Response to Specific Pathogens That Can Improve With Subsequent Exposure Is

Learning Objectives

• Define memory, master response, secondary response, and specificity
• Distinguish between humoral and cellular immunity
• Differentiate betwixt antigens, epitopes, and haptens
• Describe the structure and function of antibodies and distinguish betwixt the different classes of antibodies

Clinical Focus: Ezra, Part ane

Ezra, a one-year one-time infant, is brought to the emergency room by his parents, who report his symptoms: excessive crying, irritability, sensitivity to calorie-free, unusual sluggishness, and vomiting. A physician feels swollen lymph nodes in Ezra's pharynx and armpits. In addition, the expanse of the abdomen over the spleen is bloated and tender.

• What do these symptoms suggest?
• What tests might be ordered to endeavor to diagnose the problem?

We'll return to Ezra'southward example in subsequently pages.

Adaptive immunity is defined by two of import characteristics: specificity and memory. Specificity refers to the adaptive allowed organisation's power to target specific pathogens, and memory refers to its ability to quickly respond to pathogens to which it has previously been exposed. For example, when an individual recovers from chickenpox, the trunk develops a retentivity of the infection that volition specifically protect information technology from the causative agent, the varicella-zoster virus, if it is exposed to the virus again later on.

Figure 1. Click for a larger image. This graph illustrates the primary and secondary allowed responses related to antibody production after an initial and secondary exposure to an antigen. Notice that the secondary response is faster and provides a much higher concentration of antibody.

Specificity and memory are achieved by essentially programming certain cells involved in the immune response to respond rapidly to subsequent exposures of the pathogen. This programming occurs as a result of the first exposure to a pathogen or vaccine, which triggers a primary response. Subsequent exposures result in a secondary response that is faster and stronger as a effect of the torso'south memory of the get-go exposure (Effigy 1). This secondary response, however, is specific to the pathogen in question. For example, exposure to one virus (e.k., varicella-zoster virus) volition not provide protection confronting other viral diseases (e.g., measles, mumps, or polio).

Adaptive specific immunity involves the actions of two distinct cell types: B lymphocytes (B cells) and T lymphocytes (T cells). Although B cells and T cells arise from a common hematopoietic stem cell differentiation pathway (see Effigy 1 in Cellular Defenses), their sites of maturation and their roles in adaptive amnesty are very different.

B cells mature in the bone marrow and are responsible for the product of glycoproteins called antibodies, or immunoglobulins. Antibodies are involved in the trunk's defence against pathogens and toxins in the extracellular surround. Mechanisms of adaptive specific immunity that involve B cells and antibody production are referred to as humoral immunity. The maturation of T cells occurs in the thymus. T cells function equally the key orchestrator of both innate and adaptive immune responses. They are also responsible for destruction of cells infected with intracellular pathogens. The targeting and destruction of intracellular pathogens by T cells is called prison cell-mediated immunity, or cellular immunity.

Think about It

• Listing the ii defining characteristics of adaptive amnesty.
• Explain the difference between a primary and secondary immune response.
• How do humoral and cellular immunity differ?

Antigens

Activation of the adaptive immune defenses is triggered by pathogen-specific molecular structures called antigens. Antigens are like to the pathogen-associated molecular patterns (PAMPs) discussed in Pathogen Recognition and Phagocytosis; however, whereas PAMPs are molecular structures found on numerous pathogens, antigens are unique to a specific pathogen. The antigens that stimulate adaptive amnesty to chickenpox, for example, are unique to the varicella-zoster virus just significantly unlike from the antigens associated with other viral pathogens.

The term antigen was initially used to draw molecules that stimulate the product of antibodies; in fact, the term comes from a combination of the words antibody and generator, and a molecule that stimulates antibody product is said to be antigenic. However, the part of antigens is not limited to humoral immunity and the product of antibodies; antigens also play an essential function in stimulating cellular immunity, and for this reason antigens are sometimes more accurately referred to equally immunogens. In this text, notwithstanding, we will typically refer to them equally antigens.

Pathogens possess a multifariousness of structures that may contain antigens. For case, antigens from bacterial cells may be associated with their capsules, prison cell walls, fimbriae, flagella, or pili. Bacterial antigens may likewise be associated with extracellular toxins and enzymes that they secrete. Viruses possess a diversity of antigens associated with their capsids, envelopes, and the spike structures they use for attachment to cells.

Figure two. An antigen is a macromolecule that reacts with components of the immune arrangement. A given antigen may comprise several motifs that are recognized by immune cells.

