• The lymphocytes and APCs for adaptive immunity are distributed throughout the body in the blood, lymph, and
epithelial and connective tissues.
• Lymphocytes are formed initially in primary lymphoid organs (the thymus and bone marrow) , but most lymphocyte
activation and proliferation occur in secondary lymphoid organs (the lymph nodes, the spleen, and diffuse lymphoid
tissue found in the mucosa of the digestive system, including the tonsils, Peyer patches, and appendix).
• The immune cells located diffusely in the digestive, respiratory, or urogenital mucosae comprise what is collectively
known as mucosa-associated lymphoid tissue (MALT).
• Proliferating B lymphocytes in the secondary structures of MALT are arranged in small spherical lymphoid nodules.
• Innate immunity is nonspecific, involves a wide variety of effector mechanisms.
• Among the cells mediating innate immunity are most of the granulocytes and other leukocytes .
• Adaptive immunity aims at specific microbial invaders, is mediated by lymphocytes and antigen-presenting cells
(APCs) and produces memory cells that permit a similar, very rapid response if that specific microbe appears again.
• Components of Immune system :
1) Immune cells
2) Lymphoid tissue
3) Lymphatic organs
4) Immunocompetent molecules
• Functions ;
a) Have the ability to distinguish "self" from "nonself" .
b) To protects the body from invasion and damage by microorganisms and foreign substances, and abnormal cells
Major Histocompatibility Complex ( MHC )
▪ The major histocompatibility complex (MHC) is a complex of chromosomal loci encoding several proteins known as
class I and class II MHC molecules.
▪ Antigens recognized by lymphocytes are often bound to specialized integral transmembrane protein complexes on
cell surfaces.
▪ These abundant antigen-presenting proteins are parts of the major histocompatibility complex (MHC) , which
includes the two key types called MHC class I and class II.
▪ MHC molecules are made in the rough ER and Golgi apparatus.
▪ Before leaving the ER , MHC class I proteins bind a wide variety of proteasome-derived peptide fragments
representing the range of all proteins synthesized in that cell.
▪ All nucleated cells produce and expose on their surfaces MHC class I molecules presenting such “self-antigens,”
which T cells recognize as a signal to ignore those cells .
1
, ▪ All nucleated cells have class I proteins, but class II proteins exist on only a small group of cells operationally called
antigen-presenting cells (APCs).
▪ By this same mechanism, some virally infected cells or cells with proteins altered by gene mutation also have MHC
class I proteins displaying peptides that T cells do not recognize as “self,” helping lead to the elimination of such
cells.
▪ MHC class II proteins are synthesized and transported to the cell surface similarly but only in cells of the
mononuclear phagocyte system and certain other cells under some conditions.
▪ Before joining the plasmalemma, the Golgi-derived vesicles with the MHC class II complexes first fuse with
endolysosomal vesicles containing antigens ingested by receptor-mediated endocytosis, pinocytosis, or
phagocytosis.
▪ This allows the class II proteins to bind fragments of whatever proteins the cells had ingested, including those from
dead, infected, or abnormal cells and atypical proteins of all kinds.
▪ At the surface of these cells, the class II complexes display the peptides from these potentially pathogenic cells,
signaling T lymphocytes and activating their responses against sources of these antigens.
▪ MHC molecules are integral membrane proteins present on the cell surface.
▪ They are synthesized by the rough ER like regular membrane proteins.
▪ However, on their way to the cell surface, they couple with small peptides of 10–30 amino acids whose origin differs
depending on whether class I or class II molecules are involved.
▪ In most cases, class I MHC molecules form complexes with peptides derived from cytosolic proteins synthesized in
that cell.
▪ Proteins encoded by virus nucleic acid in an infected cell are an important example of this kind of cytosolic protein.
▪ The proteins are targeted by ubiquitin to be degraded by proteasomes, producing small peptides.
▪ These peptides are transported membranes of the ER where they bind class I MHC molecules ; the resulting complex
is moved to the cell surface, exposing the peptides to the extracellular space
Antigen-Presenting Cells
▪ Most specialized APCs are part of the mononuclear phagocyte system, including all types of macrophages and
specialized dendritic cells in lymphoid organs.
▪ Features common to all APCs are an active endocytotic system and expression of MHC class II molecules for
presenting peptides of exogenous antigens.
▪ Besides dendritic cells (not to be confused with cells of nervous tissue) and all monocyte-derived cells,
“professional” APCs also include thymic epithelial cells .
▪ During inflammation transient expression of MHC class II is induced by interferon-γ in certain local cells that can be
considered “nonprofessional” APCs, including fibroblasts and vascular endothelial cells.
