Immunology. Richard Coico

5 IMMUNOGENS AND ANTIGENS

      INTRODUCTION

      Immune responses arise as a result of exposure to foreign stimuli. The compound that evokes the response is referred to either as antigen or immunogen. The distinction between these terms is functional. An antigen is any agent capable of binding specifically to components of the immune system, such as the B‐cell receptor (BCR) on B lymphocytes and soluble antibodies. By contrast, an immunogen is any agent capable of inducing an immune response and is therefore immunogenic. The distinction between the terms is necessary because there are many compounds that are incapable of inducing an immune response, yet they are capable of binding with components of the immune system that have been induced specifically against them. Thus all immunogens are antigens, but not all antigens are immunogens.

      This difference becomes obvious in the case of low molecular weight compounds, a group of substances that includes many antibiotics and drugs. By themselves, these compounds are incapable of inducing an immune response but when they are coupled with much larger entities, such as proteins, the resultant conjugate induces an immune response that is directed against various parts of the conjugate, including the low molecular weight compound. When manipulated in this manner, the low molecular weight compound is referred to as a hapten (from the Greek hapten, which means “to grasp”); the high molecular weight compound to which the hapten is conjugated is referred to as a carrier. Thus a hapten is a compound that, by itself, is incapable of inducing an immune response but against which an immune response can be induced by immunization with the hapten conjugated to a carrier.

      Immune responses have been demonstrated against all the known biochemical families of compounds, including carbohydrates, lipids, proteins, and nucleic acids. Similarly, immune responses to drugs, antibiotics, food additives, cosmetics, and small synthetic peptides can also be induced, but only when these are coupled to a carrier. In this chapter, we discuss the major attributes of compounds that render them antigenic and immunogenic.

REQUIREMENTS FOR IMMUNOGENICITY

      A substance must possess the following characteristics to be immunogenic: (1) foreignness; (2) high molecular weight; (3) chemical complexity; and, in most cases, (4) degradability and interaction with host major histocompatibility complex (MHC) molecules.

      Foreignness

Animals normally do not respond immunologically to self. Thus, for example, if a rabbit is injected with its own serum albumin, it will not mount an immune response; it recognizes the albumin as self. By contrast, if rabbit serum albumin is injected into a guinea pig, the guinea pig recognizes the rabbit serum albumin as foreign and mounts an immune response against it. To prove that the rabbit, which did not respond to its own albumin, is immunologically competent, it can be injected with guinea pig albumin. The competent rabbit will mount an immune response to guinea pig serum albumin because it recognizes the substance as foreign.

      Thus, the first requirement for a compound to be immunogenic is foreignness. The more foreign the substance, the more immunogenic it is. In general, compounds that are part of self are not immunogenic to that individual. However, there are exceptional cases in which an individual mounts an immune response against his or her own tissues. This condition is termed autoimmunity (see Chapter 12).

      High Molecular Weight

      The second feature that determines whether a compound is immunogenic is its molecular weight. In general, small compounds that have a molecular weight of less than 1000 Da (e.g., penicillin, progesterone, aspirin) are not immunogenic; those of molecular weights between 1000 and 6000 Da (e.g., insulin, adrenocorticotropic hormone [ACTH]) may or may not be immunogenic; and those of molecular weights greater than 6000 Da (e.g., albumin, tetanus toxin) are generally immunogenic. In short, relatively small substances have decreased immunogenicity whereas large substances have increased immunogenicity.

      Chemical Complexity

      The third characteristic necessary for a compound to be immunogenic is a certain degree of physicochemical complexity. Thus, for example, simple molecules such as homopolymers of amino acids (e.g., a polymer of lysine with a molecular weight of 30,000 Da) are seldom good immunogens. Similarly, a homopolymer of poly‐γ‐D‐glutamic acid (the capsular material of Bacillus anthracis) with a molecular weight of 50,000 Da is not immunogenic. The absence of immunogenicity is because these compounds, although of high molecular weight, are not sufficiently chemically complex. However, if the complexity is increased by attaching various moieties (such as dinitrophenol or other low molecular weight compounds), which by themselves are not immunogenic, to the ε amino group of polylysine, the entire macromolecule becomes immunogenic. The resulting immune response is directed not only against the coupled low molecular weight compounds but also against the high molecular weight homopolymer. In general, an increase in the chemical complexity of a compound is accompanied by an increase in its immunogenicity. Thus copolymers of several amino acids, such as polyglutamic, alanine, and lysine (poly‐GAT), tend to be highly immunogenic.

Because many immunogens are proteins, it is important to understand the structural features of these molecules. Each of the four levels of protein structure contributes to the molecule’s immunogenicity. The acquired immune response recognizes many structural features and chemical properties of compounds. For example, antibodies can recognize various structural features of a protein, such as its primary structure (the amino acid sequence), secondary structures (the structure of the backbone of the polypeptide chain, such as an α‐helix or β‐pleated sheet), and tertiary structures (formed by the three‐dimensional configuration of the protein, which is conferred by the folding of the polypeptide chain and held by disulfide bridges, hydrogen bonds, hydrophobic interactions, etc.) (Figure 5.1). They can also recognize quaternary structures (formed by the juxtaposition of separate parts, if the molecule is composed of more than one protein subunit) (Figure 5.2).

      Degradability

      In contrast to B cells, in order for antigens to activate T cells to stimulate immune responses, interactions with MHC molecules expressed on antigen‐presenting cells (APCs) must occur (see Chapter 9). APCs must first degrade the antigen through a process known as antigen processing (enzymatic degradation of antigen) before they can express antigenic epitopes on their surface. Epitopes are also known as antigenic determinants. They are the part of an antigen that is recognized by the immune system and are the smallest unit of antigen that is capable of binding with antibodies and T‐cell receptors. Once degraded and noncovalently bound to MHC, these epitopes stimulate the activation and clonal expansion of antigen‐specific effector T cells.

      A protein antigen’s susceptibility to enzymatic degradation largely depends on two properties: (1) it has to be sufficiently stable so that it can reach the site of interaction with B cells or T cells necessary for the immune response, and (2) the substance must be susceptible to partial enzymatic degradation that takes place during antigen processing by APCs. Peptides composed of D‐amino acids, which are resistant to enzymatic degradation, are not immunogenic, whereas their L‐isomers are susceptible to enzymes and are immunogenic. By contrast, carbohydrates are not processed or presented and are thus unable to activate T cells, although they can directly activate B cells.

      In general,