240 
reticular and loose connective tissues and 
owe their functional importance to their 
wide distribution over the body, their 
capacity for phagocytosis, their ability to 
secrete enzymes and antibodies, and their 
retention of various mesenchymal potencies 
to develop into other blood and connective 
tissue cells. Their functions in immunity 
are, to a large extent, and may be entirely 
an accentuation of their activities in nor¬ 
mal metabolism. 
Many factors have contributed to the 
present complex and, in part, almost cha¬ 
otic classification of the cells of the blood 
and connective tissue. Chief among these 
are: (1) cells now believed to have identi¬ 
cal functions have long-standing different 
names which date from early histological 
descriptions; (2) different groups of work¬ 
ers, such as anatomists, pathologists and 
hematologists, often use different termi¬ 
nologies; and (3) most important of all, 
there is no consensus of opinion as to the 
identification, classification, developmental 
potencies or even existence of some of the 
important cells. 
The following simplified classification is 
taken largely from the author (1941) with 
special reference to the cells functional in 
malarial immunity. The views regarding 
the developmental potencies and relation¬ 
ships of the cells follow Maximow (1927a, 
1927b) and Bloom (1938a). A more de¬ 
tailed consideration of the cells in relation 
to malaria is given in Taliaferro and Mulli¬ 
gan (1937). 
Predominantly Fixed Connective 
Tissue Cells 
Of the many connective tissue cells which 
are predominantly fixed, the most impor¬ 
tant in defense reactions are macrophages, 
as defined in this paper, fibroblasts and 
endothelial cells. 
(1) Macrophages. The term macro¬ 
phage is used in this chapter in much the 
original sense intended by Metchnikoff to 
denote any large mononuclear cell that is 
phagocytic or can immediately become 
phagocytic without any pronounced change 
in morphology. It includes the following 
four categories of cells: the reticular cells, 
littoral cells and pericytes, which retain 
the embryonic or mesenchymal potency to 
develop into all other types of cells of the 
blood and connective tissue, and the macro¬ 
phages of the loose and dense connective 
tissue, which are believed to have fewer 
potencies for heteroplastic development. 
In the older pathological literature, all 
these cells were frequently termed endo¬ 
thelial cells (see below). The concept that 
cells in the connective tissue of the adult 
body retain mesenchymal potencies is due 
largely to the work of Marchand (1924) 
and Maximow (1927a, 1927b). It is not 
known to what extent they lose their poten¬ 
cies for heteroplastic development when 
they become engorged with foreign ma¬ 
terial. From the viewpoint of immunity 
it is important that all these cells can be 
phagocytic in their fixed position (fixed 
macrophages) or after rounding up and 
becoming free (free macrophages). They 
are, especially as free macrophages, indis¬ 
tinguishable from cells developed from the 
lymphoid free mesenchymal cells (lympho¬ 
cytes or hemocytoblasts). They can all 
proliferate by mitosis and under suitable 
stimuli can become transformed into fibro¬ 
blasts (see below). Both mitosis and trans¬ 
formation into fibroblasts are seen to a 
limited extent after they contain phago- 
cytosed material. In the following brief 
descriptions, the four categories of macro¬ 
phages are arranged in order of their func¬ 
tional importance in malaria. 
1. The reticular cells, together with 
fibers, form the stroma of all reticular 
(myeloid and lymphatic) tissues. It is 
probable that the stroma cells of various 
lymphoid tissues, such as the lamina pro¬ 
pria of the gut, may have identical poten¬ 
cies and functions. 
The reticular cells of the spleen and, to 
a lesser extent, of the bone marrow are of 
paramount importance in immunity to 
malaria. Except under unusual circum¬ 
stances, as in the occlusion of blood vessels, 
local hemorrhage and the like, the reticular 
cells of the lymph nodes or stroma cells of 
the gut are not important. 
