Antibodies Flashcards
What are the five classes of antibodies in humans?
IgG, IgA, IgM, IgD, and IgE.
What are the two main fragments produced when antibodies are proteolytically cleaved?
When antibodies are proteolytically cleaved, they yield two Fab fragments (the antigen-binding part) and an Fc fragment (responsible for effector functions like complement activation).
What is the difference between the V and C regions of antibody chains?
V (Variable) Regions are regions located at the tips of the antibody’s arms and are responsible for binding to specific antigens. Each antibody has unique V regions that determine its antigen-binding capacity.
C (Constant) Regions: These regions are more conserved and do not vary as much between different antibodies. The C regions determine the antibody’s class (IgG, IgA, IgM, IgD, IgE) and mediate effector functions, e.g. complement activation and binding to cell receptors [dictates the fate of the antigen].
What are the basic components of an antibody’s polypeptide chain unit?
All antibodies have a basic unit of four polypeptide chains: two light (L) chains and two heavy (H) chains. One L-chain is bound to one H-chain by a disulfide bridge and noncovalent interactions. The two H-chains are bound together by covalent disulfide bridges and noncovalent hydrophilic and hydrophobic interactions.
What determines the class of an antibody and what are the types of heavy and light chains?
The class of an antibody is determined by the type of heavy chain it has. There are five different kinds of heavy chains, which correspond to the five classes of antibodies:
µ (mu) chain: IgM
δ (delta) chain: IgD
γ (gamma) chain: IgG
ε (epsilon) chain: IgE
α (alpha) chain: IgA
In addition to heavy chains, antibodies also have light chinas. there are two types of light chains:
κ (kappa) chain
λ (lambda) chain
Each antibody unit can have either κ or λ light chains, but not both. These light chains pair with the heavy chains to form the complete antibody molecule.
What is the valence of an antibody?
The valence of an antibody is the maximum number of antigenic determinants with which it can react. For example, IgG antibodies have a valence of two, meaning they can bind two molecules of antigen or two identical sites on the same particle.
How does valence affect antibody binding affinity?
The higher the valence, the higher the avidity.
[This is because having multiple binding sites for an antigen increases the overll strength of binding (avidity) due to the combined effect of all the binding interactions. This makes the antibody more effective at neutralizing antigens like bacteria or viruses.]
Briefly discuss IgG antibodies.
🦠 IgG antibodies are the most abundant in the blood.
🦠 They provide the bulk of immunity to most bloodborne infectious agents and is the only class to cross the placenta to provide passive humoral immunity to the developing fetus and thus to the infant on its birth.
🦠 IgG antibodies are primarily responsible for neutralizing pathogens, opsonizing bacteria for phagocytosis and activating the complement system.
🦠 IgG antibodies provide long-term immunity by persisting in the body after an infection or vaccination.
Briefly discuss IgM antibodies.
🦠 IgM is the first antibody produced in response to an infection.
🦠 In the bloodstream, IgM typically exists as a pentamer, meaning it is composed of five four-chain units held together by disulfide bridges and a J-chain. Despite having relatively low-affinity binding sites, the pentameric structure of IgM allows it to bind multiple antigens simultaneously, resulting in high avidity. This makes IgM very effective at neutralizing pathogens.
🦠 On the surface of B cells, IgM is expressed as a monomer and acts as an antigen receptor, helping the immune system recognize and respond to pathogens.
Briefly discuss IgA antibodies.
🦠 IgA exists in two main forms: monomerc IgA (found in the blood) and dimeric IgA (found in the mucosal areas).
🦠 It is the major immunoglobulin present in external secretions such as colostrum, mucus, tears, milk and saliva.
🦠 IgA helps in the immune exclusion of pathogens by trapping them in mucus and facilitating their removal from the body.
Briefly discuss IgD antibodies.
🦠 IgD exists as a monomer and is primarily found on the surface of immature B cells, where it functions as a receptor for antigens [it is co-expressed with IgM]. It plays a crucial role in the activation and differentiation of B cells [though its exact function is not yet understood].
🦠 It is present in very low concentrations in the blood stream.
Briefly discuss IgE antibodies.
🦠 IgE is present in the serum at very low levels.
🦠 IgE plays a crucial role in allergic reactions. It binds to allergens and triggers the release of histamine and other chemicals from mast cells and basophils [it is primarily found bound to the surface of mast cells and basophils via high-affinity IgE receptors (FcεRI)], leading to symptoms like itching, swelling and redness.
🦠 IgE is also important in the immune response against parasitic infections, such as those caused by helminths.
