Lec 5 antigens Flashcards
What are antigens?
any substances recognized by the immune system and react with T or B lymphocytes. Can be self or non-self.
Immunogen
any foreign substance which can stimulate the immune system to produce a specific immune response(humoral or cellular)
Hapten
substances of low molecular weight, cannot induce an immune response unless coupled to a protein carrier.
Examples of haptens
drugs such as penicillin, aspirin or cosmetics and soap
Criteria needed for immunogenicity
1-Foreighness.
2-Molecular weight : more than 10 kDa(kilo-Dalton).
3-Chemical complexity.(proteins,CHO3,lipids,nucleic acids)
4-Epitopes: variable multiple epitopes.
5-Degradability: susceptible to enzymatic degradation in APCs
6-Dose: high or low: number of doses(booster doses)
7-Route of administration: parenteral or oral
8-Adjuvants:substances when mixed with an antigen before administration will increase the immune response to that antigen.
Types of antigens
1-Bacterial antigens 2-Viral antigens 3-Human tissue antigens 4- Blood group antigens 5-Histocompatability antigens
Bacterial antigens can be
soluble antigens(products of a microbe): exotoxins,enzymes,hemolysins cellular antigens: capsular,flagellar,somatic antigens
Examples of soluble antigens
products of a microbe: exotoxins,enzymes,hemolysins
Examples of cellular antigens
capsular,flagellar,somatic antigens.
Viral antigens may be
- protein coat viral antigens
- soluble antigens
Examples of soluble antigens
soluble nucleoproteins in Inlfluenza viruses.
Blood group antigens examples
A,B and Rhesus antigens.
Histocompatability antigens examples
- MHC I on surface of all nucleated cells
- MHC II on APCs(B-cells,macrophages,dendritic cells and activated T-cells)
Antigens can be either ___ or ____
infectious / microbial , or noninfectious
Examples of infectious/microbial antigens
- bacterial
- viral
- protozoal
- helminths
Examples of noninfectious antigens
- self antigens
- food antigens
- plant products
- dust
- cell surface proteins
- synthetic chemicals
- venoms
- insect toxins
What are antigen epitopes?
the smallest part of the antigen that binds to the antigen receptor (TCR or BCR).
What may each epitope be composed of?
4-7 amino-acids or monosaccharide residues
Chemical families of antigens
1- Proteins
2- Polysaccharides
3- Nucleic acids
4- Lipids
Describe protein antigens
(most potent immunogens) and all proteins are immunogenic.
Describe polysaccharide antigens
blood group antigens(A and B antigens on RBCs) glycoproteins: potentially but not always immunogenic.
Describe nucleic acid antigens
poor immunogens ,but immunogenic when coupled to a protein carrier.
Describe lipid antigens
rarely immunogenic but lipoproteins are immunogenic.
What do T-cell receptors recognize?
Recognize fragments of protein (peptides) associated with MHC molecules on surface of APCs (MHC restricted)
What do T helper cells recognize?
-recognizes peptide associated with MHC II molecules
What do cytotoxic T cells recognize?
-recognize peptides associated with MHC I
Relationship between T cells and MHC
T-cells are MHC restricted
What do B-cells receptors =?
IgM and IgD
What do IgM and IgD recognize?
Proteins. Polysaccharides. Nucleic acids. Some lipids (glycolipids and lipoproteins) Haptens
Relationship between B cells and MHC
Do not need MHC to recognize an antigen(not MHC restricted),can recognize soluble antigens
Relationship between B cell receptors and APCs
Do not need the antigen to be processed or degraded by APCs
What are superantigens?
Class of antigens that result in excessive activation of the immune system.
Not processed by APCs.
Where do superantigens bind?
Can bind to both the variable region of ß chain on TCR of T-cells and to MHC II on APCs.
What do superantiens act as between the variable region of ß chain on TCR of T-cells and to MHC II on APCs?
They act as a clamp between the two, providing a signal for T cell activation.
What do superantigens activate?
Activates large number of T-cells resulting in massive cytokines release and massive pathological effect.
How do superantigens differ from classic antigens?
1-Superantigens are not processed. They interact with the MHC molecules outside the peptide-binding groove.
2-They are active at very low concentrations
3-Stimulate many T-cells causing the release of large amounts of cytokines. This causes systemic toxicity
Examples of superantigens
- Staphylococcal enterotoxins
- Staphylococcal Toxic Shock Toxin
- Staphylococcal Exfoliating toxin
- Streptococcal pyrogen exotoxin
