Poison ivy
Toxicodendron radicans (syn. Rhus toxicodendron, Rhus radicans; Poison ivy) is a plant in the family Anacardiaceae. The name is sometimes spelled "Poison-ivy" in an attempt to indicate that the plant is not a true Ivy (Hedera). It is a woody vine that is well known for its ability to produce urushiol, a skin irritant that causes an itching rash for most people, technically known as urushiol-induced contact dermatitis.

# Taxonomy
Poison-ivy is subject to frequent taxonomic reclassification and confusion; it is currently divided into eastern and western species in the genus Toxicodendron. At least six distinct subspecies of Toxicodendron radicans are recognized. Complicating identification and taxonomy are the fact that the species (even a particular subspecies) can be highly variable in growth habit and leaf appearance

# Habitat and range
It grows throughout much of North America, including all Canadian provinces except Newfoundland (but not the Territories) and all U.S. states except Alaska, Hawaii, and California, and is normally found in wooded areas, especially along edge areas. It also grows in exposed rocky areas and in open fields and disturbed areas.  It also grows as a forest understory plant, although it is only somewhat shade tolerant. The plant is extremely common in suburban and exurban areas of New England, the Middle Atlantic and Southeastern United States. It rarely grows at altitudes above 1,500 meters (5,000 ft), although the altitude limit varies in different locations. The plants can grow as a shrub up to about 1.2 meters (4 ft) tall, as a groundcover 1025 centimeters (410 in) high, or as a climbing vine on various supports. Older vines on substantial supports send out lateral branches that may at first be mistaken for tree limbs.
It is not particularly sensitive to soil moisture, although it does not grow in desert or arid conditions.  It grows in a wide variety of soil types, and soil pH from 6.0 (acidic) to 7.9 (moderately alkaline).  It can grow in areas subject to seasonal flooding or brackish water.
It is more common now than when Europeans first entered North America.  Real estate development adjacent to wild, undeveloped land has engendered "edge effects," enabling poison ivy to form vast, lush colonies in such places. It is listed as a noxious weed in the U.S. states of Minnesota and Michigan.

# Characteristic appearance
The leaves are ternate with three almond-shaped leaflets. The berries (actually drupes) are a grayish-white color and are a favorite winter food of some birds. Hence the mnemonic,
"Leaves of three, let it be; berries white, danger in sight."
Another version is:
"Leaves of three let it be; hairy vine, no friend of mine."
The color ranges from light green (usually the younger leaves) to dark green (mature leaves), turning bright red in fall. The leaflets of mature leaves are somewhat shiny.The leaflets are 3-12 cm long, rarely up to 30 cm. Each leaflet has a few or no teeth along its edge, and the leaf surface is smooth. 
Leaflet clusters are alternate on the vine, and the plant has no thorns. These three characteristics: (a) clusters of three leaflets, (b) alternate, and (c) lack of thorns, are sufficient to positively identify the plant. If it is growing up the trunk of a tree, the presence of copious root-hairs will identify it, leading to the "hairy vine, no friend of mine" warning.
Poison ivy spreads both  vegetatively and  sexually.  The vines put down adventitious roots, or the plant can spread from rhizomes or root crowns.  The plant flowers in May to July and produces mature fruits by August to November.  Seeds are  spread mainly by animals, and are viable after passing through the digestive tract of birds.

