4.7 MHC l vs MHC ll

The immune system utilizes antibodies and T cell receptors for defense.
- Antibodies can recognize a diverse range of chemical structures.
- T cell receptors are more specialized, recognizing peptide epitopes derived from partially degraded proteins, but only when these peptides are presented on Major Histocompatibility Complex (MHC) molecules.

Definition: An epitope is the specific part of an antigen recognized by the immune system.
- It is the component of an antigen that elicits an immune response.

MHC molecules are glycoproteins located on the surface of nearly all cells, except for red blood cells.
These molecules are unique to each individual, allowing the immune system to distinguish between self and non-self cells.
- Function: MHC molecules bind to peptide fragments and display them on the cell surface.
- These fragments can originate from self-proteins or from pathogens that have invaded the cell.
The T cell's ability to discriminate between self and non-self proteins is crucial for immune function.

Analogy: MHC molecules function similarly to billboards, presenting signs to T cells about the health status of the presenting cell.
- They can display:
- Self-antigens: indicating that everything is normal.
- Pathogenic antigens: indicating that the cell is infected and requires action from T cells.

Found on the surface of all nucleated cells, absent in red blood cells (which lack a nucleus).
Presents antigens to cytotoxic T cells (CD8+ T cells).
Characteristics:
- Presents short peptide fragments, usually 8 to 10 amino acids in length.
- Peptides may include self-proteins or viral proteins from infected cells.
Activation Process:
- If a cytotoxic T cell recognizes the peptide as foreign, it becomes activated.
- Activated cytotoxic T cells can then eliminate infected or abnormal cells by releasing granzymes and perforins, which perforate the cell membrane and trigger lysis.

Primarily located on Specialized Antigen Presenting Cells (APCs) such as macrophages, dendritic cells, B cells, and some activated T cells.
Presents antigens to helper T cells (CD4+ T cells).
Characteristics:
- Presents longer peptide fragments, ranging from 13 to 25 amino acids.
- These originate from exogenous proteins—antigens engulfed, processed, and degraded by APCs.
Antigen Presentation Process:
- The process involves phagocytosis, where APCs consume pathogens, degrade them, and display fragments on MHC II.
- If a helper T cell recognizes a foreign antigen in an MHC II molecule, it will activate.

When activated, cytotoxic T cells:
- Clone themselves into additional activated cytotoxic T cells and memory cells.
- Release perforins and granzymes to induce cell death in infected cells.

When activated, helper T cells:
- Clone into activated helper T cell populations and memory helper T cells.
- Produce cytokines that activate B cells and cytotoxic T cells.

Activated B cells can produce antibodies in response to antigens presented by activated helper T cells.
The interaction between helper T cells and B cells is essential for adaptive immunity, leading toward specific defense mechanisms against foreign antigens.

In summary, MHC molecules play a critical role in immune surveillance by presenting peptide antigens to T cells, distinguishing between self and non-self proteins, and initiating appropriate immune responses.
Cytotoxic T cells kill infected cells directly, while helper T cells activate B cells and cytotoxic T cells, creating a robust and adaptive immune response against pathogens.
The collaboration between various immunological components ensures effective defense against a myriad of infections, showcasing the intricate dynamics of the immune system.