Antibody Structure and Functions in humoral immunity

White blood cellsLymphoid organPrimary lymphoid organ: Bone marrow & fetal liverorigin of all immune cells site for development and education of B cellsThymus: site for development and education of T cellsSecondary lymphoid organLymph nodes, spleen, lymphoid tissueinduction sites for immune responsesBody tissueseffector sites for immune responsesComponents of the immune system

Blood cells derived from bone marrow cells

Blood cells migrate through blood and lymph nodes or home to tissues

Lymphoid cells

Lymphoid organWhite blood cells migrate through blood, lymph nodes and tissues.Lymphocytes recirculate between blood and lymph organ until encounter their Ag Lymph from body surfaces drains through the draining lymph nodes into the thoracic duct and then into the blood circulation.

Clonal selectionLymphocyte development and maturationLymphocyte activation

Lymphocyte proliferate and differentiate into effector and memory cellsLymphocyte stimulationLymphocytes change function and morphology on antigen stimulationEffector cells attack invading pathogens

Memory cells protect against a new infection

RecognitionDifferent pathogen require different response mechanism for detection, recognition & destroying themLymphocyte receptor:BCR (surface Ig): recognize Ag that is present outside the cellsTCR: detect Ag generated inside infected cellEach developing lymphocyte generate a unique Ag receptor by rearranging its gene segment:

Cells interaction: 2 signal are required for lymp activation

Antigen presenting cells (APCs)

Effector MechanismT Lymphocytes:

T cells are responsible for the cell mediated imm responseCTL (cytotoxic T lymp) recognize infected cells CTL tipically express molc CD8 on their surface

T cells that activate the cells they recognized are marked by molc CD4 on their surface.CD4 T lymp can be divided into 2 subset :TH1 cells: activate M (stimulating antibacterial mechanism) and release cytokine & chemokineTH2 cells: activate B cells to produce certain Ab (ch.8)

T cell help for macrophages and B cells requires recognition of peptide-MHC II

Molecules involved for antigen recognitionB cell receptor & productantibodies (Abs): immunoglobulin (Ig)T cell receptor (TCR)TCR a/b, g/dMajor histocompatibility complex (MHC)/HLAClass IClass II

Immune responsesNatural/innate/nonspecificHumoral: type I IFN (IFN-a/b), lysozyme, Complement proteinsCellular: phagocytes, NK cellsAdaptive/acquired/specificHumoral: Abs: IgM, IgG, IgA, IgE, IgDCellular: T cells:CD4+ Th, CD8+CTL

Ab molecule: structure of immunoglobulin

Immunoglobulin is a general term for antibodiessImmmunity mediated by Ab: humoral immunityB cells produce antibodies which can recognize soluble antigen An immunoglobulin molecule has two identical H-chains and two identical L-chainsThe variable regions of Abs are encoded by multiple gene fragments

Variable region determine Ag binding specificity

Macrophages and antibodies play a prominent role in the removal of antigens and cell debris

Concluding remarksAb/Ig: glycoproteins produced in membrane-bound or secreted forms by (activated) B cells

Comprises 2 identical covalently-linked Heavy (H) chains and 2 identical Light (L) chains

Fab (antigen binding region) is highly variable; Ab can be produced ca 109/indiv. with distinct specificity

Effector functions: C region of the H chain moleculeOpsonization by binding through FcR on phagocytic cell membraneComplement activation esp. via the classical pathwayAb-dependent cellular cytotoxicity (ADCC) functionAb-mediated type I hypersensitivity reaction

H chain of C region determine also the tissue distribution, ie. IgA isotypesecretion (mainly)

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