Exam 1: Antibody Structure and Function Flashcards
Antibody Overview
- Proteins that recognize and bind to foreign substances
- Comprise immunoglobulins - glycoproteins
- Present in circulation and on B-cells
B-cell Overview
- B-cells rearrange DNA to produce Ig receptors
- Once receptor expressed the B-cell is committed to that Ag
- Humoral immunity mediated by Ab and B-cells
Antigen Classes
Ab able to recognize a wide variety of antigens:
- Microbial surface Ag
- capsular components
- viral envelops
- Internal components
- capsid
- transcriptase from virus
- dsRNA
- Soluble microbial mediators
- secreted toxins
Ab & B-cell
Summary
Ig Electrophoresis
- Humans have a broad gamma peak (hence gamma globulin) due to Ab diversity
- Individuals with multiple myeloma produce a single Ab in blood
- Also have Bence Jones proteins in urine
- dimers of Ig kappa or Ig lambda light chains
- Also have Bence Jones proteins in urine
Antibody Structure
- Comprised of two heavy chains and two light chains
- Held together by disulfide and non-covalent bonds
- N-terminal variable region
- __Includes 2 antigen binding sites
- Formed by combination of the light and heavy chains
- __Includes 2 antigen binding sites
- Remaining domains termed c_onstant region_
- Hinge region joins Ag binding domain to Fc region
- Flexibility allows binding at different angles
Ab Structure Summary
Ab Genes
- Light chain
- Kappa chain on chromosome 2
- Lamda chain on chromosome 22
- Each B cell expresses one or the other
- Predominance of one type suggests malignancy
- Heavy chain
- Single gene locus on chromosome 14
- Determines Ab class
- Which gene segment is closest to the variable region on the genome determines isotype production
- μ (mu) = IgM
- γ (gamma) = IgG
- α (alpha) = IgA
- ε (epsilon) = IgE
- δ (delta) = IgD
- Which gene segment is closest to the variable region on the genome determines isotype production
- Heavy chain determines effector activities and tissue distribution
Ig Class Summary
Ab Opsonization
- Ab coats the surface of a pathogen to promote phagocytosis
- IgG potent opsonin through binding of Fc region with Fc receptors on phagocytes
Ab Complement Fixation
- IgG and IgM can activate the classicle complement pathway
- C1q binds to Fc region
- Activation leads to:
- MAC formation
- C3b binding and phagocytosis
- Release of proinflammatory mediators
- C5a
- C3a
Antibody-Dependent Cellular Cytotoxicity
(ADCC)
- Innate effector cell selectively kills a target cell coated with Ag-specific Ig
- Mostly NK cells but also neutrophils, macrophages, eosinophils, and basophils
- Cells express Fc receptors that bind to Ig
- If phagocytes are unable to take in the pathogen they are programmed to self-destruct on its surface leading to tissue damage
Neutralization
Ab defends a cell or the host from Ag or pathogen.
Ab binds to pathogen or toxin and sterically hinders their binding to cell surface receptors.
Ex. IgA prevents attachment and colonization of muscosal sites.
Induced through a number of vaccines to prevent the effects of microbial released toxins.
Agglutination
Ab binds to and promotes clumping of particulate Ag.
Ab have at least 2 Ag-binding sites - one Ab can bind two epitopes on different Ag and hold them together.
Increases likelihood of phagocytosis.
Mast Cell Degranulation
Antigen cross-links IgE already bound to high affinity FcεRI receptors on mast cells and basophils.