Question

If someone was unable to make MHC-2 proteins, how would that affect their adaptive immune response?

Answer 1

MHC: A major histocompatibility complex is a group of genes.

• This group of genes code for a specific protein on the cell surface.
• These proteins help the immune system recognize foreign substances.
• This complex is also called a human leukocyte antigen system (HLA).

There are two major types of MHC protein molecules class 1 and class 2.

MHC class I is present in the membrane of every cell of the body.

• MHC class I glycoproteins present endogenous antigens that originate from the cytoplasm.

MHC II is restricted to the cells of the immune system called macrophages and lymphocytes.

• MHC II proteins present exogenous antigens that originate extracellularly from foreign bodies such as bacteria.

The adaptive immune system, also called acquired immunity, uses specific antigens to strategically mount an immune response.

• The adaptive immunity is activated by exposure to pathogens and uses an immunological memory to learn about the threat and enhance the immune response accordingly.
• It mostly relies on B cells and T cell lymphocytes.
• It basically relies on distinguishing between host antigen present inside the host body and foreign antigens present on various pathogens.
• First, the pathogens enter the body, and Macrophages a type of WBC are activated.
• Macrophages are specialized cells involved in the detection, phagocytosis, and destruction of bacteria and other harmful organisms
• When a macrophage engulfs a microorganism, it partially digests it and displays peptide fragments of the microbe on its surface, bound to MHC molecules.
• The T-lymphocyte recognizes the foreign fragment attached to the MHC molecule and binds to it, stimulating an immune response by releasing cytokines.
• The immune response is triggered by recognizing foreign antigens.
• In uninfected healthy cells, the MHC molecule presents peptides from its own cell (self-peptides), to which T cells do not normally react.
• MHC class II molecules are heterodimeric glycoproteins composed of α and β chain.
• It also has an antigen-binding groove where the antigen or the peptide binds.
• The antigen-binding groove of MHC class II molecules is open at both ends, therefore, can bind longer peptides than class 1 MHC.
• the corresponding groove on class I molecules is closed at each end.
• having MHC class II molecules present proper peptides that are bound stably is essential for overall immune function.

Thus the inability to make MHC class 2 molecules will severely affect the acquired immunity which highly specific to a particular pathogen and specifically the cell-mediated immunity which relies on MHC class 2 proteins.

One such example is a deficiency of MHC class 2 proteins, here the body produces deficient MHC class 2 molecules.

The deficiency of MHC class 2 molecules is called Bare Lymphocyte Syndrome.

• Deficient MHC class II molecules are unable to present antigens to T cells and properly activate T cells.
• T cells are then unable to proliferate and secrete cytokines.
• The deficient MHC class II molecules affect the activation and proliferation of T cells.
• I also affect the rest of the immune response cascade which includes B cells.
• due to a decrease in the number of T cells, the T cells cannot interact and activate the B cells.
• Normally when B cells are activated they divide, proliferate, and differentiate, which includes the differentiation of these cells into plasma cells that are responsible for producing antibodies.
• But due to the inactivation of B cells there are deficient plasma antibodies, which are unable to perform and act against the foreign cells.