Antibodies (Part 1. Basic Information)

Antibodies play a central role in the body’s immune defense, with their remarkable ability to recognize specific targets or antigens, on invading pathogens or abnormal cells. In recent decades, this natural targeting mechanism has been harnessed and refined in the development of precision therapies, particularly in oncology.

As antibody-based drugs become increasingly central to targeted treatment strategies, understanding the basic biology of antibodies is essential. Here are the introduction of key antibody concepts, including structure, isotypes and therapeutic evolution, to build a foundation for their application in advanced modalities such as Antibody-Drug Conjugates (ADCs).

 

Antibody Basics

Antibodies, also known as immunoglobulins, are Y-shaped proteins produced by plasma cells – a type of white blood cell in the immune system. Their key feature is the ability to precisely recognize and bind to specific “markers” (antigens) on foreign substances such as viruses, bacteria, or cancer cells. Once bound, antibodies can directly neutralize the threat or tag it for elimination by other immune cells.

Naturally, the body produces a wide range of antibodies that recognize different parts of an antigen  - these are called polyclonal antibodies. For therapeutic purposes, however, especially in applications like ADCs, we use monoclonal antibodies (mAbs). These are lab-engineered antibodies that recognize a single, specific epitope with high precision.

Figure 1. Categories of antibodies

This specificity is critical in drug design. A monoclonal antibody can be chemically linked to a cytotoxic payload to form an ADC, guiding the payload directly to cancer cells while minimizing damage to healthy tissues. In this way, the antibody functions not just as a recognition element, but also as the vehicle for targeted delivery – providing both selectivity and therapeutic efficiency.

 

Antibody Structure

All antibodies share a common Y-shaped basic structure, composed of two identical heavy chains and two identical light chains, held together by disulfide bonds and non-covalent interactions. Each chain contains both constant and variable regions. The constant regions, e.g., constant heavy chain 1 (C_H1), constant heavy chain 2 (C_H2), and constant heavy chain 3 (C_H3) and constant light chain (C_L) form the structural backbone and mediate interactions with other components of the immune system. The variable regions like variable heavy chain (V_H) and the variable light chain (V_L) are responsible for recognizing specific antigens – they vary greatly among different antibodies and determine binding specificity.

At the tips of the “Y” are the fragment antigen-binding (Fab) regions. Each Fab region consists of a complete light chain(including its V_L and C_L parts) and part of a heavy chain (specifically its V_H and C_H1 parts). The antigen-binding site, or paratope, is formed where the V_H and V_L domains come together. It recognizes a specific region (epitope) on the antigen with high precision. The diversity of these paratopes enables antibodies to recognize a vast array of targets.

The base of the “Y” is the fragment crystallizable (Fc) region, consisting primarily of the C_H2 and C_H3 domains of the heavy chains. This region binds to Fc receptors on immune effector cells (such as macrophages and natural killer cells) and interacts with components of the complement system, helping to trigger immune responses that assist in clearing pathogens or abnormal cells.

Located between the Fab arms and Fc stem is a flexible hinge region, which allows the two arms to move and adapt to the spatial arrangement of antigens on a target cell surface.

Figure 2. A general antibody structure

 

Antibody Isotypes and Subclasses

Antibodies are classified into five main isotypes based on differences in their heavy chain constant regions: IgM, IgD, IgG, IgE, and IgA. Each isotype has distinct roles, distributions, and immune functions.

·       IgM is the first antibody produced in an immune response. It forms large pentamers with ten binding sites, providing strong early defense but is less used therapeutically due to its size.

·       IgD is mainly found on immature B cells and helps initiate their activation; its role in circulation is unclear.

·       IgG is the most abundant antibody in blood, making up about 75% of serum antibodies. It crosses the placenta to protect fetuses and is the preferred isotype for therapeutics, including ADCs, due to its stability and immune functions.

·       IgE plays key roles in allergic reactions and defense against parasites by binding mast cells and basophils, triggering mediator release.

·       IgA predominates in mucosal areas like respiratory and digestive tracts, protecting these surfaces from infection.

Table 1. Five main isotypes of antibodies.