2
epithelial and connective tissues.
• Lymphocytes are formed initially in primary lymphoid organs (the thymus and bone marrow) , but most lymphocyte
activation and proliferation occur in secondary lymphoid organs (the lymph nodes, the spleen, and diffuse lymphoid
tissue found in the mucosa of the digestive system, including the tonsils, Peyer patches, and appendix).
• The immune cells located diffusely in the digestive, respiratory, or urogenital mucosae comprise what is collectively
known as mucosa-associated lymphoid tissue (MALT).
• Proliferating B lymphocytes in the secondary structures of MALT are arranged in small spherical lymphoid nodules.
• Innate immunity is nonspecific, involves a wide variety of effector mechanisms.
• Among the cells mediating innate immunity are most of the granulocytes and other leukocytes .
• Adaptive immunity aims at specific microbial invaders, is mediated by lymphocytes and antigen-presenting cells
(APCs) and produces memory cells that permit a similar, very rapid response if that specific microbe appears again.
• Components of Immune system :
1) Immune cells
2) Lymphoid tissue
3) Lymphatic organs
4) Immunocompetent molecules
• Functions ;
a) Have the ability to distinguish "self" from "nonself" .
b) To protects the body from invasion and damage by microorganisms and foreign substances, and abnormal cells
Major Histocompatibility Complex ( MHC )
▪ The major histocompatibility complex (MHC) is a complex of chromosomal loci encoding several proteins known as
class I and class II MHC molecules.
▪ Antigens recognized by lymphocytes are often bound to specialized integral transmembrane protein complexes on
cell surfaces.
▪ These abundant antigen-presenting proteins are parts of the major histocompatibility complex (MHC) , which
includes the two key types called MHC class I and class II.
▪ MHC molecules are made in the rough ER and Golgi apparatus.
▪ Before leaving the ER , MHC class I proteins bind a wide variety of proteasome-derived peptide fragments
representing the range of all proteins synthesized in that cell.
▪ All nucleated cells produce and expose on their surfaces MHC class I molecules presenting such “self-antigens,”
which T cells recognize as a signal to ignore those cells .
1
, ▪ All nucleated cells have class I proteins, but class II proteins exist on only a small group of cells operationally called
antigen-presenting cells (APCs).
▪ By this same mechanism, some virally infected cells or cells with proteins altered by gene mutation also have MHC
class I proteins displaying peptides that T cells do not recognize as “self,” helping lead to the elimination of such
cells.
▪ MHC class II proteins are synthesized and transported to the cell surface similarly but only in cells of the
mononuclear phagocyte system and certain other cells under some conditions.
▪ Before joining the plasmalemma, the Golgi-derived vesicles with the MHC class II complexes first fuse with
endolysosomal vesicles containing antigens ingested by receptor-mediated endocytosis, pinocytosis, or
phagocytosis.
▪ This allows the class II proteins to bind fragments of whatever proteins the cells had ingested, including those from
dead, infected, or abnormal cells and atypical proteins of all kinds.
▪ At the surface of these cells, the class II complexes display the peptides from these potentially pathogenic cells,
signaling T lymphocytes and activating their responses against sources of these antigens.
▪ MHC molecules are integral membrane proteins present on the cell surface.
▪ They are synthesized by the rough ER like regular membrane proteins.
▪ However, on their way to the cell surface, they couple with small peptides of 10–30 amino acids whose origin differs
depending on whether class I or class II molecules are involved.
▪ In most cases, class I MHC molecules form complexes with peptides derived from cytosolic proteins synthesized in
that cell.
▪ Proteins encoded by virus nucleic acid in an infected cell are an important example of this kind of cytosolic protein.
▪ The proteins are targeted by ubiquitin to be degraded by proteasomes, producing small peptides.
▪ These peptides are transported membranes of the ER where they bind class I MHC molecules ; the resulting complex
is moved to the cell surface, exposing the peptides to the extracellular space
Antigen-Presenting Cells
▪ Most specialized APCs are part of the mononuclear phagocyte system, including all types of macrophages and
specialized dendritic cells in lymphoid organs.
▪ Features common to all APCs are an active endocytotic system and expression of MHC class II molecules for
presenting peptides of exogenous antigens.
▪ Besides dendritic cells (not to be confused with cells of nervous tissue) and all monocyte-derived cells,
“professional” APCs also include thymic epithelial cells .
▪ During inflammation transient expression of MHC class II is induced by interferon-γ in certain local cells that can be
considered “nonprofessional” APCs, including fibroblasts and vascular endothelial cells.
2
