Antigen and Factors Affecting Immunogenicity

A vaccine works because a harmless piece of a pathogen provokes a strong, lasting immune response. But inject that same person with a small drug molecule, such as penicillin, and usually nothing happens, until the drug attaches to a body protein, and then the immune system attacks it fiercely enough to cause a dangerous allergy.

Two molecules, two very different outcomes. What decides whether a substance provokes an immune response at all? That question is what this article answers, and the answer runs through a handful of properties: foreignness, size, chemical complexity, and how the molecule is delivered.

Introduction

The substance that induces a specific immune response and subsequently reacts with the products of specific immune response is called an antigen (it is more appropriately called an immunogen). In the case of infectious diseases, antigens are components of invading microorganism structures that are usually composed of proteins or polysaccharides.

Ag has to be recognized by the

• Immunoglobulin receptor of B cells or
• By the T cell receptor, when complexed with MHC

Among the biological macromolecules, protein is the most potent immunogen, followed by the polysaccharide. Other macromolecules such as lipids and nucleic acids generally do not serve as immunogens on their own.

For cell-mediated immunity only proteins and some lipids/glycolipids serve as immunogen.

Properties of an Immunogen

• Immunogenicity
• Antigenicity

Immunogenicity: Ability to induce a humoral and/or cell-mediated immune response.

• B cells + Ag   =           effector B cells (plasma cells) + memory B cells
• T cells + Ag    =           effector T cells (e.g., CTLs, THs) + memory T cells

Antigenicity: The ability to combine/react specifically with the final products of the above responses (i.e., antibodies and/or cell-surface receptors).

> All molecules that have the property of immunogenicity also have the property of antigenicity but Reverse not true. Remember: All Immunogen are Antigen but all Antigen are not Immunogen e.g. Hapten.

Hapten: antigenic but not immunogenic

A hapten is a small molecule that cannot provoke an immune response on its own, but can react with the products of one. On its own it is too small to be immunogenic. When it attaches to a larger carrier molecule, usually a protein, the hapten-carrier complex becomes immunogenic, and the immune system then makes antibodies that recognize the hapten itself, even without the carrier.

This is not just a textbook idea. It explains an important class of drug allergies. Penicillin is a hapten. By itself it is harmless to the immune system, but it can bind to body proteins, and the resulting complex can provoke an allergic response against the drug. This is the clearest everyday example of the hapten principle.

Epitopes: what the immune system actually recognizes

Immune cells do not recognize a whole antigen. They recognize small, specific regions of it called epitopes. One antigen carries many different epitopes, and B cells and T cells recognize different ones on the same molecule. This is why a single protein can trigger several distinct antibodies at once.

The details of epitope types, how B cell and T cell epitopes differ, and epitope spreading are covered in a separate article on epitopes.Types of antigen

Antigens are grouped in several ways. By origin, they are exogenous (entering from outside, such as microbes and pollens) or endogenous (arising within the body's own cells, such as viral proteins made inside an infected cell). By their need for T cell help, they are T-dependent (mostly proteins, needing helper T cells for a full antibody response) or T-independent (such as polysaccharides, which can activate B cells directly).

The T-dependent versus T-independent distinction is covered in detail in a separate article.

Factors influencing Immunogenicity

1. Nature of the Immunogen

• Foreignness
• Molecular size
• Chemical composition and heterogeneity
• Ability to be  processed and presented with an MHC molecule on the surface of Antigen Presenting Cells (APCs) or altered self-cell

1. The biological system that the antigen encounters

2. The genotype of the recipient animal

3. Dosage and route of administration

Ⅰ. Foreignness

Antigens must be recognized as non-self by the biological system

The degree of immunogenicity depends on the degree of foreignness, i.e., The greater the phylogenetic distances between two species, the greater the structural (and therefore the antigenic) disparity between them.

e.g., If bovine serum albumin (BSA) is injected into cow, rabbit, and chicken, the order of Immunogenicity will be:

Cow < Rabbit < Chicken (least for cow and most for chicken)

This property is governed by Tolerance to self (specific unresponsiveness to self-antigens)

Ⅱ.  Molecular Size

A correlation exists between the size of the macromolecule and its immunogenicity

As a general rule, the most potent immunogens have a molecular mass at or above 100,000 Da. Molecules in the range of about 5,000 to 10,000 Da are usually weak immunogens, and molecules below roughly 1,000 Da, such as most haptens, are generally not immunogenic on their own. These are guidelines, not strict cutoffs: a few small molecules can be immunogenic, and some large ones are not, because size is only one of several factors and chemical complexity matters alongside it.