Antigens may belong to any number of molecular classes, including carbohydrates, lipids, nucleic acids, proteins, and combinations of these molecules. Antigens of unlike classes vary in their ability to stimulate adaptive immune defenses as well as in the blazon of response they stimulate (humoral or cellular). The structural complication of an antigenic molecule is an important factor in its antigenic potential. In general, more complex molecules are more constructive as antigens. For case, the three-dimensional complex structure of proteins make them the most effective and potent antigens, capable of stimulating both humoral and cellular immunity. In comparison, carbohydrates are less circuitous in structure and therefore less effective equally antigens; they can only stimulate humoral immune defenses. Lipids and nucleic acids are the least antigenic molecules, and in some cases may only get antigenic when combined with proteins or carbohydrates to form glycolipids, lipoproteins, or nucleoproteins.

Figured iii. A typical protein antigen has multiple epitopes, shown by the ability of three different antibodies to bind to different epitopes of the same antigen.

One reason the iii-dimensional complication of antigens is and then 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. A single antigen may possess several different epitopes (Figure 2), and unlike antibodies may bind to different epitopes on the aforementioned antigen (Figure 3). For example, the bacterial flagellum is a large, circuitous 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 exist bound by specific antibodies.

An antigen's size is some other of import cistron in its antigenic potential. Whereas large antigenic structures like flagella possess multiple epitopes, some molecules are likewise small-scale to be antigenic by themselves. Such molecules, called haptens, are essentially free epitopes that are not function of the complex three-dimensional structure of a larger antigen. For a hapten to become antigenic, information technology must kickoff attach to a larger carrier molecule (usually a protein) to produce a cohabit antigen. The hapten-specific antibodies produced in response to the conjugate antigen are then able to collaborate with unconjugated free hapten molecules. Haptens are not known to be associated with whatever specific pathogens, merely they are responsible for some allergic responses. For example, the hapten urushiol, a molecule institute in the oil of plants that cause poison ivy, causes an immune response that can result in a astringent rash (called contact dermatitis). Similarly, the hapten penicillin can cause allergic reactions to drugs in the penicillin grade.

Think nearly It

• What is the difference between an antigen and an epitope?
• What factors affect an antigen's antigenic potential?
• Why are haptens typically not antigenic, and how practice they become antigenic?

Antibodies

Antibodies (also chosen immunoglobulins) are glycoproteins that are present in both the blood and tissue fluids. The bones structure of an antibody monomer consists of four poly peptide bondage held together by disulfide bonds (Figure 4). A disulfide bond is a covalent bond between the sulfhydryl R groups institute on two cysteine amino acids. The 2 largest chains are identical to each other and are called the heavy bondage. The two smaller bondage are also identical to each other and are called the low-cal chains. Joined together, the heavy and light chains form a bones Y-shaped structure.

Effigy 4. (a) The typical 4-chain structure of a generic antibody monomer. (b) The respective iii-dimensional construction of the antibody IgG. (credit b: modification of work by Tim Vickers)

The two 'artillery' of the Y-shaped antibody molecule are known as the Fab region, for "fragment of antigen binding." The far end of the Fab region is the variable region, which serves as the site of antigen binding. The amino acid sequence in the variable region dictates the iii-dimensional construction, and thus the specific 3-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 antibiotic molecule, this region displays a high degree of variability betwixt antibodies with dissimilar 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 antibiotic molecule includes the trunk of the Y and lower portion of each arm of the Y. The body of the Y is likewise called the Fc region, for "fragment of crystallization," and is the site of complement factor binding and bounden to phagocytic cells during antibody-mediated opsonization.

Recall well-nigh Information technology

• Describe the dissimilar functions of the Fab region and the Fc region.

Antibody Classes

The constant region of an antibody molecule determines its grade, or isotype. The five classes of antibodies are IgG, IgM, IgA, IgD, and IgE. Each class possesses unique heavy chains designated by Greek messages γ, μ, α, δ, and ε, respectively. Antibody classes also exhibit important differences in abundance in serum, organization, body sites of activeness, functional roles, and size (Table 1).

IgG is a monomer that is by far the most abundant antibody in human being blood, accounting for about 80% of total serum antibody. IgG penetrates efficiently into tissue spaces, and is the just antibody course with the ability to cantankerous the placental barrier, providing passive amnesty to the developing fetus during pregnancy. IgG is also the most versatile antibody grade in terms of its office in the body's defense against pathogens.

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 v monomers of IgM bound together by a protein structure called the J concatenation. Although the location of the J chain relative to the Fc regions of the five monomers prevents IgM from performing some of the functions of IgG, the ten available Fab sites associated with a pentameric IgM get in an important antibiotic in the body's arsenal of defenses. IgM is the starting time antibiotic produced and secreted by B cells during the principal and secondary allowed responses, making pathogen-specific IgM a valuable diagnostic marker during active or recent infections.

