Which statement describes many of the structures involved in the first line of defense of the immune system? they have contact with the external environment. they are internal and contact the pathogen through blood. they produce b-cells in response to a pathogen. they produce t-cells in response to a pathogen.

The correct answer would be They have contact with the external environment.  

The first line of defense refers to the outside defense system which consists of physical and chemical barriers.

They have contact with the external environment and help in preventing the entry of the infectious agent in the body.

They include skin, mucous membranes and their secretions such as tears, stomach acid, urine flow, et cetera.

the tricuspid valve is present between right atrium and right ventricle