Antigen Presenting cells(APCs)

APCs:
Introduction,
types,
function,
processing& presentation,
T-cell activation.

1. Antigen presenting cells ( APCs)

2. Antigen processing & presentation Foreign protein antigen are degraded into small antigenic peptides that from complexes with class | or class || MHC molecule.. This conversion of proteins into MHC associated peptide fragments is called antigen processing and presentation.

3. Antigen presenting cells( APCs) Can process a protein antigen, break it into peptides and present it in coconjunction with class || MHC molecules on the cell surface where it way interact with appropriate T cell receptors.. Engulf a pathogen through phagocytosis and the presenting it to the whole immune syste . So,that cell mediated and humoral immune response can build up.

4. Antigen Presenting Cells: Dendritic cells Macrophages B cells

5. Dendritic cells: Dendritic cells are the antigen presenting cells of mammalian immune system.

6. Function: Process antigen material Present it on cell surface to the T cells Act as messenger between innate and adaptive immune systems

7. Classes: There are three major subclasses

8. Langerhans cells: Dendritic cells of the skin Migrate as ‘veiled cells’ via afferent lymphatics into the paracortex of the draining lymph nodes interact with T cells Termed as Interdigitating dendritic cells Rich in class ll MHC Important for presenting antigen to TH cells

9. Interdigitating cells: Present in thymus Abundant in medulla Delete T cells that react against self antigen Negative selection

10. Follicular dendritic cells: Found in primary and secondary follicles of B cells areas of lymphoid tissue Present antigen to B cells Non migrating Express FC receptors Lack class ll MHC molecules Bind antigen via complement receptors (CD21, CD35)

11. Macrophages: Macrophages are antigen presenting cells that recognize, engulf and destroy target cells.

12. How does a macrophages act as an antigen presenting cell? After the activation, macrophages are able to express MHC class II and co-stimulatory molecules , including the B7 complex and can present phagocytosed peptide fragments to helper T cells.

13. B Cells: B cells , also known as B lymphocytes, are a type of white blood cell of small lymphocyte subtype.

14. How do B cell become antigen presenting cell? B cells can Internalize antigen; Recognize soluble antigen; then process the antigen and present peptides by using MHC class II molecules.

15. Function of different Antigen Presenting cells

16. Comparative characteristics of some APCs Dendritic cell Macrophage B lymphocyte Antigen uptake Endocytosis Phagocytosis phagocytosis Receptor- mediated endocytosis Class II MHC expression Constitutive (+++) Inducible (++) Constitutive (+++) Co-stimulatory activity Constitutive B7 (+++) Inducible B7 (++) Inducible B7 (++) T-cell activation Native T cells Effector T cells Memory T cell Effector T cell Memory T cell Native T cell Effector T cell Memory T cell

17. Antigen processing and presentation The process by which pathogens and their products are degraded to produce peptide antigens is known as Antigen Processing These peptide fragments combine with MHC molecules inside cell The MHC-peptide complex thus formed travels to the cell surface where it displays peptide fragments to T cells. This is known as Antigen Presentation.

18. Components: MHC (Major Histocompatibility Complex),CD(Cluster of Designation)4+,CD8+,CD80/86,CD28,IL-1,2..,TCR,etc.

19. Engulf of a pathogen:

20. Processing: Internalizing. Digested to small peptide fragments. MHC-II +small fragments= MHC-II Complex molecule . Transportation of complex.

21. Presentation: MHC-II complex shows antigen fragments to T cells. T cell recognization

22. T-CELL ACTIVATION

23. What is T-Cell? One type of lymphocyte Generated in the thymus. Develops in the thymus gland Play a central role in immune response.

24. T cell activation by antigen presenting cell What is the antigen presenting cell? Heterogeneous group of immune cells Presenting antigen for recognition by certain lymphocytes Dendritic cells,B cells, macrophages,Langerhans cells etc.

25. How T-cell is activated? Some direct and immune response Directly attack infected or cancerous cell

26. Important two signal by T-cell activation Signal 1: Antigen specific signal. Signal 2: Engagement of co-stimulatory molecules.