For induction of immune responses, recognition of antigens by T cells is essential. T cells cannot recognize the native and free antigens, but they do so only after the antigen is processed into smaller antigenic peptides containing specific epitopes which are subsequently combined with MHC molecules (class I or II) and presented on the host cell surface.

Antigen-presenting Cells (ApCs)

Although antigen presentation refers to presentation of antigenic peptide to both TH (helper T cells) and TC (cytotoxic T cells) by complexing with MHC-II and I respectively; however, antigen-presenting cells (APCs) in strict sense implies only to those cells (e.g. dendritic cell, macrophage, etc.) that present the antigenic peptide along with MHC class II to TH cells (Table 1).

Table1. Antigen-presenting cells (ApCs).

Cells presenting antigenic peptides along with MHC class I molecules to TC cells are not included under APCs. These cells are usually virus infected cells or tumor cells. They are often referred to as target cells as the activated TC cells cause lysis of these cells.

Dendritic cells, macrophages and B cells are the major APCs and are called professional APCs. There are some other non-professional cells that can occasionally present antigens to helper T cells (Table 1).

Antigen processing pathways

For induction of immune response (both CMI and AMI), antigens must be presented to T_H cells. In addition, for CMI induction, antigen presentation to T_C cells is essential. Two well defined pathways are used by the immune system for this purpose. They differ from each other in their mechanism and target antigen, as described below (Table.2):

 1. Cytosolic pathway: Here, the endogenous (intra cellular) antigens such as viral antigens and tumor antigens are processed and presented along with MHC class I molecules to CD8 T cells

 2. Endocytic pathway: In this pathway, the exogenous antigens (extracellular microbes and their products, e.g. toxins) are processed and complexed with MHC class II molecules and presented to T_H cells. The cells involved in endocytic pathway include the APCs such as macrophages, dendritic cells and B cells.

Table2. Differences between cytosolic and endocytic pathways of antigen presentation.

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المجمَع العلمي يكرم الأساتذة المشاركين في مشروع الدورات القرآنية الصيفية في كربلاء

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المزيد

الآخبار التكنلوجية

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المزيد

مواضيع ذات صلة

Cytokines

Structure of MHC Molecule

MHC Genes and Their Products

Macrophage

B Lymphocytes

T Lymphocytes

Immunologic Manifestations of Infectious Mononucleosis

Peripheral Lymphoid Organs

Central Lymphoid Organs

Effector Functions of Complement

Complement Pathways

Mechanisms of innate immunity