B CELLS

Innate immunity Adaptive immunity T cells B cells Intracellular pathogens Extracellular pathogens Recognition Communication Elimination Recognition Communication Elimination The simplest Schema of the immune system

THE TWO ARMS OF THE IMMUNE SYSTEM Monocytes, Macrophages, Dendritic cells, Granulocytes, NK cells and Complement components B and T cells Monocytes, Macrophages, Dendritic cells, Granulocytes, NK cells and Complement components

Overview of B cell–mediated immunity

B CELLS Structure of antibody Antigen binding/hypervariable regions Clonal proliferation B cell differentiation, memory cells, plasma cells Antibody-mediated effector functions Izotypes B cell mediated antigen presentation

IMMUNOGLOBULINS IMMUNOGLOBULINS structure and function

STRUCTURE OF ANTIBODY Light chain (L) Heavy chain (H) VL CL VH CH Symmetric core structure 2 identical heavy chain, 2 identical light chain Variable regions  antigen binding Constant regions

STRUCTURE heavy and light chains disulfide bonds – inter-chain – intra-chain hinge region carbohydrate disulfide bond C H1 VLVL CLCL VHVH C H2 C H3

variable and constant regions hinge region domains –V L & C L –V H & C H1 - C H3 (or C H4 ) oligosaccharides hinge region carbohydrate disulfide bond C H1 CLCL VHVH C H2 C H3 VLVL immunoglobulin domenSTRUCTURE

ANTIBODY DOMAINS AND THEIR FUNCTIONS Constant domain Effector functions Antigen recognition Variable domain Two identical binding site Heavy chain and light chain compose the antigen binding surface

THE ROLE OF THE HINGE REGION

RIBBON STRUCTURE OF IGG

B cells Structure of antibody Antigen binding/hypervariable regions Clonal proliferation B cell differentiation, memory cells, plasma cells Antibody-mediated effector functions Izotypes B cell mediated antigen presentation

VL VH ANTIGEN BINDING Variable domens responsible for antigen binding

DIFFERENT VARIABLE REGIONS  DIFFERENT ANTIGEN-BINDING SITES  DIFFERENT SPECIFICITIES

3 CDR regions in a V domain V H & V L domains  3+3 CDR HYPERVARIABLE REGION – COMPLEMENTARY DETERMINING REGION (CDR) COMPLEMENTARY DETERMINING REGION (CDR)

HYPERVARIABLE REGION – COMPLEMENTARY DETERMINING REGION (CDR)

B cells Structure of antibody Antigen binding/hypervariable regions Clonal proliferation B cell differentiation, memory cells, plasma cells Antibody-mediated effector functions Izotypes B cell mediated antigen presentation

Cc. (minimum) 10 million various (10 7 ) B lymphocyte clones with different antigen-recognizing receptors Cc. (minimum) 10 – 1000 million various ( ) T lymphocyte clones with different antigen-recognizing receptors DIVERSITY OF LYMPHOCYTES All lymphocytes have a different receptor lymphocytes in our body ( B and T lymphocytes )

Several antibodies are expressed on B cells, (arround ) but all of them with the same specificity

Activation Clonal expansion B cell Antigen receptor, BCR Ag Clonal antigen receptors are expressed exclusively on T- and B lymphfocyties. ANTIGEN RECOGNITION BY SPECIFIC BCR INDUCES CLONAL EXPANSION OF THE SPECIFIC B CELLS.

ANTIGEN RECOGNITION BY SPECIFIC BCR INDUCES CLONAL EXPANSION AND DIFFERENTIATION OF THE SEPCIFIC B CELLS.

Primary lymphoid organs : - Bone marrow - Thymus Secondary lymphoid organs: - Spleen - Lymphatic vessels - Lymph nodes - Adenoids and tonsils - MALT (Mucosal Associated Lymphoid Tissue) GALT (Gut Associated Lymphoid Tissue) BALT (Bronchus Associated Lymphoid Tissue) SALT (Skin Associated Lymphoid Tissue) NALT (Nasal Associated Lymphoid Tissue) LYMPHOID ORGANS

LYMPHOCYTES REACTING WITH SELF ANTIGEN DURING THEIR DEVELOPMENT IN THE PRIMARY LYMPHOID ORGANS, BECOME INACTIVATED OR DIE BY APOPTOSIS.

