I wanted to know where and how does eczema start?

Eczema: There are many different kinds of eczema and dermatitis. 1. Contact dermatitis--an allergy, such as poison ivy. Typically, there are red bumps or blisters arranged in a line or in the areas in which contact with the substance is made. 2. Atopic dermatitis--there is scale, itching and redness that most commonly affected the front of the elbow and the back of the knees. 3. Seborrheic dermatitis.