Microbiology 12 - Antibodies PDF

Microbiology Immunology Antibody characteristic features Microbiology| Antibody characteristic features Contents : Antibodies 3 Classes of Antibodies 7 Microbiology| Antibody characteristic features Antibodies (Ab) are Y- shaped protein structures produced by cells of the B- lymphocyte lineage, comp...

Microbiology Immunology Antibody characteristic features Microbiology| Antibody characteristic features Contents : Antibodies 3 Classes of Antibodies 7 Microbiology| Antibody characteristic features Antibodies (Ab) are Y- shaped protein structures produced by cells of the B- lymphocyte lineage, composed of two large heavy chains and two smaller light chains, stabilized with disulphide bonds. Each chain is composed of variable (V) and constant (C) domains based on variability in the amino acid sequence. Microbiology| Antibody characteristic features The light chain is composed of one constant (CL) and one variable region (VL). In contrast heavy chain is composed of one variable (VH) domain and several constant domains (CH1, CH2, CH3, etc.) based on the class of the antibody. The main functional domains are the relatively constant Fc region, which interacts with cellular Fc receptors and complement and the Fab region, which contains the antigen binding site. Microbiology| Antibody characteristic features Fc and Fab fragments of an antibody molecule (immunoglobulin G). Antibody structure Microbiology| Antibody characteristic features The smallest part that maintains the antigen binding affinity is the variable fragment (Fv), which in recombinant form is commonly stabilized with a 15-amino acid peptide linker. The variable domain of both heavy and light chains is composed of hypervariable loops, known as complementarity determining regions (CDRs) that determine the antibody specificity to bind antigen. Microbiology| Antibody characteristic features There are five different classes of antibodies:IgG, IgM, IgA, IgD, IgE classified according to the differences in the amino acid sequences in the constant region of their heavy chains. 1. IgG antibodies have heavy chains named γ-chains. 2. IgM have μ-chains. 3. IgA have α-chains. 4. IgE have ε-chains. 5. IgD have δ-chains. Microbiology| Antibody characteristic features Based on small differences in the constant region of the heavy chain, antibody classes are further divided into subclasses, for instance subclasses of IgGs are IgG1, IgG2, IgG3 and IgG4 in humans. There are also two light chain types, κ and λ, which will be of the same type in any particular antibody molecule. Microbiology| Antibody characteristic features As mentioned above, the development of hybridoma techniques, that involve immunizing mice with a target antigen and then fusing mouse lymphocytes with myeloma cell line cells, is used to generate murine monoclonal antibodies. However, as human anti- mouse antibody (HAMA) responses develop in patients treated with murine antibodies, chimeric monoclonal antibodies were generated using genetic engineering technology, where the mouse variable region (V) of both heavy and light chains are combined with the constant region (C) of the heavy and the light chains from human antibodies. Microbiology| Antibody characteristic features Although these chimeric monoclonal antibodies exhibited reduced immunogenicity, however, the HAMA responses developed in patients were still considerable. In the next leap forward, humanized monoclonal antibodies were generated from grafting of only the complementary determining regions (CDRs) of a mouse antibody onto human variable regions. Microbiology| Antibody characteristic features With the advent of phage display technology, this was taken further toward generation of fully human antibodies by which genes encoding for the Fab or Fv fragments of human antibodies are expressed in bacteriophage, and subsequently selected and expressed in E.coli. In addition to phage display, fully human antibodies are also generated after immunizing transgenic mouse strains that express human variable domains. Microbiology| Antibody characteristic features The immunogenic potential is significantly reduced with both humanized and fully human antibodies that have similar properties to human endogenous IgGs. The transgenic mice have impaired endogenous murine Ig machinery; therefore only human antibodies are expressed. Microbiology| Antibody characteristic features Monoclonal antibodies and polyclonal antibodies are the two varieties of antibodies, which are used in therapeutics as well as in research applications. Both monoclonal and polyclonal antibodies interact with the same antigen. The main difference between monoclonal and polyclonal antibodies is that monoclonal antibodies are produced by the same clone of plasma B cells, and they bind to a unique epitope whereas polyclonal antibodies are produced by different clones of plasma B cells, and they bind to the different epitopes in the same antigen. Microbiology| Antibody characteristic features In general, naked antibody molecules are rarely potent cytotoxic agents, therefore they can be linked to cytotoxic drugs, toxins, or radionuclides to enhance their anti-cancer activity. The optimal characteristics for efficacious and safe antibody-based therapy are high affinity to the target antigen, internalization following binding, nonimmunogenicity and limited normal tissue expression of the target antigen. Microbiology| Antibody characteristic features Moreover, the isotype of the mAb is another key point to consider in Ab-based therapy design as it will affect whether the targeted therapy has the potential to kill cells via antibody-dependent cytotoxicity (ADCC) and/or complement dependent cytotoxicity (CDC) in addition to the killing induced by cytotoxic agents or toxin. Microbiology| Antibody characteristic features Murine antibody (entirely of murine origin). Chimeric antibody (murine variable region combined with human constant region). humanized antibody (human antibodies with murine CDR region grafted onto the human variable region). Fully human antibody (entirely of human origin). Microbiology| Antibody characteristic features Antibody production :