Antibodies are natural products that appear on the cell surface as receptors and selectively react with the antigen. Lymphocyte receptors are variable and carry various antigen-recognizing receptors. ‘Non-self’ antigens/pathogens encounter the existing lymphocyte pool (repertoire). Antigens select their matching receptors from the available lymphocyte pool, induce clonal proliferation of specific clones and these clones differentiate to antibody secreting plasma cells. The clonally distributed antigen-recognizing receptors represent about ~10 7 – 10 9 distinct antigenic specificities. MACFARLANE BURNET ( ) CLON SELECTION HYPOTHESIS

- Lymphocytes are monospecific cells - Antigen engagemnt result in the activation of lymphocytes - Activated lymphocytes differentiate and proliferate but keep their antigen specificity - Lymphocytes reacting with self antigen during their development in the primary lymphoid organs, become inactivated or die by apoptosis.

1.BCR (cell surface antibody) recognize the antigen 2. Clonal proliferation of specific B cells 3. Differenciation of activated B cells to plasma cell (antibody production) or memory cell 4. To distinguish self-nonself is with the selection and killing of self dangerous clones Activated B cells Plasma cells Ag

B cells Structure of antibody Antigen binding/hypervariable regions Clonal proliferation Antibody-mediated effector functions Izotypes B cell mediated antigen presentation

B cell: Antibody on the cell surface (BCR) function: cell activation Plasma cell: production of antibody antibod y-mediated effector functions TWO FORMS OF ANTIBODY -cell surface (BCR) -soluble (on the surface of plasma cells antibody is not expressed) Cell surface and soluble antibodies recognize the same antigen

ANTIBODY BCR (B cell receptor) MEMBRANE BOUND! Associated chains for signaling Transmembrane domain Cytoplasmic domain Antigen recognition and B cell activation SOLUBLE (freely circulating) Antigen recognition and effector functions. Produced by plasma cells

B cell B CELL ACTIVATION BCR oligomerization results in B cell activation, proliferation and differentiation !

Soluble antibody-mediated functions

VL VH ANTIGEN BINDING Variable domens responsible for antigen binding

detect antigen block the active sites of toxins or pathogen-associated molecules block interactions between host and pathogen- associated molecules The variable domain can:

immune complex

Complement proteins The conception of immune complex

Complement proteins IMMUNCOMPLEX complexe of (1)antigens-(2)antibodies (3)complement components complex

detect antigen Neutralization: block the active sites of toxins or pathogen-associated molecules block interactions between host and pathogen- associated molecules The variable domain can:

NEUTRALIZATION Covering of the pathogen’s surface prevents replication and growth

Crucial factors of anti-viral response 1.Type I. Interferons 2.NK cells 3.Cytotoxic T cells 4.Neutralizing antibodies

ANTIBODY-MEDIATED EFFECTOR FUNCTIONS: 1. Neutralization (variable domen) Fc part: 2. Complement activation Via opsonization: 3. Phagocytosis 4. ADCC (antibody dependent celular cytotoxicity) 5. (mast cell degranulation)

WHY DO ANTIBODIES NEED AN FC REGION? inflammatory and effector functions associated with cells inflammatory and effector functions of complement the trafficking of antigens into the antigen processing pathways Fc region can activate

IMMUNOGLOBULIN FRAGMENTS: STRUCTURE/FUNCTION RELATIONSHIPS antigen binding complement binding site placental transfer binding to Fc receptors Fc or constant region

Pathogen recognition by innate immune system 1.Directly via PRR 2.Indirectly via opsonization Receptors for opsonin recognition

FC RECEPTORS RECOGNIZE THE CONSTANT (FC) PART OF ANTIBODIES !

