Why does poison ivy or oak itch?

Poision ivy has an oil on its leaves that when they come in contact with our skin, it makes a chemical burn or rash. Poision oak has a pollen that when magnified, looks like a porcupine and when it makes contact with our skin, the skin will expand around the pollen and make a small liquid filled environment to contain the pollen. Both of these are the plants natural defenses. The poison oak is more contageous if it has little clusters of white berries on it. Also because G-D said so! it should also get bubbly and gross. the bubbles might look like Pizza. speaking of pizza i love pizza. I wonder if poison ivy like pizza.

This is completely wrong. First of all it is POISON ivy not poision. Poison Ivy and oak both contain urishiol oil that binds to the proteins on the keratin layer of our skin. While the oil is completely harmless; our immune systems become confused resulting in 90% of humans responding with an allergic reaction. Patrolling T cells recognize it as an allergen and release lymph fluid over the affected skin cells in an attempt to rid the skin of the oil. In the process many skin cells are killed, affecting nerve cells and causing us to itch in response. It is not a defense mechanism (how would a delayed allergic reaction - sometimes 3 to 5 days after initial contact - in any way defend the plant?), and actually the only part of poison ivy that doesn't contain the oil, is the pollen.