Antigens are substances that can trigger an immune response in the body. They are typically proteins or polysaccharides found on the surface of pathogens like bacteria, viruses, and fungi. Antigens can also be present on the surface of cells, including red blood cells, which helps the immune system distinguish between self and non-self. The characteristics of antigens include their size, shape, and complexity. Larger and more complex antigens tend to provoke a stronger immune response. Additionally, antigens can be classified as exogenous, originating from outside the body, or endogenous, produced within the body, such as in the case of cancer cells.