Expression of Fc receptors on the cell surface is constitutive (relativelly) Fc receptors are not activated by free/lonely antibody but by immunocomplexes FC RECEPTORS

ANTIBODY-MEDIATED EFFECTOR FUNCTIONS: 1. Neutralization (variable domen) Fc part: 2. Complement activation Via opsonization: 3. Phagocytosis 4. ADCC (antibody dependent celular cytotoxicity) 5. (mast cell degranulation)

COMPLEMENT ACTIVATION GENERATES INFLAMMATION OPSONIZATION KlLLING of PATHOGEN REMOVE IMMUNKOPLEXES

ANTIBODY-MEDIATED EFFECTOR FUNCTIONS: 1. Neutralization (variable domen) Fc part: 2. Complement activation Via opsonization: 3. Phagocytosis 4. ADCC (antibody dependent celular cytotoxicity) 5. (mast cell degranulation)

OPSONIZATION Flagging a pathogen Antigen binding portion (Fab) binds the pathogen, the Fc region binds phagocytic cells Fc- receptors speeding up the process of phagocytosis Opsonins: ANTIBODY Complement components Acute phase proteins

ANTIBODY-MEDIATED EFFECTOR FUNCTIONS: 1. Neutralization (variable domen) Fc part: 2. Complement activation Via opsonization: 3. Phagocytosis 4. ADCC (antibody dependent celular cytotoxicity) 5. (mast cell degranulation)

DEGRANULATION OF NK CELLS ANTIBODY DEPENDENT CELLULAR CYTOTOXICITY (ADCC)

ANTIBODY-MEDIATED EFFECTOR FUNCTIONS: 1. Neutralization (variable domen) Fc part: 2. Complement activation Via opsonization: 3. Phagocytosis 4. ADCC (antibody dependent celular cytotoxicity) 5. (mast cell degranulation)

INNATE IMMUNITY Killing: Phagocytosis Soluble mediators Complement system NK cells Antibody-mediated effector functions accelerates and facitlitates the effector functions of innate immune system

B cell memory:  Quicker response  Increase in the number of specific B cell  The amount of antibody are higher  Higher affinity antibodies (‘more specific’)  Isotype switch In case of T dependent B cell activation

During Memory response:

B cells Structure of antibody Antigen binding/hypervariable regions Clonal proliferation Antibody-mediated effector functions Izotypes B cell mediated antigen presentation

ANTIBODY DOMAINS AND THEIR FUNCTIONS Constant domain Effector functions Antigen recognition Variable domain

HUMAN IMMUNOGLOBULIN CLASSES HUMAN IMMUNOGLOBULIN CLASSES encoded by different structural gene segments (isotypes) IgG - gamma ( γ ) heavy chains IgM - mu ( μ ) heavy chains IgA - alpha ( α ) heavy chains IgD - delta ( δ ) heavy chains IgE - epsilon ( ε ) heavy chains light chain types kappa ( κ ) lambda (λ)

Expression of Fc receptors on the cell surface is constitutive (relativelly) Different cells express various Fc receptors Antibodies with diferent izotype activates distinct cells, effector functions Fc receptors are not activated by free/lonely antibody but by immunocomplexes FC RECEPTORS

FcE receptor (for IgE) on mast cells initiate allergic rections

IZOTYPE SWITCH

MAIN CHARACTERISTICS OF ANTIBODY ISOTYPES free IgM pentamer (star shape) Antigen bound IgM (crab shape)

B cells Structure of antibody Antigen binding/hypervariable regions Clonal proliferation Antibody-mediated effector functions Izotypes B cell mediated antigen presentation

The antigen presentation of B cells is a BCR-dependent process!

ANTIGEN PRESENTATION OF B CELLS

+++ B CELL-MEDIATED ANTIGEN PRESENTATION

ANTIBODY-MEDIATED FUNCTIONS: Cell surface (BCR): -antigen recognition -B cell activation -(antigen presentation) Soluble: effekctor functions 1. Neutralization (variable domen) Fc part: 2. complement activation Via opsonization: 3. Phagocytosis 4. ADCC

cell surface antigen receptor on B cells (BCR) allows B cells to sense their antigenic environment connects extracellular space with intracellular signalling machinery secreted antibody neutralization opsonization complement fixation NK cell –mediated killing IMMUNOGLOBULIN STRUCTURE-FUNCTION RELATIONSHIP

Innate immunity Adaptive immunity T cells B cells Intracellular pathogens Extracellular pathogens Recognition Communication Elimination Recognition Communication Elimination The simplest Schema of the immune system