Antigens and Major Histocompatibility Complex (MHC)
Antigens and Major Histocompatibility Complex (MHC)
# Antigens (Ag)
Definition:
Antigens are substances that, when introduced into the body, are recognized by the immune system and stimulate the production of corresponding antibodies. These antibodies can be either humoral or cellular antibodies, and they have the property of specifically binding to that antigen.
Characteristics of Antigens:
- Specificity: Each antigen is only recognized by the antibody produced against it, not by antibodies produced against other antigens.
- Immunogenicity: The ability to stimulate the body’s immune system to produce antibodies.
Factors that contribute to the specificity of an antigen:
- Foreignness: The more foreign an antigen is to the host, the stronger its ability to stimulate antibody production.
- Immunogenicity:
- Only complete antigens (multivalent) have the ability to induce antibody production.
- Hapten (partial antigen) with a small molecular weight must bind to a protein molecule to be able to produce antibodies.
Factors affecting antibody production:
- Antigen dose:
- Too low a dose is insufficient to stimulate an immune response.
- Too high a dose can lead to immune paralysis.
- Vaccination with small doses, repeated several times, can create a high and lasting immunity.
- Route of antigen entry into the body: Intradermal, subcutaneous, intramuscular, intravenous. If the antigen enters through the digestive system, it must not be structurally altered by digestive enzymes.
- Role of adjuvant: The immunogenicity of a substance can be enhanced by combining it with an adjuvant (e.g., paraffin oil, lanolin) when sensitizing.
- Role of genetic background: The immune system varies with species and the degree of immune differentiation of each species.
Classification of Antigens:
- Based on ability to induce antibody production:
- Complete antigens: Have multiple antigenic determinant groups.
- Incomplete antigens: Must bind to a protein to be immunogenic.
- Based on genetic homology:
- Heterogeneic antigens: Have the highest degree of foreignness, with a very strong ability to induce antibody production.
- Homogeneic antigens: Determine the differences between individuals within the same species and create antibodies in individuals who do not carry that antigen.
- Autoantigens: Present on the cells of the individual or self-generated antibodies against the individual’s own body.
- Based on chemical properties:
- Protein antigens: Strong immunogenicity when they have a high molecular weight. Peptides with fewer amino acids are less immunogenic.
- Pure lipid antigens: Do not have the ability to produce antibodies, but they do when bound to a protein.
- Nucleic acid antigens: Partial antigens that do not have the ability to induce antibodies but do when bound to a protein.
- Based on T-B cell interaction:
- Thymus-dependent antigens: Require the activity of the thymus to produce antibodies (require T lymphocytes). The nature of these antigens is protein.
- Thymus-independent antigens: Examples: bacterial lipopolysaccharide, bacterial polysaccharides…
- Based on antigen origin:
- Antigens originating from animals.
- Plant antigens.
- Viral antigens.
- Bacterial antigens.
Types of cellular antigens:
- Red blood cell antigens: Exist on the red blood cell membrane, are produced by red blood cells, and exhibit agglutination with corresponding antibodies.
- Rh antigens: Only those with Rh- are sensitized to produce antibodies against Rh+. For example, hemolytic disease in infants due to the mother being Rh-, the father being Rh+, and the child being Rh+ from the father, leading to the mother producing antibodies against the child.
- White blood cell antigens (Major Histocompatibility Complex – MHC):
# Major Histocompatibility Complex (MHC)
Definition:
MHC (Major Histocompatibility Complex) is a major histocompatibility complex, which plays a crucial role in antigen presentation and immune responses. MHCs are essentially protein molecules on the cell surface of various cell types in the body.
In humans, MHC is the human leukocyte antigen (HLA) group.
MHC Class 1:
- Location: Present on all nucleated cells (antigen-presenting cells).
- Structure: Glycoproteins composed of two polypeptide chains (alpha and beta chains), divided into four regions:
- Antigen-binding region.
- Transmembrane region.
- Cytoplasmic region.
- Ig-like region.
- Function: Bind peptide fragments (intracellular antigens) and present them on the cell surface for recognition by cytotoxic T cells (CD8+ T cells) and activation.
MHC Class 2:
- Location: On macrophages, B lymphocytes.
- Structure: Glycoproteins composed of two polypeptide chains (alpha and beta chains), divided into four regions:
- Antigen-binding region.
- Transmembrane region.
- Cytoplasmic region.
- Ig-like region.
- Function: Bind peptide fragments (extracellular antigens) and present them on the cell surface for recognition by helper T cells (CD4+ T cells) and activation.
# Tissue and organ transplantation
- Syngeneic transplantation: Tissue exchange between two genetically identical individuals (e.g., twins).
- Allogeneic transplantation: Tissue transplantation between two individuals of the same species but genetically different.
- Xenogeneic transplantation: Tissue transplantation between two individuals of different species.
- Autologous transplantation: Tissue transplantation from one site to another in the same individual (e.g., skin).
# Role of MHC and HLA
- The survival or rejection of a tissue graft is determined by proteins encoded by genes located within the major histocompatibility complex (MHC).
- The genes controlling the HLA and MHC systems are located on chromosome 6.
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