When microorganisms enter the body, the immune system responds by producing which of the following?

Antigens are substances that provoke an immune response, typically found on the surface of pathogens. While they are crucial for the immune system's recognition of invaders, they are not produced by the immune system; rather, they are components of the microorganisms themselves that stimulate the production of antibodies.

Hormones are chemical messengers produced by glands in the body that regulate various physiological processes, including growth, metabolism, and immune responses. However, they are not directly involved in the immune response to microorganisms; thus, they do not serve as a product of the immune system in this context.

Infection refers to the invasion and multiplication of pathogens in the body, which triggers an immune response. However, it is not a product of the immune system but rather the condition that necessitates the immune system’s action.

Thrombocytes, or platelets, are cell fragments involved in blood clotting rather than immune responses. They do not play a role in the production of antibodies or the direct fight against microbial infections.