Dendritic Cells and Antigen Processing

Three Professional Antigen Presenting Cells (APCs):

• Dendritic Cells (DCs): These are the most efficient APCs ($100$ times more efficient than others) and are unique because they can activate naïve $T$ cells to initiate primary adaptive immune responses. They capture antigens in tissues and migrate to lymphoid organs for presentation.

• Macrophages: These cells present antigens primarily to memory $T$-helper cells. While they are less efficient in a resting state, their presentation capability increases significantly when activated by substances like $INF\gamma$.

• B-cells: These cells also present antigens to memory $T$-helper cells. They are more pivotal in secondary immune responses, becoming highly effective APCs after being activated by $T$-helper cells.

Differences Between Immature and Mature Dendritic Cells:

• Location: Immature DCs are located in tissues, while mature DCs are found in lymphoid organs.

• MHC II Expression: Immature DCs have low surface $MHC II$ but high intracellular levels; mature DCs have high surface $MHC II$ for optimal presentation.

• Receptors: Immature DCs have high levels of $Fc$ receptors ($FcR$) for antigen capture, whereas mature DCs have low $FcR$.

• Co-stimulatory Molecules: Mature DCs express high levels of $CD40, CD80, and CD86$ and produce high amounts of $IL-12$, which are all low in immature DCs.

• Special Markers: Mature DCs express $DC-SIGN$.

Langerhans Cell Definition:

• Langerhans cells are specialized dendritic cells specifically located in the skin (epithelial tissue) that function as part of the immune system's sentinel network.

Differentiating Follicular Dendritic Cells (FDCs) from Dendritic Cells (DCs):

• Origin: DCs are derived from bone marrow stem cells (myeloid and plasmacytic precursors). FDCs are structurally unique (resembling octopuses with tentacle-like dendrites) and remain stationary.

• Antigen Presentation: DCs process exogenous antigens and present them via $MHC II$ molecules to $T$ cells. FDCs do not process antigens and lack $MHC II$; instead, they use $Fc$ and complement receptors to retain antigen-antibody complexes to present to $B$ cells.

• Location: DCs are found in epithelial tissues and migrate to various lymphoid organs (lymph nodes, spleen, thymus). FDCs do not migrate and are located strictly within lymphoid follicles ($B$-cell areas).