In opsonization, the pathogens are marked prior to being destroyed while in neutralization, the effect of the pathogen is neutralized. Immunologic responses can be innate or adaptive. Pathogens possess pathogen recognition receptors, which make it easier to be identified by the host.
In opsonization, the host produces opsonins. However, in neutralization, the host produces neutralizing antibodies to neutralize the effect of the antibody-antigen reaction.
Overview and Key Difference 2. What is Opsonization 3. What is Neutralization 4. Similarities Between Opsonization and Neutralization 5. Opsonization is the process which removes pathogens from the system upon being marked by means of opsonins. Opsonins are molecules that can recognize pathogens. Pathogens possess pathogen recognition receptors. Moreover, opsonins are present in phagocytes and participate in recognizing pathogen recognition receptors.
Some examples of opsonins are receptors such as Fc receptor and complement receptor 1 CR1 , etc. Opsonins also have the capability to induce the complement pathway and activate phagocytosis. Opsonins bind to the epitope of a pathogen. When the opsonins bind to the pathogen, the phagocytes attract to the pathogen and facilitate phagocytosis.
Opsonization can also activate adaptive immune responses. In this regard, the antibody IgG binds to the opsonized pathogen. Thus, this allows the antibody-dependent cell-mediated cytotoxicity in cells. In the absence of opsonins, inflammation can take place and damage healthy tissues during infection.
In immunology, neutralization means neutralizing the effect of an antigen by an antibody. The antibodies that participate in these reactions are called neutralizing antibodies. Lipids and nucleic acids are the least antigenic molecules, and in some cases may only become antigenic when combined with proteins or carbohydrates to form glycolipids, lipoproteins, or nucleoproteins.
One reason the three-dimensional complexity of antigens is so important is that antibodies and T cells do not recognize and interact with an entire antigen but with smaller exposed regions on the surface of antigens called epitopes. For example, the bacterial flagellum is a large, complex protein structure that can possess hundreds or even thousands of epitopes with unique three-dimensional structures. Moreover, flagella from different bacterial species or even strains of the same species contain unique epitopes that can only be bound by specific antibodies.
Whereas large antigenic structures like flagella possess multiple epitopes, some molecules are too small to be antigenic by themselves.
Such molecules, called haptens, are essentially free epitopes that are not part of the complex three-dimensional structure of a larger antigen. For a hapten to become antigenic, it must first attach to a larger carrier molecule usually a protein to produce a conjugate antigen. The hapten-specific antibodies produced in response to the conjugate antigen are then able to interact with unconjugated free hapten molecules. Haptens are not known to be associated with any specific pathogens, but they are responsible for some allergic responses.
For example, the hapten urushiol, a molecule found in the oil of plants that cause poison ivy, causes an immune response that can result in a severe rash called contact dermatitis. Similarly, the hapten penicillin can cause allergic reactions to drugs in the penicillin class. Antibodies also called immunoglobulins are glycoproteins that are present in both the blood and tissue fluids. A disulfide bond is a covalent bond between the sulfhydryl R groups found on two cysteine amino acids. The two largest chains are identical to each other and are called the heavy chains.
The two smaller chains are also identical to each other and are called the light chains. Joined together, the heavy and light chains form a basic Y-shaped structure. The amino acid sequence in the variable region dictates the three-dimensional structure, and thus the specific three-dimensional epitope to which the Fab region is capable of binding.
Although the epitope specificity of the Fab regions is identical for each arm of a single antibody molecule, this region displays a high degree of variability between antibodies with different epitope specificities. Binding to the Fab region is necessary for neutralization of pathogens, agglutination or aggregation of pathogens, and antibody-dependent cell-mediated cytotoxicity.
The constant region of the antibody molecule includes the trunk of the Y and lower portion of each arm of the Y. The constant region of an antibody molecule determines its class, or isotype. IgG penetrates efficiently into tissue spaces, and is the only antibody class with the ability to cross the placental barrier, providing passive immunity to the developing fetus during pregnancy. IgM is initially produced in a monomeric membrane-bound form that serves as an antigen-binding receptor on B cells.