# Effects on the body
The reaction caused by poison-ivy, urushiol-induced contact dermatitis, is an allergic reaction. Around 15% to 30% of people have no allergic response, but most if not all will become sensitized over time with repeated or more concentrated exposure to urushiol. Note that reactions that worsen over time may progress to anaphylaxis and can therefore be dangerous, even life-threatening.
For those who are affected by urushiol, it causes a very irritating rash. In extreme cases, corticosteroids can be needed to treat rashes and severe itching.  The first symptom of contact is a severe itching of the skin that develops into reddish colored inflammation or non-colored bumps, and then blistering of the skin occurs. In severe cases, clear fluids ooze from open blistered sores. Once the urushiol poison has had contact with the skin, it is quickly bound to the skin.
The oozing fluids released by itching blisters do not spread the poison. The appearance of a spreading rash indicates that some areas received more of the poison and reacted sooner than other areas. The blisters and oozing result from blood vessels that develop gaps and leak fluid through the skin; if the skin is cooled, the vessels constrict and leak less. If poison ivy is burned and the smoke then inhaled, this rash will appear on the lining of the lungs, causing extreme pain and possibly fatal respiratory difficulty. If poison ivy is eaten, the digestive tract, airway, kidneys or other organs can be damaged.
Understanding why new lesions may develop for two weeks (studied on forearm) after one exposure was made clear by a University of Miami scientist: larger amounts have earliest onset and largest reaction, smallest produce a delayed reaction. The overall severity 'progresses' with the combined active lesions. Therefore, the last new lesion should occur at two weeks after last exposure, the total rash (untreated) may go on for 3-4 weeks.
Urushiol oil can remain active for several years, so handling dead leaves or vines can cause a reaction.  In addition, oil transferred from the plant to other objects (such as pet fur) can cause the rash if it comes into contact with the skin.
People who are sensitive to poison-ivy can also experience a similar rash from mangoes.  Mangoes are in the same family (Anacardiaceae) as poison ivy; the sap of the mango tree and skin of mangoes has a chemical compound similar to urushiol. 
Similar reactions have been reported occasionally from contact with the related aromatic sumac or Japanese lacquer tree.

# Confusion with other plants
- Boxelder Maple (Acer negundo) saplings can look almost indistinguishable from poison ivy. While Boxelder Maples often have five or seven leaflets, three leaflets are also common. The two can be differentiated by the fact that Poison-ivy has alternate leaves, while the maple has opposite leaves; in other words, by observing where the leaf stalk (the "branch" the three leaflets are attached to) meets the main branch. Another leaf stalk directly on the opposite side is characteristic of Boxelder Maple. If the three-leaflet leaves alternate along the main branch, it may be Poison-ivy.
- Virginia creeper (Parthenocissus quinquefolia) vines can look like Poison-ivy. The younger leaves can consist of three leaflets but have a few more serrations along the leaf edge, and the leaf surface is somewhat wrinkled. Most Virginia creeper leaves have five leaflets, however. Virginia creeper and poison ivy very often grow together, even on the same tree.
- Western Poison-oak (Toxicodendron diversilobum) leaflets also come in threes on the end of a stem, but each leaflet is shaped somewhat like an oak leaf. Western Poison-oak only grows in the western United States and Canada, although many people will refer to poison ivy as poison-oak. This is because poison ivy will grow in either the ivy-like form or the brushy oak-like form depending on the moisture and brightness of its environment. The ivy form likes shady areas with only a little sun, tends to climb the trunks of trees, and can spread rapidly along the ground.
- Poison Sumac (Toxicodendron vernix) has compound leaves with 715 leaflets. Poison sumac never has only three leaflets.
- Kudzu (Pueraria lobata) is a non-toxic edible vine that scrambles extensively over lower vegetation or grows high into trees. Kudzu is an invasive species in the southern United States. Like poison ivy it has three leaflets, but the leaflets are bigger than those of poison ivy and are pubescent underneath with hairy margins.
- Blackberry and raspberry vines bear a passing resemblance to poison ivy, with which may share territory. The chief difference between blackberry vines and poison ivy is that blackberry vines have spines on them, whereas poison ivy is smooth. Also, the three-leaflet pattern of blackberry vine leaves changes as the plant grows: the two bottom leaves both split into two leaves, for a total of five in a cluster. They have many teeth along the leaf edge, and the top surface of their leaves is very wrinkled where the veins are, and the bottom of the leaves is light minty - greenish white, while poison ivy is all green. The stem and vine of poison ivy are brown and woody, while blackberry stems are green with thorns.
- The thick vines of grape, with no rootlets visible, differ from the vines of poison ivy, which have so many rootlets that the stem going up a tree looks furry.

# Treatment