Lec 11 MHC and Antigen Presentation

These flashcards cover key concepts related to MHC and antigen presentation, focusing on their structure, function, and interactions relevant for understanding the immune response.

What is the primary function of MHC molecules?

To present processed peptide antigens to T cells.

What types of cells act as professional antigen-presenting cells (APCs)?

Dendritic cells, macrophages, and B cells.

Which chains make up the T cell receptor (TCR) in 95% of T cells?

One alpha chain and one beta chain.

How do T cells differ from B cells in terms of antigen receptors?

T cells do not secrete their antigen receptors, while B cells can differentiate and secrete antibodies.

What is MHC restriction?

A given T cell recognizes a peptide only when presented by a particular self MHC molecule.

Which class of MHC molecules is expressed on nearly all nucleated cells?

MHC Class I.

What is the peptide binding cleft of MHC molecules?

A region where peptide antigens bind, formed by two alpha helices and a beta-pleated sheet.

What process is used to degrade endogenous proteins for MHC Class I presentation?

Proteasomal degradation into short peptides.

What initiates the presentation of exogenous antigens by MHC Class II?

Endocytosis and degradation of proteins within antigen-presenting cells.

What molecule blocks peptide binding in MHC Class II before it encounters antigen?

Invariant chain (li).

What is the function of HLA-DM in MHC Class II processing?

To remove CLIP and allow peptide binding.

What characteristic of MHC genes contributes to their diversity?

MHC genes are polygenic, co-dominant, and polymorphic.

What does the Ubiquitin Proteasome System (UPS) do?

It degrades unneeded or damaged proteins into short peptide fragments.

What kind of antigens do CD8+ T cells recognize?

Peptides presented by MHC Class I molecules.

What is a superantigen?

A type of antigen that can activate a large percentage of T cells by binding directly to TCRs.

Compare the expression and structure of Class I and Class II MHC molecules and their role in antigen presentation.

What are Class I MHC molecules and their primary function?

Class I MHC molecules are expressed on almost all nucleated cells and present endogenous antigens, typically from intracellular pathogens, to CD8+ cytotoxic T cells.

What are Class II MHC molecules and their primary function?

Class II MHC molecules are primarily expressed on professional antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells and present exogenous antigens to CD4+ helper T cells.

How do Class I and Class II MHC molecules differ in terms of structure?

Class I MHC molecules consist of a single alpha chain and a beta-2 microglobulin, while Class II MHC molecules have two alpha and two beta chains, creating a larger binding groove.

What is the role of Class I MHC in antigen presentation?

Class I MHC molecules present intracellular antigens to CD8+ T cells, which can lead to the recognition and killing of infected or malignant cells.

What is the role of Class II MHC in antigen presentation?

Class II MHC molecules present extracellular antigens to CD4+ T cells, fostering the activation and differentiation of helper T cells, which then aid in orchestrating the immune response.

What is the antigen processing pathway for Class I MHC molecules?

The Class I MHC antigen processing pathway involves the degradation of intracellular proteins into peptides by the proteasome, followed by transport of these peptides into the endoplasmic reticulum (ER), where they bind to nascent Class I molecules and are subsequently presented on the cell surface.

What is the antigen processing pathway for Class II MHC molecules?

The Class II MHC antigen processing pathway involves the uptake of extracellular proteins through endocytosis or phagocytosis, which are then degraded in endosomal/lysosomal compartments. The resulting peptides are then loaded onto Class II MHC molecules before being transported to the cell surface for presentation to CD4+ T cells.

How do the antigen processing pathways of Class I and Class II MHC molecules differ?

The Class I pathway primarily processes endogenous antigens (from within the cell), while the Class II pathway processes exogenous antigens (from outside the cell).

What are anchor residues?

Anchor residues are specific amino acids within a peptide that interact with the binding groove of MHC molecules, stabilizing the peptide-MHC complex and determining which peptides can be effectively presented by MHC class I and class II molecules.

What are superantigens?

Superantigens are a class of antigens that can activate a large number of T cells by binding directly to the T cell receptor (TCR) and major histocompatibility complex (MHC) class II molecules simultaneously, bypassing the normal antigen processing and presenting pathway.

Why do superantigens activate a higher proportion of CD4+ T cells?

Superantigens can activate a higher proportion of CD4+ T cells because they engage the TCR in a non-specific manner, directly binding to T cell receptors (TCRs) and MHC molecules outside the peptide-binding groove, leading to a non-specific, polyclonal T cell activation.