Conduction of heart beat

Explanation:

The cardiac conduction system are specialized cardiac muscle cell that are located in the walls of the heart and sends signals to the heart muscle. These signals cause the heart to contract. The main components of this system are AV node, bundle of His, SA node, bundle branches, and the Purkinje fibers.

Antigens cause the production of antibodies, and antibodies attack antigens.

Explanation:

Antigen is any foreign molecule that are recognized by the immune system and have ability to stimulate the antibodies production. The antibodies are the cells of immune system that serve to target and kill the antigens to prevent any spread of infection.  Hence, antigens cause the production of antibodies, and antibodies attack antigens.