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metapopulations of parasitic nematodes such as Ascaris may evolve a set
of polymorphisms in important antigens and immune evasion genes that
suit success in a given metapopulation of humans. Also, with time, a host
population may become reactive to the range of antigens expressed by its
parasite population, and those parasites will then be replaced by immi-
grant worms bearing different antigens or immune evasion factors.
At risk of overextending the possible existence and importance of
genetic polymorphisms in parasites, the next section discusses and chal-
lenges some ideas about concomitant immunity in helminth infections.
POLYMORPHISMS IN PARASITE ANTIGENS
A ND CONCOMITANT IMMUNITY
One of the puzzles in helminth infections is that they are so persistent
and without strong evidence of protective immunity, while in the face of
continual recruitment by new parasites (the average lifetime of an Ascaris
adult worm is estimated to be about 1 year, yet infections can persist for
decades). 14,37 This could be because of immunomodulation and immu-
nosuppression by the parasites themselves, but another aspect could be,
as introduced above, polymorphisms in the parasite population in their
antigens and/or immunomodulatory factors. While diversity within
populations of parasitic nematodes has been noticed before, 66 it has not
been established yet to what degree antigenic polymorphism contributes
to the slow or zero acquisition of immunity in people infected with
nematodes. Another problem is that, with parasites such as Ascaris that
contaminate the environment so effectively with their transmission stages
(eggs), how is it that hosts do not more frequently become lethally over-
populated with worms?
One concept that arose to explain this latter problem in the context of
schistosome infections is concomitant immunity. 67 By this hypothesis,
adult parasites that are established somehow immunize against incoming
larvae so as to avoid overcrowding and death of the host. Mathematical
consideration of concomitant immunity falls in favor of this being
a successful reproductive strategy for the parasite population. 68 But does
not concomitant immunity essentially infer altruism between parasites?
That is, for the concomitant immunity system to work, adult worms
would be eliciting immune responses that would be lethal to their own
offspring but to the benefit of unrelated adults that are already present.
Also, a larva invading a host is that parasite's one and only chance to
reproduce, so selection would favor the evolution of larvae that evade
the system. Another problem is that concomitant immunity would limit
the colonization of a host and thereby reduce the outbreeding potential
of a sexually reproducing parasite. Why should we expect parasites to
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