Antigen processing and presentation involves antigens being internalized by antigen presenting cells (APCs) like macrophages and dendritic cells. The antigens are then degraded into peptides and combined with either class I or class II MHC molecules. Class II MHC-peptide complexes are expressed on the surface of APCs and recognized by TH cells to trigger an immune response against extracellular pathogens. Class I MHC-peptide complexes are expressed by all nucleated cells and recognized by TC cells to trigger responses against intracellular pathogens. The route an antigen takes, either extracellularly or intracellularly, determines whether it engages the exogenous or endogenous processing pathways.

1. MINHAJ UNIVERSITY LAHORE
PRESENTATION : IMMUNOLOGY
TOPIC : ANTIGEN PROCESSING AND PRESENTATION
PRESENTED BY : YAHYA ZAIN & IHTISHAM HUSSAIN
PRESENTED TO : MAM ANUM TARIQ

2. Antigen processing and presentation

3. Antigen processing and presentation
 Antigen
 APCs
 Immune cell

4. Antigen Presenting Cells
 Antigen-presenting cells (APCs) are specialized cells, which include
macrophages, B lymphocytes, and dendritic cells, are distinguished by two
properties.
 They express class II MHC molecules on their membranes.
 They are able to deliver a co-stimulatory signal that is necessary for TH-cell
activation.

5.  Antigen-presenting cells first internalize antigen, either by phagocytosis or
by endocytosis, and then display a part of that antigen on their membrane
bound to a class II MHC molecule.
 The TH cell recognizes and interacts with the antigen–class II MHC
molecule complex on the membrane of the antigen-presenting cell leading
to the initiation of a costimulatory signal by the antigen-presenting cell,
leading to activation of the TH cell.

6. Major Histocompatibility Complex
(MHC)
 The MHC loci encode two major classes of membrane-bound
glycoproteins: class I and class II MHC molecules.
 They function as antigen-recognition molecules.
 TH cells recognize antigen combined with class II molecules, whereas TC
cells generally recognize antigen combined with class I molecules.

7.  MHC class I molecules, are based on all nucleated cells (not just
professional APCs) and generally present intracellular antigens such as
viruses.
 MHC class II molecules, are only based on APCs and generally
present extracellular antigens such as bacteria.

8. Antigen Processing & Presentation
 Foreign protein antigen are degraded into small antigenic peptides that
form complexes with class I or class II MHC molecules.
 This conversion of proteins into MHC-associated peptide fragments is
called antigen processing and presentation.
 Whether a particular antigen will be processed and presented together
with class I MHC or class II MHC molecules appears to be determined by
the route that the antigen takes to enter a cell.

9. Exogenous pathway
 Exogenous antigen is produced outside of the host cell and enters the cell
by endocytosis or phagocytosis.
 Antigen presenting cells (macrophages, dendritic cells, and B cells)
degrade ingested exogenous antigen into peptide fragments within the
endocytic processing pathway.

10.  Class II MHC molecules are expressed within the endocytic processing
pathway and that peptides produced by degradation of antigen in this
pathway bind to the cleft within the class II MHC molecules.
 The MHC molecules bearing the peptide are then exported to the cell
surface to be recognized by TH cells.

11. Processing of Exogenous Antigen

12. Endogenous pathway
 Endogenous antigen is produced within the host cells.
 Endogenous antigens are degraded into peptide fragments that bind to
class I MHC molecules within the endoplasmic reticulum. The peptide
class I MHC complex is then transported to the cell membrane.

13. Since all nucleated cells express class I MHC molecules, all cells producing
endogenous antigen use this route to process the antigen.
These cytotoxic T cells attack and kill cells displaying the antigen–MHC class I
complexes for which their receptors are specific.

15. Overview of the Immune