Which chemical involved in immune response is secreted by T lymphocytes?

Cytokines are the correct choice as they are specific signaling molecules secreted by T lymphocytes during an immune response. These proteins play a vital role in cell signaling and can influence the behavior of other immune cells, promoting communication between them. T lymphocytes, particularly helper T cells, release cytokines to activate and direct other immune cells, thereby coordinating the body's response to pathogens.

Immunoglobulins and antibodies are terms often used interchangeably, but they refer specifically to the components that B cells produce in response to an antigen. Histamine is primarily related to allergic responses and is released by mast cells and basophils, not T lymphocytes. Thus, while all these substances are important in the immune system, the specific role of T lymphocytes in secreting signaling molecules is fulfilled by cytokines.