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30         The Rice Institute Pamphlet

Reaction of Host to Parasite

Having reviewed the present state of knowledge regarding the be-
havior of the parasite we are in a better position to discuss the question of
immunity in amebiasis. Among the available facts the first in order are
the well-known classical experiments of Walker & Sellards (1913), who
had shown that human beings varied in their susceptibility to infection
with
E. histolytica, one and the same strain of which produced symptom-
less infection in some individuals and clinical manifestations in others.
These results were interpreted as evidence for the existence of natural or
innate immunity in certain persons (Dobell & Low, 1922; Craig, 1944;
Craig & Faust, 1951; Frye, 1955). Similarly, the absence of clinical symp-
toms in carriers has also been attributed to natural resistance of the host
(Craig, 1944). On the other hand, there is no clear evidence of acquired
immunity to amebiasis in man, for persons who had recovered from or
were cured of an infection are liable to be reinfected (Craig, 1944; An-
derson
et al., 1953; Porter, 1953; Frye, 1955), and it is significant in this
connection that amebic infections are remarkably persistent, lasting some-
times for years (Dobell & Low, 1922; Brumpt, 1928, 1949). That the hu-
man host does not acquire immunity to reinfection has recently been
demonstrated experimentally (Beaver
et al., 1956), though dogs which
had recovered from the disease appear to be resistant (Simic, 1935;
Swartzwelder & Avant, 1952).

Among the barriers which an intestinal parasite like E. histolytica must
overcome to gain access to the tissues is the normal mucosa of the gut
wall. When its resistance is lowered by various factors mentioned above,
this barrier may be broken at some points, exposing the tissues to attack
by the amebae (Westphal, 1937, 1938; Fischer & Reichenow, 1952; Ander-
son
et al., 1953; Chandler, 1957). However, as noted already, it is not
clear whether or not the amebae are capable of penetrating through this
barrier without the aid of extraneous agents.

More tangible—though indirect—indications of some immunological re-
sponse to amebic infection is provided by serological reactions, the most
important of which is the complement fixation test. Though much work
has been devoted to this test, the results are conflicting, some of the dis-
crepancies probably being due to variations in the preparation of anti-
gens and in technical procedure, or to antigenic differences between
strains of
E. histolytica used for the test (Hussey & Brown, 1950; Bozice-
vich, 1950). However, there seems to be a consensus that when the
amebae are restricted to the lumen of the gut (as in symptomless infec-
tions) they do not stimulate antibody formation and the reaction is there-
fore negative, but when the parasite invades the tissues there is a positive



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