Answer
The Heart.
Explanation
Bone marrow produces B lymphocytes.
Lymphocytes are a type of white blood cell involved in the production of antibodies (protein markers that can cause pathogens to cling together).
The skin has several layers which act as a barrier against the entry of pathogens.
The lymph system is a network of tissues and organs that consists of lymph vessels. It’s primary function is to transport lymph, a clear, colourless fluid containing white blood cells that helps rid the body of toxins.
