antigens Flashcards
how was the word antigen come about
it was a combination of the words antibody and generator
what are antigens
- antigens are molecules or pieces of molecules that bind to antibodies, MHC molecules or TCRs
- Molecules that induce an immune response when introduced into the body
what are the two sub categories of antigens
- immunogens
- haptens
what are immunogens
immunogens are a substance capable of stimulating an immune response
what are haptens
haptens are substances that will not by themselves stimulate an immune response but can do so when complexed with a larger molecules, such as a host protein
what are the two cardinal features of antigens
- Immunogenicity
- antigenicity
what is immunogenicity
antigens are capable of inducing an immune response (t cells, B cells - Ab)
What is antigenicity
antigenicity is when antigens are capable of binding with products of the immune response that they induced (i.e., Lymphocytes or antibodies)
What are the functions of Epitope or antigenic determinant
Epitope or antigenic determinant is a part of an antigen that is actually responsible for inducing the immune response and binding to the products of the immune response (lymphocyte receptors and antibodies)
what are the sources of antigens
- Infectious agents such as (microbial antigens)
- bacteria
- viruses
- parasites
- Fungi
- Altered self-antigens
- Tumors
what are the factors that affect the immunogenicity of antigens
- Foreignness
- size
- chemical composition
- Physical properties
- Degradability
- genetic factors
- age
How does Foreignness affect the immunogenicity of antigens
The immune system distinguishes between self and non-self and only self substances are immunogenic and induce immune response (with exceptions)
How does size affect the immunogenicity of antigens
there is no absolute size at which antigens become more immunogenic, however, the larger the antigen the more immunogenic is can be
How does chemical composition affect the immunogenicity of antigens
complex molecules are usually immunogenic (chance of diverse antigenic determinants)
what are the two physical properties that affect immunogenicity of antigens
- Particulate antigens
- are more immunogenic than soluble antigens
- Denatured antigens
- are more immunogenic than native forms (denaturation exposes more antigenic determinants)
How does Degradability affect the immunogenicity of antigens
antigens that are phagocytosed and easily degraded are usually highly immunogenic
How does Genetic factors affect the immunogenicity of antigens
- some antigens are more immunogenic for a given species than for the other. Similarly, some antigens are more immunogenic for an individual than the other
- species or individuals may have genetic variations in genes encoding antigen receptors or both T or B cells
How does age affect the immunogenicity of antigens
young or old individuals usually have less capability to mount an immune response compared to the middle-aged in relation to the same antigen
what are the chemical nature of antigens we are concerned with
- Proteins
- by far the largest group of antigens
- polysaccharides
- Nucleic acids
- Lipids
how do the proteins affect the chemical nature of antigens
most proteins are highly immunogenic (pure proteins, glycoproteins or lipoproteins)
how do the polysaccharides affect the chemical nature of antigens
pure polysaccharides and lipopolysaccharides are usually good immunogens (pneumococcal polysaccharides may require conjunction)
how do the nucleic acids affect the chemical nature of antigens
weak antigens may become highly immunogenic when conjugated to proteins
how do the Lipids affect the chemical nature of antigens
generally not immunogenic but some may be heptanes
what are adjuvants
a substance that enhances the body’s immune response to an antigen
what are exogenous antigens
antigens that have entered the body from outside (i.e. inhalation, ingestion, or infection
what are endogenous antigens
antigens that are generated within cells as a result of normal cell metabolism or because of viral or intracellular bacterial infection
what are the 5 types of antigens
- T-independent antigens
- T-dependent antigens
- superantigens
- Haptens
- Autoantigens
what are T independent antigens
antigens that directly stimulate B lymphocytes do produce antibodies without the need of helper T cells, (ie, lipopolysaccharides are T independent antigens)
what are the properties of T-independent antigens
- have a polymeric structure
- the same antigenic determinants repeats several times throughout the length of the antigen
- May activate lymphocytes polyclonally
- I.e. B cells of different antigenic specificity may be activated
- Resistant to degradation
- do not easily degrade, therefore, may last longer and stimulate B lymphocytes much longer
what are T- dependent antigens
antigens that indirectly stimulate B lymphocytes to produce antibodies
what are the characteristics of T-dependent antigens
- The presence of helper T cells is required for stimulation, hence the name T-dependent
- Protein are mostly T-dependent antigens
- structurally, they contain a few copies each of various antigenic determinants throughout the length of the antigen
what is the purpose of superantigens
superantigens can activate a large number of lymphocytes at one time (non-specifically)
where are superantigens mostly derived from
superantigens are mostly derived from bacteria or viruses
where do superantigens bind to t cells
they bind to the variable domain of the Beta(Vbeta) chain in the T cell receptor (TCR) of CD4+ T cells and the alpha chain of the MHC class 2 molecule on antigen-presenting cells
what is the result of superantigens to the TCR and MHC class 2 molecules
induces a very strong signal that activates a large number of T cells, polyclonally
Polyclonal activation of helper T lymphocytes leads to
excessive production of IL-1, IL-2, IL-6, TNFalpha, MIP-1alpha, and MIP-1beta which may lead to systemic toxicity
what are haptens
small non-immunogenic substances (I.E. metabolites of antibiotics such as penicilloyl from penicillin)
Can Haptens induce an immune response
Haptens on their own cannot induce an immune response
how can haptens induce an immune response
only if they become conjugated to a carrier (I.E. protein)
why are haptens incomplete antigens
because they lack immunogenicity
what are autoantigens
autoantigens are natural constituents of the body, as opposed to foreign antigens
what is the function of Autoantigens
they play a role autoimmunity- reactions of the immune system against host antigens
