Final answer:
Antigens are substances that trigger an immune response and come in various molecular forms. Proteins are potent antigens that can provoke both humoral and cellular responses. Antigen presentation can occur via MHC molecules or as free antigens, and the molecular class of the antigen determines whether it is T cell-dependent or independent.
Step-by-step explanation:
In biology, antigens are substances that can induce an immune response in the body. These antigens can belong to various molecular classes such as carbohydrates, lipids, nucleic acids, and proteins. Antigens vary in their capacity to stimulate the adaptive immune defenses and in the types of response they stimulate, which are categorized as humoral or cellular.
The structural complexity of an antigen is a critical factor in determining its antigenic potential. Proteins, with their three-dimensional complexity, are highly effective antigens that can evoke both humoral and cellular immune responses. On the other hand, carbohydrates are simpler and tend to only stimulate humoral responses, while lipids and nucleic acids are typically the least antigenic. However, when these less antigenic molecules combine with proteins or carbohydrates, they can form complexes such as glycolipids or nucleoproteins that are more likely to provoke an immune response.
Antigens can be presented to the immune system in various forms such as MHC I molecules, MHC II molecules, or as free antigens. Additionally, haptens are small molecules that may only become antigenic when attached to a larger carrier. The molecular class of the antigen also influences immune cell activation. For instance, protein antigens are T cell-dependent as they require processing and presentation by an Antigen-Presenting Cell (APC) to activate B cells. Conversely, antigens like polysaccharides are T cell-independent, capable of activating B cells without the need for antigen processing and T cell help.