Biomedical Engineering Reference
In-Depth Information
are transported throughout the body. The cells of the adaptive immune system
are a type of white blood cell called lymphocytes. These cells are transported by
the blood but can exit the blood stream and crawl through the tissues of the
body. They return to the blood in the lymph fluid that bathes cells and that is
collected in a system of ducts called lymphatics, which ultimately connect with
the blood stream.
The best-studied immune systems are those of the mouse and the human.
The mouse has about 10
8
lymphocytes, while a human has about 10
12
. Immune
systems are found in all vertebrates, and thus organisms as small as a tadpole
(body mass on the order of 10
-1
g) and as large as an elephant (10
6
g) or a whale
(10
8
g) have immune systems. Here we address the question of whether there
exist any scaling principles that can guide our understanding of the operation of
the immune system in organisms that differ by nine orders of magnitude in
mass. Scaling principles are introduced in Part II, chapter 3 (by Savage and
West) of this volume.
For much of its history immunology has been a subfield of microbiology
and closely linked with medicine. As such, studies of the immune systems of
diverse species have not attracted much attention or much funding, and there is a
paucity of data about the immune system of most species. We know of only one
other group that has theoretically addressed the question of the scaling of the
immune system (1). Thus, the goal of this chapter is to raise questions, and pro-
vide a brief overview of what is known about scaling laws in the immune sys-
tem.
1.1. The Protecton Hypothesis
Langman and Cohn (1) suggested that the immune system has a modular
structure and is built of basic units called "protectons." Each protecton guaran-
tees an adequate immune response in some volume element of the animal. Thus,
in the Langman-Cohn view, a big animal simply has more protectons in its im-
mune system than a small animal. Langman and Cohn also estimated, based on
the concentration of antibody needed to protect an animal, that a protecton con-
tains about 10
7
B cells in a volume of about 1 ml. In terms of scaling, the protec-
ton idea suggests that the size of the immune system scales as the mass of the
organism.
While modularity is a desirable property of any large complex system, we
argue against this strict point of view and the simple scaling proportional to
mass (~
M
) for a number of reasons:
(1) Transport of lymphocytes depends on the circulatory system.
As shown by West, Brown, and Enquist (2), properties of the
circulatory system do not scale ~
M
, but rather as ~
M
3/4
.