The secreted form of IgM assembles into a pentamer with five monomers of IgM bound together by a protein structure called the J chain. IgM is the first antibody produced and secreted by B cells during the primary and secondary immune responses, making pathogen-specific IgM a valuable diagnostic marker during active or recent infections.
IgA can also be found in other secretions such as breast milk, tears, and saliva. Secretory IgA is assembled into a dimeric form with two monomers joined by a protein structure called the secretory component. One of the important functions of secretory IgA is to trap pathogens in mucus so that they can later be eliminated from the body. Similar to IgM, IgD is a membrane-bound monomer found on the surface of B cells, where it serves as an antigen-binding receptor.
However, IgD is not secreted by B cells, and only trace amounts are detected in serum. These trace amounts most likely come from the degradation of old B cells and the release of IgD molecules from their cytoplasmic membranes. IgE is the least abundant antibody class in serum.
Like IgG, it is secreted as a monomer, but its role in adaptive immunity is restricted to anti-parasitic defenses. The Fc region of IgE binds to basophils and mast cells. The Fab region of the bound IgE then interacts with specific antigen epitopes, causing the cells to release potent pro-inflammatory mediators. The inflammatory reaction resulting from the activation of mast cells and basophils aids in the defense against parasites, but this reaction is also central to allergic reactions see Diseases of the Immune System.
These functions include neutralization of pathogens, opsonization for phagocytosis, agglutination, complement activation, and antibody-dependent cell-mediated cytotoxicity. For most of these functions, antibodies also provide an important link between adaptive specific immunity and innate nonspecific immunity.
Neutralization involves the binding of certain antibodies IgG, IgM, or IgA to epitopes on the surface of pathogens or toxins, preventing their attachment to cells.
For example, Secretory IgA can bind to specific pathogens and block initial attachment to intestinal mucosal cells. Similarly, specific antibodies can bind to certain toxins, blocking them from attaching to target cells and thus neutralizing their toxic effects. As described in Chemical Defenses, opsonization is the coating of a pathogen with molecules, such as complementfactors, C-reactive protein, and serum amyloid A, to assist in phagocyte binding to facilitate phagocytosis.
IgG antibodies also serve as excellent opsonins, binding their Fab sites to specific epitopes on the surface of pathogens. IgG has two Fab antigen-binding sites, which can bind to two separate pathogen cells, clumping them together. When multiple IgG antibodies are involved, large aggregates can develop; these aggregates are easier for the kidneys and spleen to filter from the blood and easier for phagocytes to ingest for destruction.
The pentameric structure of IgMprovides ten Fab binding sites per molecule, making it the most efficient antibody for agglutination. Another important function of antibodies is activation of the complement cascade. As discussed in the previous chapter, the complement system is an important component of the innate defenses, promoting the inflammatory response, recruiting phagocytes to site of infection, enhancing phagocytosis by opsonization, and killing gram-negative bacterial pathogens with the membrane attack complex MAC.
Complement activation can occur through three different pathways see [link] , but the most efficient is the classical pathway, which requires the initial binding of IgG or IgM antibodies to the surface of a pathogen cell, allowing for recruitment and activation of the C1 complex. Yet another important function of antibodies is antibody-dependent cell-mediated cytotoxicity ADCC , which enhances killing of pathogens that are too large to be phagocytosed.
The effector cell then secretes powerful cytotoxins e. T cells C. B cells. B cells B. T-cell receptors B. B-cell receptors C. There are two critically important aspects of adaptive immunity.
Learning Objectives Define memory, primary response, secondary response, and specificity Distinguish between humoral and cellular immunity Differentiate between antigens, epitopes, and haptens Describe the structure and function of antibodies and distinguish between the different classes of antibodies. Clinical Focus: Part 1 Olivia, a one-year old infant, is brought to the emergency room by her parents, who report her symptoms: excessive crying, irritability, sensitivity to light, unusual lethargy, and vomiting.
What tests might be ordered to try to diagnose the problem? Explain the difference between a primary and secondary immune response.
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