III. Chemical Composition and Heterogeneity

• Chemical complexity contributes to immunogenicity
• Copolymers composed of different amino acids or sugars are usually more immunogenic than homopolymers of their constituents.
• All four levels of protein organization contribute to the structural complexity of a protein and hence affect its immunogenicity.

IV. Susceptibility to Ag Processing and Presentation

The development of both humoral and cell-mediated immune responses requires the interaction of T cells with Ag that has been processed and presented together with MHC molecules.

• Large, insoluble macromolecules are generally more immunogenic than small, soluble macromolecules because of the ease of phagocytosis and processing.
• Macromolecules that can not be degraded and presented are poor immunogens. For example, polymers of D-amino acids.

Contribution of the Biological System

1. Age: Usually the very young and the very old have a diminished ability to mount an immune response in response to an immunogen.

2. Genotype of the recipient animal

• The genetic constitution of an immunized animal influences the type of immune response the animal manifests and the degree of response.
• Genetic control of immune responsiveness is primarily confined to genes within MHC.
• MHC gene products play a central role in determining the degree to which an animal responds to an immunogen.
• Some substances are immunogenic in one individual but not in others (i.e.responders and non-responders).

Immunogen Dosage and Route of Administration

A: Amount of Immunogen

• An optimum dose is necessary to mount a good immune response.
• An insufficient dose will not stimulate an immune response (fails to activate enough lymphocytes or can induce a state of immunologic unresponsiveness or tolerance).
• Excessively high dose induces tolerance.

B: Times

• Single-dose will not induce a strong response.
• Repeated administration, such as vaccine boosters, increases clonal proliferation of antigen-specific T and B cells and expands the lymphocyte population specific for that immunogen. Why the second exposure produces a faster and stronger response than the first is covered in a separate article on the primary versus secondary immune response.

C. Routes of Administration

• Generally, the subcutaneous route is better than the intravenous or intragastric routes.
• Route of administration strongly influences which immune organs and cell populations will be involved in the response. e.g., intravenous  (spleen); subcutaneous (local lymph nodes)

How to remember the factors affecting immunogenicity

The factors split into two groups: properties of the antigen itself, and properties of the host and how the antigen is given.

Properties of the antigen: the mnemonic is "Foreign Stuff Confuses Defenses":

Foreignness: the more evolutionarily distant the source, the stronger the response. Recall the classic example: bovine serum albumin provokes the weakest response in a cow (it is self), a stronger one in a rabbit, and the strongest in a chicken, which is the most phylogenetically distant.

Size: bigger is generally more immunogenic. Above 100,000 Da is a potent immunogen; below about 1,000 Da is usually not immunogenic alone.

Complexity: a mix of different building blocks beats a repetitive one. Copolymers of several amino acids are more immunogenic than a homopolymer of just one. This is why proteins, with four levels of structure, are the most potent immunogens.

Degradability: the antigen must be able to be broken down and presented on MHC. Polymers of D-amino acids, which the body cannot degrade, are poor immunogens for exactly this reason.

Properties of the host and delivery:

Age (the very young and very old respond less), genotype (MHC genes control responder versus non-responder status), dose (too little or too much both cause tolerance rather than a response), and route (subcutaneous generally beats intravenous, and the route decides which organs respond: intravenous engages the spleen, subcutaneous engages local lymph nodes).

Key exam facts in one table

Where students get confused

"Antigen and immunogen mean the same thing." In everyday use they are swapped freely, but strictly: an immunogen provokes a response, an antigen reacts with the products of one. Every immunogen is an antigen, but a hapten is an antigen that is not an immunogen. When precision matters, the direction is: immunogenicity implies antigenicity, never the reverse.

"A hapten is a weak immunogen." No. A hapten is not an immunogen at all on its own. It has zero immunogenicity by itself. Only after it couples to a carrier does the complex become immunogenic.

"Bigger always means more immunogenic." Size helps, but it is not sufficient. A large polymer of a single repeated amino acid is a poor immunogen because it lacks chemical complexity. Size and complexity work together.

"Self molecules can be immunogens if they are big enough." No. Foreignness is required. The body is tolerant to its own molecules regardless of their size. This is why the BSA response is weakest in the cow.

"Lipids are immunogens." Alone, generally not. Lipids and nucleic acids do not provoke a response by themselves, though some lipids and glycolipids can be presented to T cells in specific contexts.

References

1. Punt J, Stranford SA, Jones PP, Owen JA. Kuby Immunology. 8th ed. W.H. Freeman; 2019. (Chapter on antigens and immunogenicity.)
2. Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 10th ed. Elsevier; 2022.
3. Delves PJ, Martin SJ, Burton DR, Roitt IM. Roitt's Essential Immunology. 13th ed. Wiley-Blackwell; 2017.