Question

When will Class II cells present a foreign antigen? (Choose all that apply.)

A. When they are infected by a pathogen.

B. When they have phagocytosed (engulfed) a pathogen.

C. When they are healthy.

D. Only when self-antigens are present.

Answer 1

Option b and c is correct answer.

explanations

option c is correct becaue antigen presenting cells (APCs) including Dendritic cells (DCs), macrophages and some B-cells function by taking in a fluid sample of the extracellular environment by (pinocytosis) or a soild sample by (phagocytosis). The material that they sample is then processed and presented on their surface via MHC class ii molecules so that T-cells of the adaptive immune response are able to recognise peptide in this form. The antigen presenting cells doing the presenting are simply just displaying all peptide they have processed, it is not a case of only presenting peptide from harmful source, they present everything in the extracellular sample (including self peptide).

[option b is correct because Antigens from the extracellular space and sometimes also endogenous ones, are enclosed into endocytic vesicles and presented on the cell surface by MHC-II molecules to the helper T cells expressing CD4 molecule. Only APCs such as dendritic cells, B cells or macrophages express MHC-II molecules on their surface in substantial quantity.

option a is incorrect because Cells of macrophage lineage represent a key target of human immunodeficiency virus (HIV) in addition to CD4-lymphocytes. Professional APC such as monocytes and macrophages can be infected in vivo and are considered as a reservoir of virus for cmv also

option d is incorrect