Ab are glycoprotein molecule producedby plasma cells in response to an Agand react specifically in an observablemannerFUNCTIONS-Ag bindingEffector functions- Complementfixtn,other cells fixation
› Soluble: secreted in blood and tissue› Membrane-bound: found on surface of B- cell, also known as a B-cell receptor (BCR)- BCR binds circulating antigen, activating the B-cell and forming plasma cells or memory B-cells- Epitope-Ag- Paratope-Ab- Idiotype-Antigenic determinant on paratope
Monomer: A flexible Y-shaped molecule withfour protein chains: 2 identical light chains 2 identical heavy chainsEach heavy and light chain has a constantand variable regionThe variable region binds the antigen in a“lock-and-key” mannerL chains : 2 forms – kappa (κ) & lambda (λ)Each molecule of Ig can have either κ or λ,but never both.
Antibodies can also be divided into tworegions based on their function › Fab (fragment, antigen binding) region. Tip of the antibody Binds the antigen › Fc (fragment, crystallizable) region › Determines biological properties of Ig molecule. Base of the antibody Can bind cell receptors, complement proteins and other molecules
H chain designated by Greek letter.– 5 different types: IgA, IgD, IgE, IgG, IgM › IgM µ (mu) › IgD δ (delta) › IgG γ (gamma) › IgA α (alpha) › IgE ε (epsilon)H chain also divided into VH & CH regions; the CH regionis further divided into CH1, CH2 & CH3.Regions also called as DOMAINS : - globular in shape - stabilized by intrachain disulphide bondsAg binding sites are located in the variable domains.
IgG- PROTECTS BODY FLUIDSIgA- PROTECTS BODY SURFACEIgM- PROTECTS BLOOD STREAMIgE- MEDIATES REAGINIC HYPERSENSITIVITYIgD- Recognisation receptor for Ag
Structure: MonomerPercentage serum antibodies: 80%Location: Blood, lymph, intestineHalf-life in serum: 23 daysComplement Fixation: YesPlacental Transfer: Yes only AbMajor Ab of secondary response, found both inserum & body fluids.4 subclasses found in humans – IgG1, IgG2,IgG3 & IgG4, each having a distinct type ofgamma chainFunctions: Enhances phagocytosis,neutralizes toxins and viruses, protects fetusand newborn.
Structure: PentamerPercentage serum antibodies: 5-10%Location: Blood, lymph, B cell surface(monomer)Half-life in serum: 5 daysComplement Fixation: YesPlacental Transfer: Noprimary immune response.Functions: First antibodies produced duringan infection. Effective against microbes andagglutinating antigens. Useful in the diagnosisof congenital infections like syphilis, rubella, HIV,dengue,toxoplasmosis etc.
Structure: Dimer second most abundunt AbLocation: Secretions (colostrum,tears, saliva,intestine, milk), blood and lymph.Half-life in serum: 6 daysComplement Fixation: NoPlacental Transfer: NoOccur in 2 forms : IgA1 & IgA2Secretory IgA is always in dimeric form – composedof 2 basic chain units, a J chain & the secretorycomponent.Secretory component helps to transport the dimerfrom the submucosa to the mucosal cell surfaceFunctions: Localized protection of mucosalsurfaces. Provides immunity to infant digestivetract.
Structure: Monomer resemble Ig GPercentage serum antibodies: 0.2%Location: B-cell surface, blood, andlymphHalf-life in serum: 3 daysComplement Fixation: NoPlacental Transfer: NoFunctions: In serum function is unknown.Occurs along with Ig M on the surface of Bcell- initiate immune response.
Structure: Monomer LOW LEVEL IN SERUMPercentage serum antibodies: 0.002%Location: linings of respiratory & intestinaltracts.Bound to mast cells and basophilsthroughout body. Blood.Half-life in serum: 2 daysComplement Fixation: NoPlacental Transfer: NoFunctions: anaphylactic type ofhypersensitivity ,Allergic reactions.Possibly lysis of worms.
› B cells develop from stem cells in the bone marrow of adults (liver of fetuses).› After maturation B cells migrate to lymphoid organs (lymph node or spleen).› Clonal Selection: When a B cell encounters an antigen it recognizes, it is stimulated and divides into many clones called plasma cells, which actively secrete antibodies.› Each B cell produces antibodies that will recognize only one antigenic determinant.
Programmed cell death (“Falling away”). › Human body makes 100 million lymphocytes every day. If an equivalent number doesn’t die, will develop leukemia. › B cells that do not encounter stimulating antigen will self-destruct and send signals to phagocytes to dispose of their remains. › Many virus infected cells will undergo apoptosis, to help prevent spread of the infection.
Structurally similar proteins in serum seenin certain pathological conditions.Bence Jones protein in multiple myeloma– light chains of Igs.Cryoglobulinemia – formation of gel or ppton cooling the serum which redissolveson warming – in myelomas, SLE etc.
An individual produces a large number ofAbs to cope with the vast number of differentAgs.This Ab diversity is due to the Ig genes.Genes coding for the variable & constantportions of the chains are separateOne or only few genes code for C regionwhereas many genes code for the V region.
Multiple V- region genes. V-J & V-D-J recombination. Junctional diversity1. Nucleotide addition – extra nucleotides may get inserted between VH & D, and between D & JH segments Somatic mutation – point mutation in the genes for V domain.