What are substances that stimulate the production of antibodies in the blood called?

Substances that stimulate the production of antibodies in the blood are known as antigens. Antigens are typically foreign molecules such as proteins, polysaccharides, or glycoproteins found on pathogens like bacteria and viruses. When these substances enter the body, the immune system recognizes them as threats and initiates an immune response by producing antibodies specifically designed to neutralize or eliminate the antigens.

In this context, pathogens refer to the disease-causing organisms themselves, while erythrocytes are red blood cells involved in oxygen transport, and enzymes are biological catalysts that speed up chemical reactions in the body. None of these options directly relate to the stimulation of antibody production in the immune system as antigens do, making antigens the correct term for substances that trigger this crucial immune response.