Detailed Notes on Antigens

Understanding Antigens

What is an Antigen?

  • Terminology: The term derives from 'anti/geno', where 'anti' means opposite or opposing properties and 'geno' refers to generation or production. This definition describes substances that can generate an immune response or create opposition against pathogens.

    • An antigen is any substance or molecule, such as proteins, polysaccharides, or nucleic acids, that can specifically bind to an antibody or T lymphocyte receptor. This interaction triggers an immune response, which is crucial for the body’s defense mechanisms.

    • Antibodies, which are glycoproteins produced by B lymphocytes, can recognize almost all types of biological molecules as antigens, but T lymphocytes primarily recognize specific peptides presented by Major Histocompatibility Complex (MHC) molecules.

Nomenclature of Antigens

  1. Homologous Antigens: These are antigens that are used to induce an immune response, confirmed by the binding with specific antibodies.

  2. Heterologous Antigens: They refer to different antigens introduced in serological tests to validate the specificity of the antibody response.

  3. Heterophile Antigens: These antigens have the capacity to provoke an immune response across various organisms, highlighting their evolutionary conserved nature.

  4. Neoantigens: These are newly arisen antigens (epitopes) that emerge as a result of mutations or post-translational modifications, often associated with tumor cells, making them potential targets for cancer immunotherapy.

Tumor Antigens

  • Tumor antigens can be classified into tumor-specific antigens, which provoke a response that specifically targets tumor cells, and tumor-associated antigens, which are usually found on both normal and tumor cells but expressed at different levels. These antigens are presented by MHC I or MHC II molecules on the surface of tumor cells and are critical for the recognition and destruction of tumor cells by cytotoxic T (Tc) lymphocytes.

  • Tumor antigens enable the immune system to discern between normal and malignant cells, potentially leading to the elimination of tumor cells prior to their proliferation or metastasis.

  • In addition, tumor antigens may be present on the tumor's surface in forms of mutated receptors, recognized by B lymphocytes, facilitating antibody-mediated responses against the tumor.

Types of Antigens

  • Immunogens: Molecules capable of inducing a robust and positive immune response that leads to the synthesis of immune products, such as antibodies. Immunogens are generally large, complex molecules that elicit strong immunogenic responses.

  • Tolerogens: These are specific antigens that can activate the immune system but do not elicit a protective immune response, often leading to immune tolerance.

  • Haptens: Small molecules (typically below 10,000 daltons) that can induce an immune response only when they bind to a larger carrier molecule. Haptens are crucial in understanding allergic reactions and drug sensitivities.

Conversion of Haptens to Immunogens

  • Upon binding to a larger carrier protein, a hapten can elicit an immune response targeting:

    1. The hapten itself

    2. The carrier molecule

    3. The overlapping portions between hapten and carrier, resulting in a more potent and well-rounded immune response.

  • The complexes formed between haptens and proteins generally promote stronger and more complete immune responses due to enhanced processing and presentation by antigen-presenting cells (APCs).

Factors Affecting Antigen Immunogenicity

  1. Genetic Relation: This includes various types of antigens based on their genetic context:

    • Xenoantigens: Antigens that are recognized as foreign due to differences between species.

    • Aloantigens: Antigens that are genetically different amongst individuals of the same species (such as blood group antigens).

    • Isoantigens: Antigens found among genetically identical individuals such as in identical twins.

    • Autoantigens: Self-antigens recognized by the immune system of the same individual, which can lead to autoimmune responses if misidentified.

  2. Molecular Size: Immunogenicity is often correlated with size; good immunogens typically have a molecular weight of around 100,000 daltons. Antigens smaller than 5,000-10,000 daltons usually exhibit poor immunogenicity.

  3. Dosage: Dosage is critical; too low a dose may fail to stimulate T and B lymphocytes, while excessively high doses may lead to immune tolerance. Strategic repeated spaced dosing is preferred for optimal immune response activation.

Antigen Complexity and Conformation

  • The complexity of an antigen, defined by its structural variability and molecular aggregation, plays a key role in immunogenic potential. Linear, unaggregated molecules may not elicit a vigorous immune response compared to more complex antigens.

  • Conformation refers to the three-dimensional arrangement of the antigen molecules, vital for its recognition by T cell receptors and antibodies. Denatured proteins may lose their antigenic properties, while complex structures tend to enhance immunogenic potential.

  • Charge: The net electrical charge of an antigen influences its interaction with antibodies, thus affecting recognition and the strength of the immune response.