IgA accounts for virtually thirteen% of total serum antibody, and secretory IgA is the most common and abundant antibody class plant in the mucus secretions that protect the mucous membranes. IgA tin also exist plant in other secretions such as breast milk, tears, and saliva. Secretory IgA is assembled into a dimeric form with 2 monomers joined by a protein construction called the secretory component. I of the important functions of secretory IgA is to trap pathogens in mucus so that they tin can later be eliminated from the body.

Similar to IgM, IgD is a membrane-bound monomer establish on the surface of B cells, where it serves every bit an antigen-bounden receptor. Withal, IgD is not secreted by B cells, and only trace amounts are detected in serum. These trace amounts most likely come from the deposition of former B cells and the release of IgD molecules from their cytoplasmic membranes.

IgE is the least abundant antibiotic class in serum. Like IgG, it is secreted equally 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 and 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).

Table one. The Five Immunoglobulin (Ig) Classes
	IgG monomer	IgM pentameter	Secretory IgA dimer	IgD monomer	IgE monomer
Construction
Heavy Chains	γ	μ	α	δ	ε
Number of antigen binding sites	2	10	4	2	2
Molecular weight (Daltons)	150,000	900,000	385,000	180,000	200,000
Percentage of total antibiotic in serum	fourscore%	6%	13% (monomer)	< 1%	< i%
Crosses placenta	yeah	no	no	no	no
Fixes complement	yes	yeah	no	no	no
Fc binds to	phagocytes				mast cells and basophils
Part	Neutralization, agglutination, complement activation, opsonization, and antibody-dependent cell-mediated cytotoxicity.	Neutralization, agglutination, and complement activation. The monomer class serves as the B-jail cell receptor.	Neutralization and trapping of pathogens in mucus.	B-cell receptor.	Activation of basophils and mast cells against parasites and allergens.

Think nearly It

• What part of an antibiotic molecule determines its class?
• What class of antibiotic is involved in protection against parasites?
• Describe the difference in structure between IgM and IgG.

Antigen-Antibody Interactions

Unlike classes of antibiotic play important roles in the torso's defense against pathogens. These functions include neutralization of pathogens, opsonization for phagocytosis, agglutination, complement activation, and antibody-dependent cell-mediated cytotoxicity. For nearly 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 cake initial attachment to intestinal mucosal cells. Similarly, specific antibodies tin bind to sure toxins, blocking them from attaching to target cells and thus neutralizing their toxic effects. Viruses can be neutralized and prevented from infecting a cell by the same machinery (Figure five).

Figure 5. Neutralization involves the binding of specific antibodies to antigens establish on leaner, viruses, and toxins, preventing them from attaching to target cells.

As described in Chemic Defenses, opsonization is the coating of a pathogen with molecules, such as complement factors, C-reactive protein, and serum amyloid A, to assistance in phagocyte binding to facilitate phagocytosis. IgG antibodies also serve as fantabulous opsonins, binding their Fab sites to specific epitopes on the surface of pathogens. Phagocytic cells such as macrophages, dendritic cells, and neutrophils have receptors on their surfaces that recognize and demark to the Fc portion of the IgG molecules; thus, IgG helps such phagocytes attach to and engulf the pathogens they have spring (Figure half dozen).

Effigy 6. Antibodies serve as opsonins and inhibit infection past tagging pathogens for destruction past macrophages, dendritic cells, and neutrophils. These phagocytic cells use Fc receptors to bind to IgG-opsonized pathogens and initiate the kickoff step of zipper before phagocytosis.

Figure seven. Antibodies, especially IgM antibodies, agglutinate bacteria by bounden to epitopes on 2 or more bacteria simultaneously. When multiple pathogens and antibodies are present, aggregates form when the binding sites of antibodies bind with separate pathogens.

Agglutination or aggregation involves the cross-linking of pathogens by antibodies to create big aggregates (Figure 7). IgG has two Fab antigen-binding sites, which tin 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 claret and easier for phagocytes to ingest for destruction. The pentameric structure of IgM provides ten Fab binding sites per molecule, making information technology the well-nigh efficient antibiotic for agglutination.

Another important function of antibodies is activation of the complement pour. Equally discussed in the previous affiliate, the complement organization is an important component of the innate defenses, promoting the inflammatory response, recruiting phagocytes to site of infection, enhancing phagocytosis past opsonization, and killing gram-negative bacterial pathogens with the membrane attack circuitous (MAC). Complement activation can occur through three different pathways (meet Figure 2 in Chemical Defenses), only the most efficient is the classical pathway, which requires the initial binding of IgG or IgM antibodies to the surface of a pathogen cell, assuasive for recruitment and activation of the C1 circuitous.

