Immunology: Superantigens Overview

Questions and Answers

Which type of cell is classified as a professional antigen-presenting cell?

Epithelial cells

Endothelial cells

Dendritic cells (correct)

Fibroblasts

What is the role of MHC class I molecules in antigen presentation?

They present peptides to CD4+ T cells.

They activate macrophages directly.

They present lipid antigens to B cells.

They present peptides to CD8+ T cells. (correct)

Which type of antigen-presenting cell requires activation through phagocytosis?

B cells

Macrophages (correct)

Dendritic cells

Epithelial cells

What characterizes nonprofessional antigen-presenting cells?

They can present antigens for limited times during inflammation. Signup and view all the answers

In which pathway are intravesicular pathogens processed?

Vesicular (endocytic) pathway Signup and view all the answers

Which cells present antigens in association with MHC class II molecules?

Professional antigen-presenting cells Signup and view all the answers

What type of cells do CD1 molecules present lipid antigens to?

Clonally diverse T cells Signup and view all the answers

What is the main function of professional antigen-presenting cells?

To present antigens and activate T cells. Signup and view all the answers

What is the role of superantigens in T cell activation?

They polyclonally activate a large fraction of T cells. Signup and view all the answers

Which of the following is an example of a superantigen?

Staphylococcal enterotoxins Signup and view all the answers

What distinguishes endogenous antigens from exogenous antigens?

Endogenous antigens are processed within the cell's cytoplasm. Signup and view all the answers

Which T cell type recognizes antigens presented by MHC class I molecules?

CD8+ T cells Signup and view all the answers

What is a characteristic feature of T-independent antigens?

They exhibit a polymeric structure with repeated antigenic determinants. Signup and view all the answers

What is the main function of antigen processing?

To digest proteins into peptides for MHC display. Signup and view all the answers

Which statement accurately describes the size of antigenic determinants recognized by B cells?

They typically consist of 4-8 residues. Signup and view all the answers

How do CD4+ T cells recognize antigens?

In the context of MHC class II molecules. Signup and view all the answers

What is a potential consequence of the polyclonal activation of B cells by T-independent antigens?

B cells can activate clones specific to other antigens. Signup and view all the answers

Which molecules serve as co-receptors for T cells?

CD4 and CD8 molecules Signup and view all the answers

How are T-dependent antigens primarily characterized?

They have few copies of many different antigenic determinants. Signup and view all the answers

What is a consequence of the hyperactivation of the immune system due to superantigens?

Excess release of cytokines by T cells. Signup and view all the answers

How do superantigens differ from conventional T-dependent antigens?

They activate nearly all T cells in the population. Signup and view all the answers

What is a primary requirement for T cells to recognize processed antigenic determinants?

MHC presentation of processed antigens. Signup and view all the answers

Which of the following compositions do T cell-recognized antigenic determinants mainly have?

Proteins and some lipids. Signup and view all the answers

In terms of antigen processing, what is essential for epitope presentation to T cells?

Loading of processed peptides onto MHC molecules. Signup and view all the answers

Study Notes

Superantigens

Superantigens can activate up to 25% of T cells, significantly more than conventional antigens.
Examples:
- Staphylococcal enterotoxins: Cause food poisoning.
- Staphylococcal toxic shock toxin: Linked to toxic shock syndrome.
- Staphylococcal exfoliating toxins: Responsible for scalded skin syndrome.
- Streptococcal pyrogenic exotoxins: Associated with shock.
Diseases caused by superantigens result from hyperactivation of the immune system and cytokine release by T cells.

Antigen Processing and Presentation

Antigen Processing: Digests proteins into peptides for display with MHC class I or II molecules on cell membranes.
Endogenous Antigens: Processed within the cell's cytoplasm.
Exogenous Antigens: Internalized via phagocytosis or endocytosis and processed in the endocytic pathway.
T cells recognize antigens as short peptides presented by MHC class-I (for CD8+ T cells) or MHC class-II (for CD4+ T cells).

T Cell Recognition

CD4+ T cells interact with MHC class-II; CD8+ T cells interact with MHC class-I, determined by co-receptors.
CD4 molecule binds to the β2 domain of MHC class-II.

T-independent and T-dependent Antigens

T-independent Antigens:
- Directly stimulate B cells, commonly polysaccharides.
- Characterized by repeated polymeric structures.
- Induce polyclonal activation of B cells and are resistant to degradation.
T-dependent Antigens:
- Require T cell assistance for antibody production, typically proteins.
- Feature a few copies of numerous antigenic determinants.

Antigenic Determinants

Recognized by B cells:
- Composed of proteins, polysaccharides, nucleic acids with both linear and conformational determinants.
- Typically sized 4-8 residues, with a focus on immunodominant epitopes.
Recognized by T cells:
- Mainly proteins (some lipids), processed and presented via MHC.
- Typically sized 8-15 residues and limited to those that can bind to MHC.

Antigen-Presenting Cells (APCs)

Only cells presenting peptides with MHC class II to CD4 T cells are classified as APCs; those presenting MHC class I are target cells.
Types of APCs:
- Professional APCs:
  - Dendritic cells: Most effective APCs, high MHC class II and co-stimulatory molecule expression.
  - Macrophages: Require activation via phagocytosis for MHC class II expression.
  - B cells: Constitutively express MHC class II but need activation for co-stimulatory expression.
- Nonprofessional APCs: Can act as APCs temporarily during inflammation (e.g., epithelial and endothelial cells).

Pathways of Antigen Processing

Two distinct processing pathways:
- Cytosolic (Endogenous) Pathway: For pathogens replicating in the cytosol, presenting to CD8+ T cells via MHC class I.
- Vesicular (Endocytic) Pathway: For intravesicular pathogens, presenting to CD4+ T cells via MHC class II.

CD1 Molecules

Group 1 CD1 molecules present lipid antigens to a diverse range of T cells, aiding adaptive immunity against various microbial lipid antigens.

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Description

Explore the fascinating world of superantigens and their role in T cell activation. This quiz covers various examples of superantigens, including those from Staphylococcus and Streptococcus bacteria, and their implications in diseases such as food poisoning and toxic shock syndrome.