Antigens, Antigen Processing, and Presentation- Thompkins

Question 1

define antigen

Answer 1

antigen: any substance that binds specifically to an antigen receptor (either BCR, TCR, or antibody); can be protein, carbohydrate, or other polymer

Question 2

define immunogen

Answer 2

an antigen that can induce an immune response from a B or T cell

Question 3

define haptogen

Answer 3

an antigen that cannot induce an immune response ALONE but when attached to a more complex molecule (carrier) is immunogenic

Question 4

give an example of a haptogen

Answer 4

penicillin and other drugs can be haptogens; they form complexes in the host to elicit an immune response and future actions

Question 5

define epitope; give two other names

Answer 5

the part of an antigen that is recognized by BCRs, TCRs, or antibodies; also called determinant or antigenic determinant

Question 6

describe the relation between antigen receptors and epitopes

Answer 6

different antigen receptors can recognize the same antigen but different epitopes on that one antigen! B cells and T cell have distinct, different requirements for antigen recognition

Question 7

understand properties that make a molecule antigenic (5)

Answer 7

1. complex
2. organic
3. degradable
4. large
5. foreign (but context is important!)

Question 8

why is context important when considering foreign as a good quality to make a molecule antigenic?

Answer 8

bovine serum albumin and ovalbumin can be great immunogens BUT ovalbumin will not elicit an immune response in chickens (comes from chicken) and BSA will not elicit a response in cattle

Question 9

what kind of epitopes can B cells recognize? (3)

Answer 9

1. linear/continuous
2. nonlinear/conformational
3. neo-epitopes (require modification before recognition)

Question 10

what kind of epitopes can T cells recognize?

Answer 10

LINEAR/CONTINUOUS ONLY

Question 11

understand how APCs relate to MHC

Answer 11

MHC class I is expressed on all nucleated cells, while MHC II is only on professional APCs (DCs, macrophages, B cells); MHC I’s peptide source is intracellular and presents to CD8 T cells and MHC II’s is extracellular and presents to CD4

Question 12

understand how APCs process and present antigen to T cells

Answer 12

all nucleated cells can utilize MHC I expression to present antigen to T cells
the professional APCs: all are capable of normal MHC I processing and presentation
macrophages: express MHC II in low levels in response to PAMPs, DAMPs, and cytokines, present extracellular antigens via MHC II, are in the residents in lymphoid and connective tissue and body cavities
DCs: always express MHC II, present both intracellular (MHC I) and extracellular (MHC II) antigens, located in lymphoid, epithelial, connective, and peripheral tissues
B cells: always express MHC II but only specifically for their epitopes; extracellular antigen binds to antigen-specific Ig receptors and is presented via MHC II; found in lymphoid tissue and blood

Question 13

understand how APCs process and present antigens to T cells and the role of MHC I in the process

Answer 13

1. proteasomes in the cell are garbage disposals and ensure that any cytosolic proteins (foreign/unwanted) are degraded into peptides
2. these peptides are transported by transporter associated with antigen processing (TAP) from cytoplasm to ER and loaded onto MHC I proteins
3. MHC I is transported to cell surface to present to CD8+ cells
   MHC can present peptides from self or nonself cutosolic proteins as the proteasome, MHC I, and TAP don’t discriminate!

Question 14

understand how APCs process and present antigens to T cells and the role of MHC II in the process

Answer 14

1. extracellular proteins are endocytosed by professional APCs, whose endosomes fuse with lysosomes and degrade the proteins within
2. MHC II proteins are folded in the ER, but do not load peptide
3. the invariant chain(li) blocks the binding groove and targets class II to endosomal/lysosomal compartments so class I can’t load into class II
4. in the endosome the invariant chain is degraded and peptides produced in the endosome are loaded into MHC II
5. MHC class II is transported to the cell surface to present antigens to CD4+ cells

Question 15

know which cells express MHC I and II when T cells are activated

Answer 15

CD4+ cells express MHC II
CD8+ cells express MHC I
once activated!

Question 16

understand basics of MHC restriction, alloreactivity, and superantigens (all intertwined)

Answer 16

MHC restriction refers to the fact that TCRs can only recognize their antigen in the context of self-MHC so MHC restricts/determines the antigens that T cells will get to see and respond to;
alloreactivity refers to how foreign MHC can still present peptides and potentially activate T cells, and thanks to MHC polymorphism, the MHC may contain non-self peptides, leading to indirect alloreactivity as the T cells may not have seen those non-self peptides and will react to self as if it is foreign;
superantigens are toxins and viral proteins that cause excessive activation of the immune system and bind outside the peptide-binding groove and cause nonspecific activation of large numbers of T cells resulting in polyclonal T cell activation and a massive cytokine release