Question

Arrange the steps in antigen processing by endocytic pathway in correct order :
A. Recognition by CD4\(^+\) T cells
B. Antigen uptake by APC
C. Transport of MHC-peptide complex to cell surface
D. Peptide loading onto MHC class II molecule
E. Fusion of phagosome with lysosome
Choose the correct answer from the options given below :

• A. \(A, B, C, D, E\)
• B. \(B, E, D, C, A\)
• C. \(E, A, D, B, C\)
• D. \(C, E, A, D, B\)

Hint:

Remember: \[ \text{Exogenous antigen} \rightarrow \text{MHC class II} \rightarrow CD4^+ \text{ T cells} \] while: \[ \text{Endogenous antigen} \rightarrow \text{MHC class I} \rightarrow CD8^+ \text{ T cells} \]
• APCs process extracellular antigens through the endocytic pathway.

Answer 1

The Correct Option is B

Concept: The endocytic pathway processes exogenous antigens for presentation through: \[ \text{MHC class II molecules} \] This pathway occurs mainly in:
• Macrophages
• Dendritic cells
• B lymphocytes The processed antigen-MHC II complex is recognized by: \[ CD4^+ \text{ helper T cells} \]

Step 1: Antigen uptake by APC. The process begins when an antigen-presenting cell (APC) internalizes extracellular antigen by:
• Phagocytosis
• Endocytosis Thus, the first step is: \[ \boxed{B} \]

Step 2: Fusion of phagosome with lysosome. The phagosome containing antigen fuses with lysosome: \[ \boxed{E} \] where proteolytic enzymes digest the antigen into peptides.

Step 3: Peptide loading onto MHC class II molecule. Processed antigenic peptides bind to: \[ \text{MHC class II molecules} \] inside vesicular compartments. Thus: \[ \boxed{D} \]

Step 4: Transport of MHC-peptide complex to cell surface. The peptide-MHC II complex is transported to the plasma membrane: \[ \boxed{C} \]

Step 5: Recognition by CD4\(^+\) T cells. Finally: \[ CD4^+ \text{ helper T cells recognize the MHC II-peptide complex} \] Thus: \[ \boxed{A} \] Therefore, the correct sequence is: \[ \boxed{B \rightarrow E \rightarrow D \rightarrow C \rightarrow A} \] Hence, the correct answer is: \[ \boxed{(B)\ B, E, D, C, A} \]