A relative afferent pupillary defect (RAPD), also known as a Marcus Gunn pupil, is a medical sign observed during the swinging-flashlight test whereupon the patients pupils dilate when a bright light is swung from the unaffected eye to the affected eye. The affected eye still senses the light and produces pupillary sphincter constriction to some degree, albeit reduced. Depending on severity, different symptoms may appear during the swinging flash light test:
Mild RAPD will presents as a weak pupil constriction initially, after which dilation continues to happen. When RAPD is moderate, pupil size will remain, after which it dilates
When RAPD is severe, the pupil will dilate quickly

Cause
The most common cause of Marcus Gunn pupil is a lesion of the optic nerve (between the retina and the optic chiasm) due to glaucoma, or severe retinal disease, or due to multiple sclerosis. It is named after Scottish ophthalmologist Robert Marcus Gunn. A second common cause of Marcus Gunn pupil is a contralateral optic tract lesion, due to the different contributions of the intact nasal and temporal hemifields. Diagnosis
The Marcus Gunn pupil is a relative afferent pupillary defect indicating a decreased pupillary response to light in the affected eye.In the swinging flashlight test, a light is alternately shone into the left and right eyes. A normal response would be equal constriction of both pupils, regardless of which eye the light is directed at. This indicates an intact direct and consensual pupillary light reflex.
They play an important role in the immune defence against bacteria and parasites. By releasing chemical "alarms" such as histamine, mast cells attract other key players of the immune defense system to areas of the body where they are needed.Mast cells seem to have other roles as well. Because they gather together around wounds, mast cells may play a part in wound healing. For example, the typical itching felt around a healing scab may be caused by histamine released by mast cells. Researchers also think mast cells may have a role in the growth of blood vessels (angiogenesis). No one with too few or no mast cells has been found, which indicates to some scientists we may not be able to survive with too few mast cells.Mast cells express a cell surface receptor, c-kit (CD117), which is the receptor for stem cell factor (scf). In laboratory studies, scf appears to be important for the proliferation of mast cells. Mutations of the gene coding for the c-kit receptor (mutation KIT(D816V)), leading to constitutive signalling through the receptor is found in >90% of patients with systemic mastocytosis. Diagnosis
Diagnosis of urticaria pigmentosa (cutaneous mastocytosis, see above) can often be done by seeing the characteristic lesions that are dark brown and fixed. A small skin sample (biopsy) may help confirm the diagnosis.In case of suspicion of systemic disease the level of serum tryptase in the blood can be of help. If the base level of s-tryptase is elevated, this implies that the mastocytosis can be systemic.