Final answer:
B-lymphocytes and T-lymphocytes respond to the adaptive immune challenge via MHC I and MHC II molecules. MHC I presents peptides to CD8+ T cells, while MHC II is involved in activating T helper cells through APCs, including B-cells that can present directly bound antigens.
Step-by-step explanation:
Response of B-lymphocytes and T-lymphocytes to MHC Molecules
The response of both B-lymphocytes and T-lymphocytes to the presence of MHC I and MHC II molecules is a crucial aspect of the adaptive immune response.
MHC I molecules are expressed on all nucleated cells and present both normal and abnormal peptides to CD8+ T cells.
Effector T cells, such as cytotoxic T cells, recognize these complexes and can induce apoptosis in infected cells, playing a critical role in cellular immunity.
MHC II molecules, on the other hand, are exclusively found on professional Antigen Presenting Cells (APCs), such as macrophages, dendritic cells, and B cells.
These molecules present processed nonself antigens for the activation of T cells.
Upon encountering an antigen, B cells can internalize it through their B cell receptor (BCR), process it, and present peptide fragments via MHC II molecules.
Helper T cells recognize these MHC II-antigen complexes, resulting in B-cell activation, which subsequently leads to the production of memory B cells and plasma cells that secrete antibodies.
Therefore, B cells also function as APCs but differ from T cells, as they can bind intact antigens without processing and present them to T helper cells.
This direct recognition is possible because B cells produce antibodies that must be able to identify the intact pathogen.
In some cases, like with bacterial carbohydrates and lipids, B cells can be activated without T-cell involvement.