Antigen-Presenting Cells

The main role of antigen presenting cells is to phagocytose antigen from the extracellular environment, process it and display the antigen on their surface coupled to an MHC molecule so that the antigen can be recognised by T lymphocytes. While B cells can recognise presented antigen, they are also capable of identifying raw, unmodified antigen.

Without this process of antigen presentation, T cells cannot recognised antigen. Given that the intrinsic function of cytotoxic T cells is to monitor antigen that is displayed on the surface of infected host cells, what may at first seem to be an unnecessarily complicated method of delivering antigen to the T cell by processing via antigen-presenting cells is actually crucial for the function of T cells. In order for antigen-presenting cells to accomplish the initial activation of young T cells, they have to display antigen to the young T cells in the same way that the cytotoxic T cells will be required to detect antigen in the future.

The interposition of antigen-presenting cells between antigen and the first episode of activation of either a cytotoxic T cell or a helper T cell also acts as a safety mechanism for ensuring that T cells are not stimulated inappropriately: the antigen-presenting cells should not be processing and displaying host antigen.

Some antigen-presenting cells are specifically configured for only this task, while others include antigen presentation as only one of several or many functions. Dendritic antigen-presenting cells constitute the former group and tend to reside in places where antigen entry into the body is likely or through which antigen that has already gained access is likely to pass. Thus they are found in the skin (Langerhans cells) and mucosal surfaces in the former case and lymph nodes, the spleen and the sinusoids of the liver (Kupffer cells) in the latter (the liver has a high blood flow and is also immediately downstream from the gastrointestinal tract so will meet any antigens that have evaded the gastrointestinal lymphoid tissue).

Extranodal dendritic cells can migrate from their normal location to lymph nodes if they are stimulated by cytokines, proteins released from damaged cells, proteins associated with bacteria or double stranded viral RNA. The migrating dendritic cells take any antigen they have sampled with them and thus deliver it to the lymphocytes in the lymph nodes.

The name dendritic cell refers to the complex network of slender branching extensions of cytoplasm that protrude from the body of the dendritic cell. This shape greatly increases the surface area of the dendritic cell and the volume of the extracellular environment it can monitor.

Despite their name the follicular dendritic cells of the germinal centres of lymph nodes are not members of the family of antigen-presenting dendritic cells. While they do have an important role in helping B cells to develop, the term dendritic in their name alludes to their morphology. Unlike antigen-presenting dendritic cells, they are not believed to originate from haematopoietic cells in the bone marrow.

Cells which number antigen presentation as one of a collection of abilities include B lymphocytes and the versatile cell that is the macrophage.

Stimulation of a young helper T cell that has never been activated before requires the intercession of the expert dendritic antigen-presenting cells; part-time antigen presenting cells like macrophages and B cells cannot accomplish this initial activation but can only restimulate a T cell that has already been inducted.