A student is trying to understand cellular immune responses. She observes that one cell receptor, after binding to an antigen on a virus-infected cell membrane MHC complex, immediately causes the breakdown of the infected cell. The breakdown is most likely the direct result of action by a(n)

Tc cell also called cytotoxic T cell.

Explanation:

The cytotoxic T cells are a kind of white blood cell that destroys are infected or damaged, or cancer cells. Tc cells recognize specific antigens. When a cell with antigens is brought to a Tc cell, the infected cell will expose in its surface the antigen by binding it to the MHC I of the cell. Once this antigen and MHC I are exposed, the T cell receptor will recognize it, if it is the specific one for that antigen and destroy the infected cell.        

T cell

Explanation:

A T cell can be defined as the type of cell which are vital and essential to the human body because it carried out or play a major role in the immune system due to the fact that they are one of the main components of an adaptive system which are essential  in helping to host  an immune response against any kind of pathogens such as viruses and bacteria. They also help to destroy several virus infected and tumor cell thereby causing their breakdown which is why T cell plays a major and important because they tend to always  act as a defence against those pathogens. It also help to destroy tissue grafts that exist in the cytosol.

Therefore according to the student understanding and observation the breakdown is most likely the direct result of action by a(n) T cell.

proteins is the answer