L1: Antigens & immunoglubulins Flashcards
What is the definition of an antigen?
Substance recognized by immune system which may be:
✓ Simple or complex
✓ Carbohydrate, lipid, protein, nucleic acid, phospholipids
What type of antigens are recognized by B cells and T cells?
- B cell recognize any biological Ag
- T cell recognize peptide Ag presented on MHC
What is the definition of epitopes (antigenic determinant)?
Smallest part on Ag which bind with BCR & T cell receptors
What is an antigen with multiple epitopes?
it is called multivalent Ag
What are the types of antigens – epitopes and what are they classified according to?
Depending on the nature of immune responses they trigger, antigens/epitopes are divided into 3 fuctional types:
- Immunogens
- Haptens
- Tolerogens “all the body” “cells that attack them are killed”
What is the definition of immunogens?
Large Ag with epitopes capable of binding with immune receptor & inducing immune response.
(Notice that not all antigens are immunogens)
What is the definition of Haptens?
- A small Ag with epitopes capable of binding with immune receptor & without inducing immune response
- BUT can produce immune response only when conjugated with large carrier molecule (as a protein) → immune response against epitopes of hapten & carrier.
What is the definition of Tolerogens?
Self Ag (MHC) normally “but may cause in pathological cases” not stimulate immune system
What are the factors that influence immunogenicity?
size:- Proteins > 10 KDs are more immunogenic
Complexity:- Complex proteins with numerous, diverse epitopes are more to induce an immune response than are simple peptides that contain only one or few epitopes.
Conformation and accessibility:- Epitopes must be “seen by” and be accessibile to the immune system. “Particles in the eye are not accessible”
Chemical properties:
✓ A protein is good immunogens.
✓ Many charbohydrates, steroids, and lipids are poor immunogens.
✓ Amino acids and haptens are, by themselves, not immunogenic
What are the types of Antigens according to dependency on T-cells?
- T-cell independent Ag (TI): Activate B cells without help from T cell; e.g. polysaccharides (Pneumococcal polysaccharide, LPS)
- T-cell dependent Ag: Requires T cell help for B cell activation, e.g. proteins (microbial proteins & non-self or altered-self proteins).
what are the steps of production and distribution of antibodies?
- In lymph node → Ag stimulation of B cells with help of T helper cytokines → B cell proliferate → differentiate into plasma cell which secrete antibodies → enter circulation → site of infection
- Also mature B cell in Bone Marrow express membrane bound antibodies (BCR)
- So antibodies are produced in lymphoid tissue & bone marrow
What are the forms of antibodies?
Secreted: “99%”
✓ In plasma & mucosa & interstitial fluids of tissues
Membrane-bound Ig:
✓ Expressed on B cell surface (IgM & IgD) as BCR for Ag
✓ If bind with Ag, initiate B cell response
What is the structure of antibodies?
- Y shaped molecules of 4 polypeptide chains
- 2 identical heavy chain (1 variable domain (VH) and 3 or 4 constant domains (CH)
- 2 identical light chain (1 variable domain (VL) and 1 constant domain (CL)
- Each variable domain (VL or VH) contains 3 hypervariable regions called complementary determining repeats (CDR)
- Disulfide bonds connect heavy chain with light chain & heavy chain with heavy chain.
What are the regions of antibodies according to proteolytic fragments?
Hinge region:
• Flexible region lies between Fab & Fc to give mobility to both Fab to accommodate different Ag
Fc (fragment crystalline) • Tend to crystallize in solution • 1 in number • Contain remaining of both heavy chains C domain. • Effector & biological function.
Fab = Fragment antigen binding
- Contain whole light+VH+ CH1
- 2 in number
- Ag recognition and binding
What are immunoglobulin classes (isotypes)?
- Immunoglobulins →divided into five different classes → according to the difference in structure in constant domains of heavy chain
Gamma heavy chains → IgG Alpha heavy chains → IgA Mu heavy chains → IgM Epsilon heavy chains → IgE Delta heavy chains → IgD
- Different classes and subclasses of antibodies perform different effector functions
What are the types of light chains of antibodies?
- There are two types of light chains, called κ (kappa) and λ (lambda).
- An antibody has either two κ or two λ light chains.
What is heavy chain class switching?
- Is the switch from one Ig isotype to another.
- After activation of B lymphocytes, a specific clone of B cells proliferate and differentiate into progeny that secrete antibodies; some of the progeny secrete IgM, and other progeny produce antibodies of different isotypes
