IMMUNITY TO HUMAN MALARIA: CHAR¬ 
ACTERISTICS OF IMMUNITY 
By W. B. REDMOND 
DEPARTMENT OF BIOLOGY, EMORY UNIVERSITY, GA. 
Immunity to human malaria should be 
considered from two standpoints. (1) A 
study of the literature reveals the impor¬ 
tance of the protection resulting from years 
of infection in areas of hyperendemie ma¬ 
laria. This protection is apparent espe¬ 
cially in the adult population, in contrast 
to the high infection in all below the age 
of 15 years. A single infection under these 
conditions affords no protection probably 
because of the numerous heterologous 
strains present and the constant rate of 
reinfection by mosquitoes. (2) A differ¬ 
ent picture is presented in experimental 
malarial infections. Complete protection 
against reinfection with the same strain is 
afforded in about one case out of every 
three, but when the reinoculated parasites 
are of a different strain the protection is 
not so marked. The resulting infection 
may be lower in number of parasites, and 
the febrile attacks are generally fewer and 
less severe. There seems to be no protec¬ 
tion, however, to heterologous strains of P. 
falciparum. 
The use of malarial therapy in the treat¬ 
ment of paresis first employed by Wagner- 
Jauregg initiated the laboratory study of 
human malaria. Previous to any experi¬ 
mental work on human malaria it was 
realized by some malarialogists and phy¬ 
sicians in highly malarious districts that 
some persons could live among infected 
peoples in conditions optimum for the 
transfer of malaria and still not be in¬ 
fected. Laveran (1908), however, stated 
that a first attack of malaria did not pro¬ 
duce immunity, and that some times fol¬ 
lowing the primary attack an individual 
was more susceptible than previously. Bass 
(1919) came to the conclusion that no effec¬ 
tive and lasting immunity to malaria was 
developed. He found that 50 per cent of 
231 
the population in certain parts of Miss¬ 
issippi had malaria each year; about half 
of these were relapses and half were rein¬ 
fections. The adult population had a very 
high rate of infection, but not as high as 
the children. His criterion of immunity 
was based on a failure to find parasites in 
the blood. 
Persons who appeared to be resistant to 
attacks may or may not have had histories 
of previous infection. Bass reported that 
blood examinations of individuals without 
any previous history of malaria showed 16 
per cent to be harboring parasites in the 
blood. They were without symptoms and 
were unaware of the fact that they were 
infected. Daniels (1901) and Gill (1914) 
noted the enlargement of the spleen and 
were able to correlate the spleen rate with 
the immunity. Children under two or 
three years of age had the highest infec¬ 
tions and most severe attacks. During this 
period the spleen increased in size, but with 
the decrease in malarial attacks after 5 
years of infection the spleen rate began to 
diminish. In adults attacks were infre¬ 
quent and mild, but the high spleen rate 
was indicative of continued infection. Ac¬ 
cording to Daniels this freedom from 
febrile attacks is associated with continued 
inoculation of sporozoites without inter¬ 
ruption over a period of years. Gill con¬ 
cluded that a constant spleen rate over an 
extended period was significant of a definite 
immunity. "Where the inoculation of para¬ 
sites is interrupted by conditions not suit¬ 
able for the natural transfer by mosquitoes, 
such as seasonal changes, the fluctuating 
infections do not result in a very effective 
immunity. A low spleen rate is considered 
to be indicative of a high susceptibility, 
and an infection rate of 5 per cent in such 
an area could result in an epidemic should 
