> What does this mean?

histoimmunological - having to do with the cells that make up the
immune
system, i think.  people have immune system cells, like B or T
lymphocytes,
that produce antibodies identify and destroy  invading antigens.  An
individual
lymphocyte has only one kind of antigen receptor that fights antigens.

So individuals have a spectrum of these cells that make up a profile
of
their immune systems.

An individual may have gaps in their profile.  By mating with someone
who
has a complimentary profile, the children will tend to have the full
suite
of antibodies, and so be healthier.