Accessibility and Degradability of Antigens

  • Accessibility: For an immune response to occur, antigen epitopes must be readily exposed; if they are hidden in the structure of the molecule, they cannot effectively elicit a response from immune cells.

  • Degradability: Only antigens that can be broken down (degradable molecules) induce effective immune responses as they are recognized when processed and presented by MHC class II molecules to CD4+ T cells, facilitating helper T cell activation.

Administration Routes and Their Effects on Immunity

  • Different routes of administration influence which lymphoid organs the antigen reaches and consequently the type of immune response elicited:

    • Intravenous (IV): Primarily reaching the spleen, typically stimulating IgG production.

    • Subcutaneous (SC) / Intramuscular (IM): Generally activates lymph nodes, promoting a robust IgG response.

    • Intradermal (ID) and Mucosal (such as oral or nasal) routes may lead to more specific regional immune responses, such as the production of IgA antibodies in mucosal tissues.

T-Dependent vs. T-Independent Antigens

T-Dependent Antigens

T-Independent Antigens

Require T cell help for B cell activation

Do not require T cell help

Produce primary IgM and secondary IgG responses

Produce poor IgM responses without memory

Typically proteins

Usually polysaccharides or lipopolysaccharides

Superantigens

  • Superantigens are a unique class of exogenous or endogenous proteins produced by certain bacteria and viruses that can nonspecifically stimulate T lymphocytes (particularly CD4+ cells).

  • These antigens can evoke severe systemic toxic responses, leading to conditions like toxic shock syndrome, by binding outside the conventional peptide-binding groove of MHC class II proteins and interacting with TCR Vβ regions, thereby activating a large number of T cells simultaneously.

Effects of Superantigens

  • The massive activation of T cells can lead to exaggerated immune responses, resulting in severe inflammatory reactions and shock. Superantigens might also activate autoreactive T cells, worsening autoimmune diseases such as rheumatoid arthritis.

Recombinant Antigens

  • Recombinant antigens are generated by cloning genes encoding immunogenic proteins into bacterial, yeast, or mammalian cells. Examples include the hepatitis B vaccine and COVID-19 vaccine, which utilize recombinant technology to produce safe and effective immunogens.

  • These antigens are typically processed exogenously and are unable to directly activate T cells without the assistance of professional antigen-presenting cells.

Mitogens

  • Mitogens are substances that non-specifically activate T and B cells through binding to various surface molecules, such as T cell receptors (TCR) and CD2, stimulating polyclonal cell activation and proliferation.

  • Types of Mitogens: Include substances like lectins, lipopolysaccharides (LPS) derived from Gram-negative bacteria, and staphylococcal protein A.

  • Known mitogens include Concanavalin A (ConA), phytohemagglutinin (PHA), and pokeweed mitogen (PWM), all of which are used in laboratory settings to study immune responses.

Effects of Lipopolysaccharides (LPS)

  • In low concentrations, LPS can stimulate specific antibody production, whereas high concentrations may act as nonspecific polyclonal activators for B cells, leading to widespread activation regardless of the specific antigen recognition.

Adjuvants

  • Adjuvants are substances that are co-administered with antigens to enhance the immunogenicity and effectiveness of vaccines. They are particularly important when the antigen alone is weakly immunogenic or present in low quantities.

  • Effects of Adjuvants: They can prolong the persistence of antigens at the injection site, enhance co-stimulatory signals necessary for full T cell activation, intensify local inflammatory responses, and stimulate the proliferation of non-specific lymphocytes, thereby amplifying the overall immune response.

Examples of Adjuvants

  1. Alum: The only universally approved adjuvant for human use, known to prolong antigen persistence and enhance adaptive immune responses.

  2. Freund's Adjuvants: Available in complete and incomplete forms, these are potent in boosting immune responses but can cause local inflammation at the injection site.

  3. Synthetic Polyribonucleotides: These are used to stimulate non-specific proliferation of lymphocytes, contributing to the overall immune response.

  4. Liposomal adjuvants: Formulations that encapsulate antigens to facilitate sustained immune signaling and improve antigen presentation.

Antigen Isotypes

  • Antibody isotypes are distinguished among various classes of immunoglobulins (IgG, IgA, IgM, IgE, IgD) based on their constant regions, each serving distinct functional roles in the immune system’s arsenal.

  • Allotypes refer to minor genetically determined variations in the constant region among individuals, and Idiotypes describe variations among the variable regions of antibodies that are crucial for binding specificity, indicating the diversity of antibody responses possible in immunological reactions.