Final answer:
Antigens can be processed in two ways depending on their molecular class and complexity. Intracellular antigens are processed in the cytosol by a complex known as the proteasome, while protein antigens require antigen-presenting cells to activate B cells.
Step-by-step explanation:
Antigens can be processed in two ways depending on their molecular class and complexity. Intracellular antigens, which are typical of viruses and certain intracellular parasites and bacteria, are processed in the cytosol by an enzyme complex called the proteasome.
These antigens are then brought into the endoplasmic reticulum by the transporter associated with antigen processing (TAP) system, where they interact with MHC class I molecules and are eventually transported to the cell surface for presentation. On the other hand, protein antigens require antigen-presenting cells (APCs) to function as intermediaries. The B cells, a type of APC, present protein epitopes with MHC class II molecules to helper T cells, resulting in the activation of B cells as part of the immune response.