9- Dendritic Cells & Antigen Processing Flashcards
What are the three major APCs?
1) Dendritic cells (DCs)
2) Macrophages
3) B-cells
Of the 3 major APCs, which can present a pathogen to naïve T cells?
Dendritic cells (primary immune response)
How do APCs capture and process antigens?
Capture foreign microbes or their products and process large proteins by breaking them into peptides and presenting on their surfaces attached to specialized antigen-presenting structures, called Major Histocompatibility Complex (MHCs)
Once antigen is broken down and loaded onto MHC on the surface of an APC, how do T cells bind?
T cells scan the surface of APCs for their specific antigen, then bind through their TCRs (T Cell Receptor)
Why are DCs the only cell type to present antigen to naïve T cells?
DC have adherence molecules that allow the antigen to remain attached to the DC longer for the naïve T cells to recognize the antigen and mount a primary immune response
Which two types of APCs present antigen to memory TH cells and are important in secondary immune response?
B cells and macrophages
Where are Dcs primarily present in the body?
Epithelial tissues (skin.mucosa) and lymphoid organs (lymph nodes, spleen, thymus)
What are the major functions of DCs?
1) Serve as sentinel cells- activate innate defenses
2) Process exogenous antigens- initiate adaptive immune system
3) Regulate adaptive immunity
Explain the differences between immature and mature DCs
Immature: Antigen uptake/processing
- Low surface MCH II
- high intracellular MHC II (chopping antigen & loading it to MHC before moving it to the surface of mature DC)
- high FcR
Mature: antigen presentation
- high surface MHC
- low FcR
How are follicular DCs different from other DCs?
- they do not migrate (stay in lymph nodes)
- are located in lymphoid follicles (B cell area)
- lack MHC II molecules on their surface
- carry many complement and Fc receptors
- do not process antigens
- primary function is to present antigen to B-cells
What are iccosomes? What is their role?
Iccosomes are spherical bodies made up of immune complexes on the ‘tentacles’ of follicular dendritic cells
- iccosomes break off from the dendrites and subsequently attach to B-cells
- iccosomes are ingested by activated B-cells with BCRs specific for the antigen
- the antigen is processed, and the B-cell presents the antigen on MHC II molecules to activated T-helper cells
How are antigens detected in extracellular fluid?
In the epithelium, DCs are constantly taking in extracellular fluid by a process called macropinocytosis to sample for signs of pathogens and their products
If DCs pickup antigens at the site of infection, where do they take the antigens?
DC take antigens to an environment that is full of immune cells
- activated DCs stop phagocytosis, move into the interstitial space, and are carried by lymph flow to the nearest lymph node (stimulated by inflammatory cytokine TNF-alpha)
When DCs stimulate T-helper cells, they provide three signals. What are they?
1) T cell antigen receptors bind antigen fragments attached to MHC molecules
2) Co-stimulatory molecules like CD40 and CD80/86
3) Provided by cytokine secreted by DCs in response to microbial stimulus (tells T cells what type of antigen/pathogen they are dealing with)
