Antigens and Antibodies Overview

Antigens and Antibodies Overview

Antigens

  • Definition of Antigen:

    • A substance recognized by the body as foreign capable of initiating an immune response (I.R.).

  • Types of Antigens:

    • Antigen: Initiates formation and reacts with specific antibodies.

    • Immunogen: Specifically initiates an immune response.

    • Transfusion Medicine Context:

      • Antigens are primarily detected in relation to blood group antigens.

Function of Antigens

  • Stimulation of Antibody Formation:

    • Antigens provoke the formation of antibodies.

    • Ability to bind with the antibody.

  • Immunogenicity vs Antigenicity:

    • Immunogenic: Capable of eliciting an immune response.

    • Antigenic: Capable of binding with an antibody, which may or may not provoke an immune response.

  • Foreign Structures:

    • Antigens contain structures recognized as foreign by the immune system.

Antigenic Determinants (Epitopes)

  • Definition:

    • Parts of the antigen that bind with antibodies or T-lymphocyte receptors (TCR).

  • Special Note:

    • Not all parts of the antigen act as epitopes; only prominent determinants are recognized by the immune system.

Major Histocompatibility Complex (MHC) Antigens

  • Definition:

    • Also known as histocompatibility antigens or human leukocyte antigens (HLA).

  • Characteristics:

    • Present on the surface of most nucleated cells.

  • Immune Response Role:

    • Can provoke responses if transfused into genetically different individuals (allogenic).

    • Significant in organ rejection; crucial for graft survival post-transplant.

Factors Influencing the Immune Response (I.R.)

  • Host Factors:

    • Genetic Makeup, Age, Race, Diet, Hormones, Other Diseases, Environmental Exposure.

  • Antigen Characteristics:

    • Size: Larger antigens have more potential determinants.

    • Complexity: More complex antigens (such as proteins) are better at eliciting responses than simpler forms (like polysaccharides or lipids).

    • Conformation: Antibodies are specific to the 3D shape.

    • Charge: The net charge (positive, negative, or neutral) impacts responses.

    • Accessibility: Antigens must be reachable by the immune system.

    • Solubility/Digestibility: Less soluble/digestible antigens are less likely to invoke an I.R.

    • Chemical Composition:

      • Different blood group antigens vary in immunogenicity; prominent examples include ABO and D antigens.

Immune Response to Blood Group Antigens

  • Potential Reactions:

    • Foreign RBC transfusion leads to hemolytic transfusion reactions (HTR).

    • Exposure of fetal red cells can lead to hemolytic disease of the fetus/newborn (HDFNB).

    • Sources such as IV drug abuse should be acknowledged.

Antibodies (Immunoglobulins)

  • Definition:

    • Proteins that specifically bind to antigens; vital role in neutralizing toxins and pathogens.

  • Functions of Antibodies:

    • Neutralization, binding to complement, facilitation of phagocytosis, and destruction of larger pathogens.

Structural Overview of Immunoglobulins

  • Basic Structure:

    • Consists of 4 polypeptide chains: 2 light chains and 2 heavy chains.

    • Chains linked by disulfide bonds (S-S links).

    • Contains variable (V) and constant (C) regions with 1 Fc and 2 Fab fragments.

Variations in Immunoglobulin Classes

  • Types of Variations:

    • Isotypes: Heavy chain classes and subclasses, takes into account structural differences.

    • Allotypes: Genetic variations within a species.

    • Idiotypes: Variations in the variable regions of antibodies, unique to individual clones.

Classes of Immunoglobulins

IgG - Gamma

  • Characteristics:

    • Major immunoglobulin in serum, crosses placenta.

    • Makes up 70-75% of serum Ig.

    • Predominant in secondary immune responses.

    • Normal serum levels: 800-1800 mg/dL.

IgM - Mu

  • Characteristics:

    • Pentameric structure linked by a J chain, making it the most efficient for complement activation.

    • Primarily found early in immune responses.

    • Represents about 10% of total serum Igs.

IgA - Alpha

  • Characteristics:

    • Predominant in secretions (tears, saliva, breast milk).

    • Exists both as monomers and polymers.

    • Protects body surfaces from microorganisms, critical for mucosal immunity.

    • Precaution for IgA deficient patients during transfusions.

IgD - Delta

  • Characteristics:

    • Found on B cells, aids in maturation but has unclear functionality.

    • Present in low plasma concentrations with short half-life.

IgE - Epsilon

  • Characteristics:

    • Mediates hypersensitivity reactions (allergies).

    • Functions in parasite protection and binds to mast cells.

    • Found only in trace amounts in serum.

Antibody Response Phases

Primary vs Secondary Responses

Primary (1°) Response

  • Sequence: Lag Time → Log Phase → Plateau → Decline.

  • Key Features:

    • IgM precedes IgG production; no antibodies risk initial detection.

Secondary (2°) Response (Anamnestic)

  • Faster due to memory cells; exhibits shorter lag phase.

  • Key Features:

    • Longer plateau phase and predominance of IgG antibodies; higher antibody titre than primary response.

Monoclonal Antibodies (MAbs)

  • Nature:

    • Highly purified antibodies derived from a single clone, distinct from polyclonal antibodies.

  • Production:

    • B-cells from an immunized animal are fused with myeloma cells, creating hybridomas that replicate indefinitely.

Uses of Monoclonal Antibodies

  • Identifying and quantifying hormones, typing tissues and blood, infectious agents detection, cancer treatments, and immunotherapy.

Antigen-Antibody Interaction

  • Affinity: Initial attraction between epitope and antibody.

  • Avidity: Overall binding strength influenced by multiple binding sites.

  • Specificity: Distinct ability of an antibody to recognize specific antigens.

  • Cross-Reactivity: Similar determinants may allow antibodies to react with different antigens.

Avidity Explained

  • Involves total attractive forces, particularly in multivalent interactions, enhancing overall stability of complex formation.

Detection Methods for Antigens/Antibodies

  • Common Techniques:

    • Hemagglutination, Precipitation, Agglutination Inhibition, Hemolysis.

    • ELISA, EIA, Western Blotting, Immunofluorescence for quantification.

  • Automation: Many methods are conducted on automated platforms.

Agglutination Methods

  • Principle: Resulting from soluble antigen-antibody complexes forming visible aggregates.

  • Stages: Sensitization followed by lattice formation that leads to agglutination.

Factors Affecting Agglutination

  • Concentration ratio, temperature, pH, type of immunoglobulin, and enhanced media to improve agglutination responses.

Grading Agglutination

  • Based on observable strength: 0 to 4+. Factors such as presence of hemolysis or rouleaux formations must be considered.