Antigens are foreign molecules recognized by the immune system that can bind to antigen-specific receptors on antibodies and T cells. An immunogen is an antigen that induces an immune response by binding to B-cell or T-cell receptors, while a hapten is too small on its own but can become immunogenic when attached to a carrier protein. Epitopes are the specific parts of antigens that are recognized by the immune system. Exogenous antigens come from outside the body and endogenous antigens are generated inside cells.

1. Antigens
Antigens are foreign molecules that are recognized by the immune system.
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Interaction between receptor and ligand.
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Strength of binding is important in activation.
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They can bind to antigen-specific receptors (antibodies and T cell receptors).
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Antigens that do not bind, do not induce an immune response.
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Antigens are usually proteins, peptides, polysaccharides or lipids.
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immunogen
An immunogen is an antigen that induces an immune response.
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An antigen binds the highly variable immunoreceptor products (B-cell receptor or T-cell receptor).
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hapten
Hapten is a small molecule that is antigenic but not immunogenic.
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It is univalent and must be attached to a carrier to become immunogenic.
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It is only when attached to a large carrier such as a protein
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For example, penicillin, when bound to a carrier, may activate an allergic response.
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epitope
The epitope is the part of the antigen that interacts with the antigen-specific molecules.
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They are typically 8 to 20 amino acids long, and a single protein antigen can have multiple epitopes.
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is recognized by the immune system, specifically by antibodies, B cells, or T cells.
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exogenous antigens
antigen processing cells such as macrophage and dendritic cells.
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The immune system's response to exogenous antigens is often subclinical.
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By endocytosis or phagocytosis, exogenous antigens are taken into the antigen- presenting cells (APCs) and
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processed into fragments.
examples; bacteria, viruses, fungi
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endogenous antigens
these are body's own cells or sub fragments or compounds or the antigenic products that are produced.
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are generated within normal cells as a result of normal cell metabolism, or because of viral or intracellular
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bacterial infection.
Endogenous antigens include xenogenic (heterologous), autologous and idiotypic or allogenic (homologous)
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antigens.
examples: blood group antigens, HLA(Histocompatibility Leukocyte Antigens), etc.
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Autoantigens
usually normal protein or complex of proteins that is recognized by the immune system of patients suffering
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from specific autoimmune disease.
examples: nucleoproteins, nucleic acids
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Viral antigens
For virus-associated tumors, such as cervical cancer and a subset of head and neck cancers, epitopes derived
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from viral open reading frames contribute to the pool of neoantigens
Tumor antigens
are those antigens that are presented by MHC class I or MHC class II molecules on the surface of tumor cells.
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2. Major Histocompatibility Complex
MHC is a coding region for surface proteins, such as HLA's essential for the acquired immune system to
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recognize foreign molecules in vertebrates, which in turn determines histocompatibility.
This region has been highly associated with a great many polygenic, complex diseases and character traits due
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to its extremely high level of variation within individuals compared to other areas of their genome.
Presentation of antigenic peptides
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Adaptive immune response
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T cells recognize antigen presented by MHC I or II
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CD4 binds to MHC II, CD8 binds to MHC I
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Innate immune response
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NK cells have KIR that binds to MHC I= inhibitory signal
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MHC class I
The largest region
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CD8 T cell recognize Ag/MHC class 1
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The MHC I molecule is composed of a SINGLE polypeptide chain plus non MHC protein called the beta
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microglobulin protein which is coded for at another genetic location-> SINGLE domain
>Peptide-binding groove (alpha and beta)
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Three genes that code for an MHC class I polypeptides
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>HLA-A, HLA-B, and HLA-C (HLA= Human leukocyte Antigen)
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Present processed ENDOGENOUS antigen to CD8 T Cell
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Class I including extended region
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which contains genes such as:
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HFE - causes haemochromatosis, associated with diseases such as MS
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UBD - associated with gastrointestinal malignancies
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HLA-G - associated with Pemphigus vulgaris in Jewish
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HLA-A - associated with autoimmunity
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PSORS1C1 and PSORS1C2 - associated with Psoriasis
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Classical HLA class I: HLA-A, -B, -C
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highly polymorphic and ubiquitous tissue expression
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Non classical class I: HLA-E, -F, -G
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limited polymorphism and tissue expression
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MHC class II
CD4 T cell recognize Ag/MHC class 2
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The MHC II molecule is composed of a TWO polypeptide chains: Alpha and Beta chain.
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Class II including extended region
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which contains genes such as:
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HLA-DR loci - associated with autoimmunity, particularly arthritis and diabetes
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HLA-DQ loci - associated with autoimmunity particularly narcolepsy
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BRD2 - associated with Juvenile myoclonic epilepsy
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COL11A2 - causes autosomal dominant deafness and other diseases
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TAPBP - causes bare lymphocyte syndrome type I
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Classical HLA class II: HLA-DR, -DQ, -DP
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highly polymorphic and limited tissue expression
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Non classical class II: HLA-DO, -DM
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oligomorphic, accessory molecules, not expressed on cell surface
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