Name	Properties	Structure
IgM	First antibodies secreted by B lymphocytes after infection. Forms groups of five (pentamers).
IgD	Found on immature B cells. Initiates B cell activation. With a currently unknown function.
IgG	Most abundant antibody type. One of the most effective at fighting infection.
IgE	Play a role in allergic reactions. Defenses against parasites
IgA	Found in mucosal areas. Protects against pathogens. Forms groups of two (dimers)

 

Among IgG antibodies, four subclasses exist: IgG1, IgG2, IgG3, and IgG4. They differ in hinge region structure and immune activation:

·       IgG1 is most common in therapies, effectively triggering immune responses and complement activation, making it ideal for cancer treatment including ADCs.

·       IgG2 is less active immunologically but more resistant to degradation.

·       IgG3 strongly activates complement but has a shorter half-life, limiting use.

·       IgG4 has weaker immune activity and can undergo “fab-arm exchange”, reducing therapeutic consistency.

 

Monoclonal Antibody Types

In the development of antibody therapeutics, scientists have continuously refined antibody structures to reduce immunogenicity and improve clinical performance. Overtime, four main types of monoclonal antibodies have emerged, evolving to become increasingly compatible with the human immune system.

Table 2. Types of monoclonal antibody.

Type	Suffix	Structure	Origin	Therapeutic Potential	Example
Murine	-onab		100% derived from mouse	Immunogenicity & delivery hurdles	Muromonab
Chimeric	-ximab		35% mouse, 65% human	Reduced immunogenicity and improved half-lives versus murine mAbs	Brentuximab
Humanized	-zumab		95% human	Reduced immunogenicity versus chimeric mAbs	Trastuzumab
Human	-mumab		100% human	Broadly reduced immunogenicity versus humanized mAbs	Belantamab

 

·       Murine Antibodies (Murine mAbs): These were the earliest antibodies, produced entirely in mice. Although they could bind to target antigens, their foreign nature triggered strong immune responses in humans, a phenomenon known as human anti-mouse antibody (HAMA) response. Murine mAbs also had short half-lives and limited therapeutic potential due to rapid clearance.

·       Chimeric Antibodies (Chimeric mAbs): To overcome the limitations of murine mAbs, researchers created chimeric antibodies by combining mouse variable regions (which recognize the antigen) with human constant regions (which interact with immune cells). This reduced immunogenicity while maintaining antigen specificity. The development of hybridoma technology by Georges Köhler and César Milstein in 1975 was instrumental in enabling large-scale production of these antibodies.

·       Humanized Antibodies (Humanized mAbs): Humanized antibodies further minimized the mouse-derived components by retaining only the complementarity-determining regions (CDRs) – the critical residues that directly bind the antigen – while replacing the rest of the structure with human sequences. This approach significantly reduced immunogenicity and improved pharmacokinetics.

·       Fully Human Antibodies (Human mAbs): The latest advancement uses genetic engineering techniques, such as phage display and transgenic animals, to produce antibodies composed entirely of human sequences. These fully human mAbs offer the lowest risk of immunogenicity and are the most compatible with human biology.

Today, humanized and fully human monoclonal antibodies are the predominant choices in antibody-drug conjugates development. They offer high target specificity, favorable serum half-lives, and excellent tolerability – key features for successful therapeutic application.

 

Conclusion

Antibodies are complex and versatile molecules, naturally evolved and further optimized through scientific innovation. Understanding their structure, isotypes, and evolution lays the foundation for their therapeutic use. As we move from basic biology to clinical application, the next section will explore how monoclonal antibodies are harnessed in the design and development of antibody-drug conjugates.

 

Reference

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2.     Baah, S.; Laws, M.; Rahman, K. M. Antibody–Drug Conjugates—A Tutorial Review. Molecules 2021, 26 (10), 2943. https://doi.org/10.3390/molecules26102943.

3.     Fu, Z.; Li, S.; Han, S.; Shi, C.; Zhang, Y. Antibody drug conjugate: the “biological missile” for targeted cancer therapy. Sig. Transduct. Target. Ther. 2022, 7, 93. https://doi.org/10.1038/s41392-022-00947-7.

4.     Sharma, P.; Joshi, R. V.; Pritchard, R.; Xu, K.; Eicher, M. A. Therapeutic Antibodies in Medicine. Molecules 2023, 28 (18), 6438. https://doi.org/10.3390/molecules28186438.