Nanosized Drug Delivery Vectors and the Reticuloendothelial System - Intracellular Delivery: Fundamentals and Applications - page 162

collagenised trabeculae extending deep into the node. Lymph flows from afferent
lymph vessels into the subcapsular sinus where it is delivered into deep cortical
sinuses that contain a network of reticular fibers, macrophages and lymphocytes
that filter pathogens and foreign material from the lymph. Filtered lymph fluid then
drains towards the efferent lymph vessel via medullary sinusoids that contain a high
density of macrophages that represent the final filtration barrier for pathogens and
particulate nanomedicines before lymph is delivered into the systemic circulation.
When examining the lymph node localization of PEGylated PAMAM dendrimers
after subcutaneous administration the greatest lymph node retention of the dose is
in the subcapsular sinus and the medulla (Mounzer et al. 2007 ).

2.4 The Lungs

As the major internal organ exposed to the air, the lungs are densely populated with
macrophages that act to prevent systemic exposure to airborne pathogens and parti-
cles. The airways are constructed such that larger airways (bronchi) branch out and
thin down to smaller bronchioles. The bronchioles supply air to thin alveolar sacs that
are composed of a single layer of alveolar endothelial cells, and a mucosal membrane
that lines the inside of the alveolar sacs to prevent the alveolar membranes sticking to
each other (Fig. 6 ). Lymphatic and blood capillaries lie on the outside of the alveoli
encased by a membranous cover (the pleura) that encloses the whole organ.
There are two distinct populations of macrophages in the lungs that are located
in the alveolar membrane and protrude out into the alveolus and interstitial mac-
rophages that account for approximately half of the macrophages in the lungs.
Macrophages resident in the alveolar mucosa act to phagocytose pathogens and

Alveoli

Capillary bed

Lymphatic
capillary

Lymph node

Pleura

Alveolar
macrophage

Interstitial
macrophage

Fig. 6 Diagram of the alveoli and localisation of macrophages