  An improved version of the symmetrical immune network theory is presented, in
which killing is ascribed to IgM antibodies, while IgG antibodies are
stimulatory. A recurring theme in the symmetrical network theory is the concept
of co-selection. Co-selection is the mutual positive selection of individual
members from within two diverse populations, such that selection of members
within each population is dependent on interaction with (recognition of) one or
more members within the other population. The theory resolves the famous I-J
paradox of the 1980s, based on co-selection involving helper T cells with some
affinity for MHC class II and suppressor T cells that are anti-anti-MHC class
II. The theory leads to an experimentally testable prediction concerning I-J. A
mathematical model that simulates IgM killing and inhibition of IgM killing by
T cells is surprisingly the same as one that models a neural network.
