Mucosal immunity: Mucosal epithelial cells are an integral part of the innate immune system, participating in the response to pathogens and capable of delivering antigens to immune cells. Epithelial cells can prevent antigenic substances with larger diameters from entering. Mucosal immunity affects various environmental and immune signals, thereby altering the function of the mucosal barrier.

Characteristics: Accumulation of white blood cells, plasma proteins, and fluid extracted from the blood at extravascular tissue sites of infection or injury. Leukocytes and plasma proteins normally circulate in the blood and are recruited to sites of infection and injury to perform a variety of effector functions, killing microorganisms and repairing tissue damage. The most abundant leukocytes recruited from the blood to sites of acute inflammation are neutrophils, and important plasma proteins that enter sites of inflammation include complement proteins, antibodies, and acute phase reactants. The accumulation of these blood-borne components into sites of inflammation relies on reversible changes in blood vessels in infected or damaged tissue, all of which are induced by mediators such as cytokines.

Cytokines: Highly active and multifunctional small molecule substances secreted by activated cells. They mainly mediate and regulate immune responses and inflammation.

Adhesion molecules: (The recruitment of white blood cells and plasma proteins from the blood to sites of infection and tissue damage is a major process in the inflammatory response). The homing and recruitment of leukocytes requires the temporary adhesion of leukocytes to the vascular endothelium. This process is mediated by a series of molecules on the surface of leukocytes and vascular endothelial cells that promote intercellular adhesion. This series of molecules is called adhesion molecules.

Immunoglobulin Ig: A globulin with antibody activity or a chemical structure similar to that of an antibody. Antibodies are immunoglobulins, but that doesn't hold true anyway.

Structure: The basic structure is two identical heavy chains (H chains) with larger relative molecular masses and two identical light chains (L chains) with smaller relative molecular masses. The heavy chains and light chains are separated by The interchain disulfide bonds form a symmetrical "Y"-shaped structure. Heavy chains can be divided into five categories based on differences in immunogenicity of the constant regions, and they are respectively involved in the composition of five types of immunoglobulins. Light chains can be divided into two categories based on their immunogenicity. The part with the most drastic sequence changes in the variable region is called the hypervariable region. The hypervariable regions of VH and VL together form the antigen-binding site, which determines the specificity of the antibody and is responsible for recognizing and binding antigens. This region forms a spatial conformation complementary to the antigenic epitope, also known as the complementarity determining region. The hypervariable region is also the idiotype or idiotype antigenic determinant of the Ig. The amino acid sequence of the region outside the hypervariable region is relatively conservative and is called the "backbone region", which can stabilize the configuration of the complementarity-determining region and facilitate the binding of the complementarity-determining region to the antigenic epitope. Constant region refers to the same type of antibodies produced by individuals of the same species against different antigens. Although the V regions are different, the amino acid sequence of the C region is relatively constant and has the same immunogenicity. Hinge region: facilitates binding of two epitopes by two arms J chain: Connect Ig monomers at the carboxyl end of the H chain to form dimers or multimers. Secretory patch SP: synthesized and secreted by mucosal epithelial cells, and binds to CH2 of dimeric IgA through covalent bonds, making it secretory IgA. After sIgA is secreted onto the mucosal surface, it exerts mucosal immunity. SP can cover the hinge region and protect it from hydrolysis by proteolytic enzymes. Papaya hydrolase: It can hydrolyze the antibody at the amino terminus of the interchain disulfide bond in the hinge region of the heavy chain, forming two identical monovalent antigen-binding fragments and one crystallizable fragment. Pepsin:

Diversity: isotypes, allotypes, and unique types.

Biological activity: 1. Specific binding to antigens. Antibodies have the function of neutralizing toxins and blocking the invasion of pathogens, that is, neutralization. However, they cannot dissolve or kill target cells carrying antigens and require complement or phagocytes. effect to clear antigens or pathological damage. 2. Activate complement. 3. Opsonization: refers to the fact that the antibody that binds to the granular antigen binds to the FcR on the surface of phagocytes through the Fc segment, activating macrophages and enhancing their phagocytic killing ability, and the "bridging" of the antibody enables the granular antigen to be anchored to the phagocyte. On the surface, it is easier to be swallowed. 4. Antibody-dependent cell-mediated cytotoxicity ADCC: refers to that after the IgG binds to the target cell antigen, its Fc segment can bind to the FcgR of effector cells such as NK cells, macrophages, and monocytes to promote the release of cytotoxic particles. , causing target cell lysis. 5. Mediate type I hypersensitivity reactions.

Characteristics and functions of five types of antibodies

Development and maturity of B cells: During this period, progenitor B cells begin to rearrange their Ig heavy chain genes, but the light chain genes have not yet begun to rearrange, and begin to express Igα and Igβ. Pre-B cells begin to express an alternative light chain, which combines with the μ chain to form the pre-B cell receptor, which is a surface marker of pre-B cells and can promote B cell proliferation. Pre-B cells continue to develop, complete Ig light chain gene rearrangement, and form mIgM expression and cell surface. The B cell antigen receptor complex composed of mIgM and Igα/Igβ heterodimer is a surface marker of immature B cells. Immature B cells undergo negative selection and continue to develop into mature B cells. The simultaneous expression of mIgM and mIgD molecules is a surface marker of mature B cells, and the variable regions of these two mIg are exactly the same.