Still some other of import function of antibodies is antibiotic-dependent cell-mediated cytotoxicity (ADCC), which enhances killing of pathogens that are also large to be phagocytosed. This procedure is all-time characterized for natural killer cells (NK cells), as shown in Figure 8, but it can besides involve macrophages and eosinophils. ADCC occurs when the Fab region of an IgG antibody binds to a large pathogen; Fc receptors on effector cells (east.g., NK cells) then demark to the Fc region of the antibody, bringing them into close proximity with the target pathogen. The effector cell and then secretes powerful cytotoxins (eastward.g., perforin and granzymes) that kill the pathogen.

Figure 8. In this example of ADCC, antibodies bind to a large pathogenic cell that is besides large for phagocytosis so demark to Fc receptors on the membrane of a natural killer jail cell. This interaction brings the NK cell into shut proximity, where it can kill the pathogen through release of lethal extracellular cytotoxins.

Think about It

• Where is IgA normally found?
• Which class of antibody crosses the placenta, providing protection to the fetus?
• Compare the mechanisms of opsonization and antibody-dependent cell-mediated cytotoxicity.

Central Concepts and Summary

• Adaptive immunity is an acquired defense against foreign pathogens that is characterized past specificity and memory. The first exposure to an antigen stimulates a primary response, and subsequent exposures stimulate a faster and potent secondary response.
• Adaptive amnesty is a dual arrangement involving humoral immunity (antibodies produced by B cells) and cellular amnesty (T cells directed confronting intracellular pathogens).
• Antigens, besides called immunogens, are molecules that activate adaptive immunity. A single antigen possesses smaller epitopes, each capable of inducing a specific adaptive allowed response.
• An antigen's ability to stimulate an immune response depends on several factors, including its molecular class, molecular complication, and size.
• Antibodies (immunoglobulins) are Y-shaped glycoproteins with two Fab sites for binding antigens and an Fc portion involved in complement activation and opsonization.
• The five classes of antibody are IgM, IgG, IgA, IgE, and IgD, each differing in size, system, location within the body, and office. The five primary functions of antibodies are neutralization, opsonization, agglutination, complement activation, and antibody-dependent cell-mediated cytotoxicity (ADCC).

Multiple Selection

Antibodies are produced past ________.

1. plasma cells
2. T cells
3. os marrow
4. B cells

Bear witness Answer

Reply a. Antibodies are produced by plasma cells.

Cellular adaptive immunity is carried out by ________.

1. B cells
2. T cells
3. os marrow
4. neutrophils

Show Answer

Answer b. Cellular adaptive amnesty is carried out by T cells.

A single antigen molecule may be composed of many private ________.

1. T-cell receptors
2. B-cell receptors
3. MHC II
4. epitopes

Testify Respond

Answer d. A single antigen molecule may exist composed of many individual epitopes.

Which class of molecules is the virtually antigenic?

1. polysaccharides
2. lipids
3. proteins
4. carbohydrates

Show Answer

Answer c. Proteins are the virtually antigenic.

Matching

Friction match the antibody form with its description.

___IgA	A. This class of antibody is the but 1 that can cantankerous the placenta.
___IgD	B. This class of antibody is the first to appear subsequently activation of B cells.
___IgE	C. This class of antibody is involved in the defense against parasitic infections and involved in allergic responses.
___IgG	D. This grade of antibody is constitute in very large amounts in mucus secretions.
___IgM	Eastward. This class of antibody is not secreted by B cells just is expressed on the surface of naïve B cells.

Fill in the Blank

There are 2 critically important aspects of adaptive amnesty. The first is specificity, while the 2nd is ________.

Show Answer

There are 2 critically of import aspects of adaptive immunity. The commencement is specificity, while the second is memory.

________ immunity involves the production of antibody molecules that bind to specific antigens.

Show Answer

Humoral immunity involves the production of antibody molecules that bind to specific antigens.

The heavy chains of an antibody molecule incorporate ________ region segments, which assist to determine its class or isotype.

Show Respond

The heavy chains of an antibody molecule contain abiding region segments, which aid to determine its class or isotype.

The variable regions of the heavy and calorie-free chains form the ________ sites of an antibiotic.

Show Answer

The variable regions of the heavy and light chains grade the antigen-binding sites of an antibiotic.

Think virtually Information technology

1. What is the difference between humoral and cellular adaptive immunity?
2. What is the departure betwixt an antigen and a hapten?
3. Describe the mechanism of antibody-dependent cell-mediated cytotoxicity.

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Source: https://courses.lumenlearning.com/microbiology/chapter/overview-of-specific-adaptive-immunity/

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