For either free HIV or HIV associated with cells to infect an
individual during sexual contact, HIV must cross the cells that
form the surface layer of the tissue (the epithelium). A significant
amount of the epithelium in both exposed male and female genital
areas is of the stratified epithelial type where there are multiple
layers of cells. This stratified epithelial layer forms a significant
physical barrier and is not very permeable to particles of the
size of an HIV virus. Although this is the predominant type of
epithelium, women may also have in the cervical region some columnar
epithelium. Columnar epithelial cells are resistant to infection
by free and cell associated HIV but they are more susceptible
to damage that exposes the underlying tissue and cells. The epithelium
present in the rectum however provides very little protection
against physical trauma during intercourse and the underlying
tissue is rich in cells to which HIV can bind.
Free and cell associated HIV may be able to cross these columnar
and stratified epithelial cell layers in a number of ways. These
include:
-
Free HIV directly infecting the epithelial cells
-
Free HIV being transported through the epithelial cell
-
Cell-associated HIV passing between the epithelial cells
-
Free HIV being bound by specialised immune cells in the
epithelial layer
-
Free HIV gaining access to underlying tissue and cells by
the effects of physical damage to the epithelial layer
