Dr Shoaib Raza Dr Shoaib Raza
Jan 18, 2016
Dr Shoaib RazaDr Shoaib Raza
B lymphocytes are also called as B-Cells Develop in the yolk sac Migrate to bone marrow Processing, differentiation and maturation take
place in Bursa of fabricius or its equivalent organ
Reside in the lymphoid follicle
B- Cells
B-Cells recognize freely circulating antigens,
that are soluble B-Cells are provided with BARC (B-Cell Antigen
Receptor Complex) IgM is an important constituent of BARC IgM present on surface occurs as monomer
(different from circulatory IgM)
B-Cells & Antigens
Upon antigenic stimulation under
influence of cytokines released from T-Cells: Antigen is perceived by sIgM Antigen-antibody complex is
internalized Primary antibody is formed Usually IgG (Primary response) Differentiation leads to formation of
IgM by plasma cells
B-Cells & Antigens
Meanwhile B-cells undergo transformational
changes to form plasma cells More cytoplasm Eccentric nucleus Amphophilic cytoplasm owing to increased
number of RNA and Golgi apparatus A perinuclear halo is characteristic of plasma cells
Plasma cells are formed within germinal center of
lymphoid follicle Cytokines and growth factors are released from
macrophages
B-Cells & Antigens
Is an immunoglobulin Formed by plasma cells (activated B-Cells) in
response to antigenic stimulation Is a protein Made up of two light and two heavy chains Binds with antigen result in:
Neutralization Agglutination Precipitation, etc.
Antibody
Composed of four chains of polypeptide Joined together by disulphide linkages Hinge region Two fragments
Fab fragment Binds with antigen
Fc fragment Performs certain cellular functions
Binds to cell receptors Complement activation others
Structure of Antibody
Four chains are Two heavy chains (alpha, gamma, mu, delta, epsilon) Two light chains (kappa and lambda)
Each heavy chain composed of four domains Three constant domains (CH) & one variable domain (VH)
Each light chain composed of one constant, and one variable domain
The variable domains of both the light and heavy chains contain a hypervariable segment (site for binding with antigen)
It varies in various antibodies An antibody molecule can bind with two molecules of
antigen
Structure of Antibody
Five classes of Antibody
IgG Gamma γ heavy chain IgA Alpha α heavy chain IgM Mu μ heavy chain IgD Delta δ heavy chain IgE Epsilon ε heavy chain
Classes of Antibody
Isotypes:
Are defined by antigenic differences in their constant regions IgG, IgM, IgA, IgD, IgE
Allotypes: Are additional antigenic features that vary
among individuals Idiotypes:
Antigenic determinants formed by specific amino acids in the hypervariable region
Isotype, Allotype, Idiotype
Two Gamma heavy chains, two kappa or lambda
light chains Secondary response Present as monomer (Bivalent) Opsonization via its Fc portion Complement activation via classic pathway May cause agglutination Four subtypes
IgG1 IgG2 IgG3 IgG4
IgG
Two mu heavy and two kappa or lambda light
chains Occur in two forms
Pentamer in circulation (decavalent) Monomer on surface of B-Cells as part of BARC
Important functions are: Agglutination Reception of antigen by B-Cells Primary response Activate complement via alternate pathway
IgM
Two alpha heavy, two kappa or lambda light
chains Usually present in secretions (secretory
antibody) Present as dimer (tetravalent) Two antibody molecules are joined together by
a J-chain (joining chain)
IgA
Two delta heavy, two kappa or lambda light
chains Occurs as monomer Traceable amounts in plasma No known biological function May be part of BARC
IgD
Two epsilon heavy, two kappa or lambda light
chains Present in trace amounts in normal individuals Levels are higher in atopic person Binds to allergen by Fab fragment Binds to mast cells through their Fc portion Induces Type I Hypersensitivity reactions
(Allergic reactions e.g. allergic asthma)
IgE
Induction of humoral immunity via antibody
synthesis from plasma cells Agglutination, neutralization, precipitation,
complement activation, opsonization, etc B-Cells are provided with both MHC I & II
molecules, therefore are one of the most potent antigen presenting cells to T-Cells, after macrophages
Functions of B-Cells
Assignment to be completed
Differences between B-Cells & T-Cells
MHC
MHC-II