Sometimes substances such as penicillin, peanut butter, bacteria, eggs, and other items will provoke an immune response. They have that will mount a response from the immune system.

antigen  

Explanation:

Antigen -  

They are referred to as some foreign bodies or toxins , which helps to generate immunity for the body , like the production of antibodies , is referred to as antigens .  

They helps to the body to fight against some external bodies , and thereby , strengthen the immunity system of the body .  

Hence , from the given information of the question ,  

The correct answer is antigen.  

Allergens

Explanation:

The immune system of the body helps to fight against the harmful pathogens and protect our body. The different immune cells present in the body are macrophages, monocytes, T cells and B cells.

The immune reaction might occur in the body due to the encounter of the antigen and antibody. Antigen may be defined as any substance that are treated as foreign substance for the body and antibodies are produced in the body to kill that antigen. The peanut butter, eggs sometimes contain the substance that might acts as allergen and generate immune response. The allergen substance evokes the immune reaction in the body.

Thus, the answer is allergen.