Questions and Answers
Which type of cell is classified as a professional antigen-presenting cell?
What is the role of MHC class I molecules in antigen presentation?
Which type of antigen-presenting cell requires activation through phagocytosis?
What characterizes nonprofessional antigen-presenting cells?
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In which pathway are intravesicular pathogens processed?
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Which cells present antigens in association with MHC class II molecules?
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What type of cells do CD1 molecules present lipid antigens to?
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What is the main function of professional antigen-presenting cells?
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What is the role of superantigens in T cell activation?
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Which of the following is an example of a superantigen?
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What distinguishes endogenous antigens from exogenous antigens?
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Which T cell type recognizes antigens presented by MHC class I molecules?
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What is a characteristic feature of T-independent antigens?
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What is the main function of antigen processing?
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Which statement accurately describes the size of antigenic determinants recognized by B cells?
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How do CD4+ T cells recognize antigens?
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What is a potential consequence of the polyclonal activation of B cells by T-independent antigens?
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Which molecules serve as co-receptors for T cells?
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How are T-dependent antigens primarily characterized?
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What is a consequence of the hyperactivation of the immune system due to superantigens?
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How do superantigens differ from conventional T-dependent antigens?
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What is a primary requirement for T cells to recognize processed antigenic determinants?
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Which of the following compositions do T cell-recognized antigenic determinants mainly have?
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In terms of antigen processing, what is essential for epitope presentation to T cells?
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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
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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.
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T-dependent Antigens:
- Require T cell assistance for antibody production, typically proteins.
- Feature a few copies of numerous antigenic determinants.
Antigenic Determinants
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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.
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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).
- Professional APCs:
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.