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42 Scientific American, April 2019 Illustration by Tami Tolpa

Skin cell

Macrophage

B cell

Aedes aegypti

mosquito

Dengue virus 1 Dengue virus 2

FIRST INFECTION SECOND INFECTION

Antibody

How Antibodies May Aggravate Dengue

Four related viruses transmitted by Aedes mosqui­

toes cause dengue disease. A theory called anti body­

depen dent enhancement, postulated in the 1970s by

Scott B. Halstead, seeks to explain why a second

bout of dengue, with a different virus, is deadlier than

the first. A first infection, with, say, dengue virus 1,

prompts B cells to make antibodies that coat the

vi ruses and deliver them to white blood cells such as

macro phages, which trap and kill them. The B cells

sub sequently become quiescent, awakening after a

second infection to make the same anti bodies. But

antibodies to dengue virus 1 cannot bind well with, for

example, dengue virus 2. The anti bodies still de liver

the invaders to the macro phages but in a way that

enables them to escape destruc tion and instead com­

mandeer the macrophages. They use the sub jugated

cells’ machinery to repli cate them selves, flooding the

body with viruses. These release a pro tein that dam­

ages blood vessels, causing serious dengue disease.

On entering the body,

dengue virus often

attacks skin cells

Antibodies attach

to virus, covering

and disarming it

Virus is trapped

and destroyed

Virus escapes

cell defenses

and replicates

New copies of virus

are released

Antibodies are

unable to attach

well, leaving virus

partially bare