Humoral Response PDF

Pre-evaluation What is adaptive immunity, and how does it differ from innate immunity? What are the primary cells involved in adaptive immunity? What are the two main types of adaptive immune responses? Can you explain how antibodies help in the defense against pathogens? What is the role of m...

Pre-evaluation What is adaptive immunity, and how does it differ from innate immunity? What are the primary cells involved in adaptive immunity? What are the two main types of adaptive immune responses? Can you explain how antibodies help in the defense against pathogens? What is the role of memory cells in adaptive immunity? How do vaccines utilize the adaptive immune system to protect against disease? What is antigen presentation, and why is it important for the activation of T cells? What are the differences between helper T cells and cytotoxic T cells? What are the different classes of antibodies, and what roles do they play? The Humoral Immune Response The Immune Response Four features characterize the immune response: specificity, the ability to respond to an enormous diversity of antigens, the ability to distinguish self from nonself, and memory. The immune response is directed against antigens that evade the nonspecific defenses. Each antibody or T cell is directed against a particular antigenic determinant.  There are two interactive immune responses: the humoral immune response and the cellular immune response. The humoral immune response employs antibodies secreted by B cells to target antigens in body fluids. The cellular immune response employs T cells to attack body cells that have been altered by viral infection or mutation or to target antigens that have invaded the body's cells. Clonal selection accounts for the rapidity, specificity, and diversity of the immune response as well as immunological memory and tolerance to self.  The Humoral Immune Response Results in production of proteins called « immunoglobulin’s » or « antibodies » Body exposed to « foreign » material termed « antigen » which may be harmful to body: virus, bacteria, etc. Antigen has bypassed other protective mechanisms, i.e. first and second line of defense  The Humoral Immune Response Activated B cells form plasma cells, which synthesize and secrete specific antibodies. The basic unit of an antibody, or immunoglobulin, is a tetramer of four polypeptides: two identical light chains and two identical heavy chains, each consisting of a constant and a variable region.  The variable regions of the light and heavy chains collaborate to form the antigen-binding sites of a specific antibody. Each antigen usually has several different antigenic determinants (binding sites for specific antibodies). The variable regions determine each antibody's specificity for a determinant; the constant region determines the destination and function of the antibody. There are five immunoglobulin classes. IgM, formed first, is a membrane receptor on B cells, as is IgD. IgG is the most abundant antibody class and performs several defensive functions. IgE takes part in inflammation and allergic reactions. IgA is present in various body secretions. Monoclonal antibodies consist of identical immunoglobulin molecules directed against a single antigenic determinant. The Cellular Immune Response T cell receptor The cellular immune response is directed against altered or infected cells of the body. TC cells attack virus-infected or tumor cells, causing them to lyse. TH cells activate B cells and influence the development of other T cells and macrophages. T cell receptors in the cellular immune response are analogous to immunoglobulins in the humoral immune response. The major histocompatibility complex (MHC) encodes many membrane proteins. MHC molecules in macrophages, B cells, or body cells bind processed antigen and present it to T cells.  In the cellular immune response, class I MHC molecules, TC cells, CD8, and cytokines collaborate to activate TC cells with the appropriate specificity. Developing T cells undergo two tests: They must be able to recognize self MHC molecules, and they must not bind to both self MHC and any of the body's own antigens. T cells that fail either of these tests die. The rejection of organ transplants results from the genetic diversity of MHC molecules. HUMORAL CELLULAR RESPONSE RESPONSE Humoral Response  Primary Antibody Response Several days to weeks lag or latent period after initial exposure to antigen – no antibody detectable in blood After B cell differentiation into plasma cells, antibody is secreted – antibody titer is measure of serum antibody concentration reciprocal of highest dilution of antiserum that gives positive reaction IgM appears first, followed by IgG 19  Secondary Antibody Response Upon secondary exposure to same antigen, B cells mount a heightened, memory response Characterized as having a shorter lag, a more rapid log phase, longer persistence, a higher IgG titer and production of antibodies with a higher affinity for the antigen 20  MHC proteins We know that certain strategies apply both to antibody and T cell receptor genes (gene recombination), others apply to antibody but not T cell receptor genes (gene hypermutation) and that the strategy underlying the ability of MHC proteins to interact with different antigens is gene polymorphism. Thus the genetics of the immune system has been largely unravelled in the last few years.  Evidence for somatic recombination (so-called rearrangement) of antibody and TCR genes The hypothesis that antibody genes resulted from the "fusion" of two different genes (a V gene and a C gene) was put forward in 1965 by W Dreyer and J Bennett. It provided an explanation for the protein sequence data that was accumulating at the time and for serological and genetic observations indicating that the same idiotype could be found in association with different isotypes. The "two genes - one polypeptide" hypothesis was proven correct in the mid seventies by N Hozumi who demonstrated that probes for the C or a V+C light chain mRNA hybridized with different restriction fragments of embryo genomic DNA but hybridized with the same restriction fragment in the case of DNA from the myeloma line. This experiment confirmed that a process of somatic rearrangement had occured in the antibody-producing cell. The Genetic Basis of Antibody Diversity Immunoglobulin heavy-chain supergenes are constructed from one each of numerous V, D, J, and C segments. The V, D, and J segments combine by DNA rearrangement, and transcription yields an RNA molecule that is spliced to form a translatable mRNA. Other gene families give rise to the light chains.  As a result of these DNA rearrangements, there are millions of possible antibodies as a result of these DNA combinations. Imprecise DNA rearrangements, mutations, and random addition of bases to the ends of the DNAs before they are joined contribute even more diversity.  Somatic hypermutation Experimental evidence supports the view that the mechanism of SHM involves deamination of cytosine to uracil in DNA by an enzyme called Activation-Induced (Cytidine) Deaminase, or AID. A cytosine: guanine pair is thus directly mutated to a uracil:guanine mismatch. Uracil residues are not normally found in DNA, therefore, to maintain the integrity of the genome, most of these mutations must be repaired by high- fidelity Base excision repair enzymes. The uracil bases are removed by the repair enzyme, uracil-DNA glycosylase. Error-prone DNA polymerases are then recruited to fill in the gap and create mutations. mRNA Splicing  Diversity of Antibodies Three mechanisms contribute to generation of antibody diversity 1. Rearrangement of antibody gene segments (combinatorial joining) - Genes are split or interrupted into many gene segments 2. Generation of different codons during antibody gene splicing 3. Somatic mutation 27  Combinatorial Joining 1. Segments clustered separately on same chromosomes – exons that code for constant regions – exons that code for variable regions 2. Exons for constant region are joined (spliced together) to one segment of the variable region 3. RAG-1 and RAG-2 are recombination enzymes – process still not fully understood – multiple enzymes involved 4. Occurs on heavy and light chains 28  Light Chain Germ line DNA for light chain contains multiple coding sequences, V and J (joining) In B cell development – one V is joined with one J region – many possible combinations formed – VJ joined with C (constant) exon after transcription 23  Heavy Chain V and J regions are joined to 3rd coding region called D (diversity) sequences VDJ joined to C region after transcription (RNA level) Antibody class switch – initial C region results in IgM but changes as the immune response progresses and B cells proliferate Region containing initial IgM C region is deleted, along with other intervening sequence – Occurs at DNA level, not RNA level (pre-transcription) – Process not fully understood yet, but depends on activation-induced cytidine deaminase (AID) enzyme and other enzymes 24 25 Antibody Diversity Splice site variability – VJ joining can produce polypeptides with different amino acid sequences Somatic mutation of V regions – V regions are susceptible to high rate of somatic mutation during an antigen challenge – produce with different epitope recognition 26