Morphometry and expression of immunoglobulins-containing plasma cells in the Harderian glands of Birds

Johann Jacob Harder first described the Harderian gland in 1694 in deer. It is found in most terrestrial animals and is located within the variable aspects of the orbit. It is believed that this gland is involved in diverse functions. Among these, it has been held to be a site of immune response, a source of thermoregulatory lipids and pheromones, act as photoprotective organ as well as part of a retinal-pineal axis. In birds, this glad was reported first in sparrow in 1918. The Harderian gland is covered by capsule and the connective tissue septa that divide the gland into numerous unequal-sized lobes and lobules. Plasma cells are found in the interacinar space and the lumina of lobules. The recent studies suggest that the Harderian gland act as an immunopotent organ in birds, and that the gland in scavenging birds contains more immunoglobulin-containing plasma cells due to their scavenging nature. Moreover, this gland shows considerable species/strain differences in terms of macro anatomy, microanatomy as well as in the dynamics of immunoglobulin-containing plasma cells among different birds. In this review, these species and strain differences are discussed based on recent studies and several goals of future research are identified.