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