Final Answer:
While T helper (Th) cells are MHC class II restricted and MHC presents denatured peptides, the recognition of tertiary structure by immunoglobulins (Igs) such as IgG is not hindered. This is because B cells endocytose BCR-bound antigens, and the presentation of these antigens on class II MHC to Th cells occurs after processing, enabling the recognition of tertiary structures beyond the limitations of MHC presentation.
Step-by-step explanation:
B cells play a crucial role in the immune response by presenting antigens to T helper (Th) cells for recognition. While it's true that MHC class II presentation involves denatured peptides (up to 30 residues), the process of endocytosis and antigen processing by B cells allows for the recognition of tertiary structures by Th cells.
The B cell internalizes antigens bound to its B-cell receptor (BCR), and within endosomes, these antigens are broken down into smaller peptides. These peptides, including portions of the tertiary structure, are then presented on the MHC class II molecules to Th cells. This mechanism ensures that Th cells can recognize and interact with a broader range of antigenic structures, facilitating the activation of B cells and subsequent isotype switching.
Additionally, the activation of B cells by previously activated Th cells is a critical step in the immune response. Once a Th cell recognizes the presented antigen on the B cell's MHC class II molecules, it provides signals for B cell activation and differentiation. This interaction leads to the production of antibodies with different isotypes, such as IgG, allowing the adaptive immune system to mount a more effective and diverse response against pathogens.
In summary, the collaboration between B cells and Th cells, along with the antigen processing capabilities of B cells, enables the recognition of tertiary structures and the subsequent production of various immunoglobulin isotypes.