn child for the first 3 months of it’s life, against all the infections that the mother has developed immunoglobulins against.
IgM: large immunoglobulins, composed of 5 antibody molecules (=pentamer). They can bind many more antigens at the some time, since they have 10 arms with highly variable antigen binding sites. These immunoglobulins are usually associated with the primary stages of the infection. These IgM immunoglobulins are so large, that they cannot pass through the placenta.
IgA: small molecules, associated mainly with mucosal immunity, hence we find large quantities in the gut and respiratory tract. The molecules can present as a dimer (i.e. 2 antibody molecules bound together) or as a monomer, which resembles the basic IgG molecule.
IgE: a monomer immunoglobulin, related to allergic responses and to protection against parasites.
IgD: not so well know. Not to be found free in serum, but found on the plasma membrane of B cells.