Antigen Presenting cells (APC):

Introduction:

All pathogenic antigens that make their way into our body must be presented to T lymphocytes of our cell mediated immunity. T lymphocytes however cannot bind its antigen directly instead the antigens must be presented to the T lymphocytes by antigen presenting cells such as macrophages, dendritic cells and B cells.
            So, Antigen-presenting cells (APC) are cells that can process a protein antigen, break it into peptides, and present it in conjunction with class II MHC molecules on the cell surface where it may interact with appropriate T cell receptors.

Mechanism of APC:

·      Capture and processing of antigen

·      Synthesis and transport of MHC II from ER

·      Formation of Peptide-MHC II complex

·      Presentation of peptide-MHC II complex to T cells

 

Types of APC: 

Macrophages:

           When infection begins, the innate immune system calls upon the macrophages. These cells engulf the pathogen by phagocytosis, digest it inside the vacuoles and present the pathogenic antigen on the MHC II (major histocompatibility complex II) found on the surface.

            T lymphocytes such as Helper T cells that contain CD4 glycoproteins on their membrane can bind to these MHC II- antigen complex and begin an immune response.

 

2)                                                                                                                              Dendritic cells:

     These are immune cells found in the tissues of our skin, lungs and GI tract.

 When they encounter antigen, they engulf them, break them down and display them on their MHC II membrane protein. They then travel to the lymph nodes or spleen and interact with the T lymphocytes that has the T cell receptor (CD4+). They then stimulate other defense mechanisms.

                                                                                                                                       B lymphocytes:

B cells are part of our humoral immunity. When they approach foreign antigens floating around the tissue, they bind to them via special receptors and take via receptor mediated endocytosis. Then they break them down and display an antigen fragment on MHC II molecule. When a helper T cell binds to the B lymphocytes, it then induces it to multiply and differentiate into plasma cells and memory cells.

                                                               T cell interaction:

              Interaction occurs between the receptors of APCs and T lymphocytes that result in         activation of T lymphocytes and then release of some chemical mediators (IL2, IL4/5, IL10, IFN2γ).