B lymphocyte activation The specific antigen signal is the first signal for B cell activation, and the co-receptor complex can enhance the first signal. The first signal can upregulate the expression of MHC class II molecules and costimulatory molecule B7 in B cells, enhancing the ability of B cells to activate Th cells. Costimulatory signal is the second signal of B cell activation. Activated Th cells can inducibly express CD40L, and CD40L interacts with CD40 on the surface of B cells to provide the strongest second signal for B cell activation. If the second signal is missing, the B cells will not only fail to be activated by the first signal, but will enter a state of incompetence. Cytokines promote B cell activation. T cell nucleus B cell interaction: Specific B cells and T cells must come into contact with each other to generate a strong antibody response. Specific T cells and B cells must meet and interact at the edge of the T cell area through specific chemotaxis in order to initiate normal humoral immunity.

Proliferation and differentiation of B cells (mainly in germinal centers) Extrafollicular activation of B cells: Extrafollicular activation of B cells initiates an early response to protein antigens and establishes a germinal center response. B cells are activated after recognizing antigens in lymph follicles, and some of them enter the extrafollicular area of ​​peripheral lymphoid tissue, differentiate into IgM-producing plasma cells, secrete IgM antibodies, and participate in the body's immediate defense response. Affinity antibodies produced by extrafollicular activated B cells can limit the spread of infection. Extrafollicular responses also contribute to the generation of follicular helper T cells. Four to seven days after the initiation of T cell-dependent B cell responses, a small number of B cells activated outside the follicles migrate back to the follicles and proliferate rapidly, forming "germinal centers." The germinal center is formed by the clonal expansion of one or several antigen-specific B cells and is divided into light and dark areas. The formation of these structures depends on the presence of FDC and the interaction between Tfh's CD40L and B-cell CD40.

Class switching: During the TD-Ag response, after activation of B cells expressing IgM and IgD, some progeny cells change the constant region of the heavy chain and produce antibodies containing different classes of heavy chains, but the specificity of the antibody remains unchanged. This process is class switching or isotype switching. Bacteria and viruses activate T cells to produce cytokines, promoting the conversion of IgM into IgG, while parasite infections activate T cells to produce IL-4, producing humoral immunity dominated by IgE antibodies. Staphylococcus aureus activates T cells to produce cytokines, which promotes the conversion of IgM into IgG types, resulting in class switching.

Classical pathway: The earliest discovered complement activation pathway, which mainly functions in the middle and late stages of primary infection. Mainly activated by antigen-antibody complexes, IgM or IgG binds to the corresponding antigen, and there must be two or more Fc regions binding to C1. It is divided into startup phase, activation phase and effect phase.

Alternative pathway: It is the oldest form of complement activation, plays a role in early infection or primary infection, and is an important effector mechanism of innate immunity.

Mannan-binding lectin pathway

Genetic characteristics: 1. Haplotype inheritance and co-dominance. 2. Polymorphism. 3. Linkage disequilibrium: refers to the phenomenon that the probability of alleles belonging to two or more gene loci appearing on the same chromosome is different from the expected value.

Classification

Molecular Structure

Biological functions: 1. Present antigens, participate in adaptive immune responses, determine the MHC restriction of T cells recognizing antigens, participate in the selection and differentiation of T cells in the thymus, determine individual differences in disease susceptibility, and participate in the formation of population immunity. Heterogeneity of response, involved in transplant rejection. 2. As regulatory molecules, they participate in the innate immune response. The classic MHC class III genes encode complement components and participate in the inflammatory response to kill pathogens. The gene products of non-classical MHC class I molecules and MIC serve as activating receptors or inhibitory receptors for NK cells. Ligand regulates the killing function of NK cells.

T cell antigen receptor complex: A TCR complex consists of a TCR molecule and a CD3 molecule.

Stages of T cell maturation 1. Double negative cell stage: DN cells are T cells before pre-T cells. At this time, T cells express neither CD4 nor CD8 molecules. It does not express T cell antigen receptors and CD3 molecules, cannot recognize antigens, and does not have any functions. Progenitor T cells begin to rearrange T cell antigen receptor genes, which is the branch point of αβ and γδ T cell differentiation. The TCRβ chain completes gene rearrangement, but the α chain genes have not yet been rearranged. The β chain expressed by αβ T cells and the α chain of pre-T cells form pre-T cell antigen receptors. Cells expressing pre-T cell antigen receptors are pre-T cells. Under the induction of cytokines such as IL-7, pre-T cells actively proliferate, express CD4 and CD8 molecules, and continue to enter the DP cell stage. 2. DP cells complete α gene rearrangement and assemble with β chains to form T cell antigen receptors. At this time, cells expressing T cell antigen receptors are immature T cells. Immature T cells undergo positive selection and further differentiate into SP cells. 3. Single-positive cell stage: SP cells undergo negative selection and further develop into MHC-restricted, self-tolerant mature T cells, which enter peripheral immune organs through blood circulation.

immune response process