- Macrophages demonstrate moderate activity in antigen processing and presentation for antigen-specific T cells.
- Under phagocytosis and antigen digestion a great amount of low-weight antigenic peptides is formed.
- They are processed inside the endosome (exo-antigens) or within special supramolecular cytoplasmic protease complex known as proteasome (endo-antigens).
- During processing antigenic peptides are coupled with the major histocompatibility complex molecules (MHC, in humans – HLA).
- The complex “Ag peptide-HLA molecule” is next transferred and expressed upon the cell membrane (antigen presentation).
- If viral or modified endo-antigen has been processed, its fragments of 8-12 amino acids residues are coupled predominantly with HLA I class molecules (HLA-A, HLA-B, HLA-C) to be presented to T-cytotoxic lymphocytes.
- Exo-antigens (peptides of 12-25 amino acids residues) are coupled mainly with HLA II class molecules (HLA-DR, HLA-DP, HLA-DQ) and presented to T helper cells.
- These interactions provide maximal stimulation for cell-mediated immunity.
- However, macrophages are not the most powerful antigen-presenting cells (APC). Dendritic cells of different origin perform this function with the greatest effectiveness. They are localized in skin, mucosal tissues, in the thymus, various zones of lymph nodes.
- Most active Langerhans cells transport the antigen from the skin to T cells in lymph nodes, where presentation occurs.
- The membrane of dendritic cells is highly enriched with MHC II class molecules. Similar follicular dendritic cells present the native antigens to B lymphocytes of B-dependent zones in peripheral lymphoid organs.
- Also, B-lymphocytes themselves can serve as efficient APCs. They present various antigens to T helper cells mostly of Th2 type.