7-8. antigens and antigen recognition by lymphocytes

immunogen

substance capable of stimulating a specific immune response and binding to immune component

examples of immunogens

• bacterial

  • O antigens

  • flagellin

  • toxins

• viral

  • capsid proteins

  • envelope glycoproteins

antigens

substance capable of binding specifically to the final products of the immune response

characteristics of haptens

• low molecular weight

• antibiotics, drugs

  • examples: penicillin, streptomycin, aspirin

• not immunogenic on their own, unless conjugated to high molecular weight compounds (carriers)

what are factors that affect immunogenicity?

• route of administration

• host genetics

• chemical stability

• foreignness

• dose

• complexity

• size of molecule

immunogenicity of chemical families

• proteins: almost always

• carbohydrates: potentially

  • glycoproteins (protein + carb): usually

• lipids: poor

• nucleic acids: poor

requirements for immunogenicity

• high molecular weight

• chemically complex

• degradable — susceptible to enzymatic degradation

• foreignness

how does the dosage of antigen affect immunogenicity?

when the dosage is too low or too high, the immune response is decreased

what is the best route of antigen administration?

SQ > IM > IP > IV

depot adjuvants

• delayed release of antigen

• prolonged antigen persistence and stimulation

• ex.

  • aluminum hydroxide

  • incomplete freund’s adjuvant (antigen in aqueous solution, mineral oil, and emulsifying agent)

immunostimulatory adjuvants

• made of PAMPS (complex microbial products)

• ex. LPS, CpG, mycobacterial extract

how do immunostimulatory adjuvants act on the immune system?

• activate macrophages and dendritic cells

  • enhance antigen processing and presentation

  • enhance innate immunity → inflammation

• stimulates lymphocytes nonspecifically

antigen recognition by B lymphocytes

• use cell surface receptors = immunoglobulin (antibody)

• do not need help from any other cells

• antibodies can only bind extracellular antigens

functions of antigen receptors

1. antigen recognition

2. signal transduction

what form(s) of antigens do antibodies (B cells) recognize?

• macromolecules (proteins, polysaccharides, lipids, nucleic acids)

• small chemicals

• conformation and linear epitopes

what type of receptors do T cells use to recognize antigens?

αβ receptors

what form(s) of antigens can T cells recognize?

• MHC-restricted antigen recognition

  • only peptides that are presented by major histocompatiblity complex (MHC) molecules on an antigen-presenting cell

• cannot recognize soluble native antigens or bind to free peptides

• derived from intracellular antigens

what is dual specificity of the MHC-peptide complex?

T cell recognizes a specific peptide bound to a specific MHC → if either is different, T cell does not recognize

what is the physiological function of MHC molecules?

• present peptides to T cells

• present in all vertebrates

MHC class I molecules present antigens to which type of T cell?

CD8+ T cells

CD8 molecules on cytotoxic T cells bind to what domain on MHC class 1 molecules?

selectively binds α3 domain

• only recognize peptides presented by the animal’s own MHC molecules → self MHC I-restricted antigen recognition

what cells express MHC class I molecules?

all nucleated cells of the body

MHC class I peptide-binding clefts can accommodate what size peptides?

8-11 amino acids

MHC class II molecules present antigens which type of T cell?

CD4+ T cells

CD4 molecules on helper T cells bind to which domain on MHC class II molecules?

selectively binds to β2 domain

•  self MHC II-restricted antigen recognition

what cells express MHC class II molecules?

dendritic cells, B cells, macrophages (professional AP cells)

• these are the only cells that express BOTH MHC I and MHC II → can interact with both CD4+ & CD8+ T cells

MHC class II peptide-binding clefts can accommodate what size peptides?

10-30 amino acids