One who treats ailments of the skin is an A dermatologist B cardiologist C neurologist D oculist?

Dermatologists treat ailments of the skin.

5its 4

Hepatitis B and C may be treated using immunoglobulin that was created in the laboratory.

The next letter is "e."

This looks like two series interspered with each other. Let's put some letters in upper-case and others in lower-case BaCbDcEdF... so the next letter would be the lower-case 'e'