Which fluid contains white blood cells and drains into the bloodstream?

Lymph is the correct answer because it is a clear fluid that circulates throughout the lymphatic system and contains a variety of immune cells, including white blood cells. These components are essential for the body's defense mechanisms, as they help to identify and fight infections and diseases. Lymphatic vessels collect excess fluid from tissues, which includes waste products and pathogens, and transport it back to the bloodstream. This process is crucial for maintaining fluid balance in the body and supporting the immune system.

The other options do not pertain to this context. Keratin is a structural protein found in skin, hair, and nails, playing a role in protection but not directly involving the drainage of fluids or white blood cells. A nodule is a small, raised mass of tissue, often associated with various health conditions, but it does not function to drain fluid. A fissure refers to a split or crack, usually in the skin or mucous membrane, and does not relate to the immune system or fluid drainage.