What are Antigen Presenting Cells?

Antigen presenting cells are a heterogeneous population of leucocytes. Through antigen presentation they play a key role in the immunological reactions. They are found primarily in the skin, lymphnodes, spleen and thymus.

They are derived from bone marrow. The migration of dendritic cells (DCs) to lymph nodes (LNs) is pivotal to the establishment of the immune response. In response to danger signals, precursors are rapidly mobilized into the circulation.

Chemokine receptors present on the dendritic cells help in their movement towards their precise regions. They hold nonspecific antigen uptake receptors. When they get activated with the antigen, they start migrating towards the T cells for presenting the antigen. When they reach the T cells, they present the processed antigen to the T cell for further action.

To cover long areas antigen presenting dendritic cells holds long cytoplasmic protrusions, like dendrites of nerve cells. Because of dendrites like protrusions they were referred as dendritic cells. Dendritic cells capture the antigens in the tissue by phagocytosis and migrate to various lymphoid organs to present the antigen to lymphocytes for immunological reactions.

The dendrities or the filamentous process may help in engulfing and presenting activity, making them better antigen presenting cells than macrophages and B cells. The high levels of MHC class II molecules on the surface of dendritic cells enhance their efficiency.

In addition to antigen presentation the activated dendritic cells secrete different cytokines including IL-7 and IL-12. IL – 7 induces CD₄ and CD₈ T cell proliferation and B cell differentiation.

Antigen presenting cells have common progenitor cells together with the monocyte macrophage lineage because both express the medullary antigen CD33 and CD35.

On the basis of tissue location dendritic cells are classified as follows.

(1) Langerhans Cells: –

These are found in the epidermal layer of the skin, where they are thought to act as sentinels for the detection of invading pathogens. Interaction with pathogens induces their migration to lymphoid regions to prime T cell immunity.

(2) Interdigitating Cells: –

The antigen presenting cells, interdigitating the T lymphocytes are localized in T cell dependent paracortex areas of lymph node and thymus.

(3) Follicular dendritic cells (FDCs): –

Follicular dendritic cells (FDCs) are the stromal cells located in the germinal centers of spleen and lymph node. They lack MHC class II molecules, but show protrusions like dendrites of nerve cells. The dendrites in the network bind immune complexes in a periodicity that is pivotal in the binding of immune complexes to B cells in the germinal centers. This binding initiates a specific antibody response to antigens.

Follicular dendritic cells (FDCs) have important functions in the selection of memory B lymphocytes during germinal center reactions (GCR). They present native antigens to potential memory cells, of which only B cells with high affinity receptors (BCR) can bind. These B lymphocytes survive, whereas non binding B cells undergo apoptosis (cell death).

FDCs are present in follicles of any secondary lymphoid organ and belong to the stromal cells of these organs. They contain large amounts of MHC class II molecules (Refer Chapter 11 for further information). In addition to MHC class II molecules they have various surface bound adhesion molecules and co stimulatory molecules to execute their function perfectly.

The development of FDCs is not restricted to secondary lymphoid organs, but it is a matter of local conditions that drives a precursor cell type into FDC- maturation. A precursor of FDCs has presently not been identified, but phenotypic marker studies, in vitro experiments with fibroblast-like cell lines and recent data on mesenchymal precursor cells from the peripheral blood suggest a close relation to fibroblast-like cells.

(4) Interstitial dendritic cells: –

They are found in most of the organs like heart liver kidney and gastrointestinal tract. They are efficient in presenting soluble antigens. Circulating antigen antibody complexes fasten to the receptors on follicular dendritic cells, thereby helping in activation of B cells in lymph nodes.