Antibody Structure Explained with a Labeled Diagram

Antibodies (also called immunoglobulins) are essential proteins of the adaptive immune system. They are produced by B cells and circulate in the blood and lymph, where they seek out and bind specific foreign molecules called antigens. Understanding the architecture of an antibody explains why it can recognize a single molecular target with high specificity while also recruiting other immune components.

Overall Architecture: The Classic “Y” Shape

The basic shape of an antibody is a Y, roughly 10 nm tall. The two arms of the Y form the antigen‑binding fragment (Fab) and the stem is the crystallizable fragment (Fc). This overall layout is shared by all five major immunoglobulin classes (IgG, IgM, IgA, IgD, IgE) but the exact geometry of the stem varies among them.⁴

The Four Polypeptide Chains

Each antibody unit consists of four polypeptide chains:

• Two heavy (H) chains – the longer chains that determine the antibody class (e.g., IgG, IgM). They are linked by disulfide bonds and each contains one variable (V_H) and several constant (C) domains.²
• Two light (L) chains – the shorter chains, each with a variable (V_L) and a single constant (C_L) domain. Light chains can be κ (kappa) or λ (lambda) type.²

All four chains are held together by inter‑chain disulfide bridges and non‑covalent interactions, creating a stable tetrameric protein of about 150 kDa.¹

Variable and Constant Domains

At the tips of the Y, the variable regions of one heavy and one light chain pair together to form the antigen‑binding site (paratope). These regions are called V_H and V_L and contain the most diverse amino‑acid sequences, generated by somatic recombination in B cells.³

Below the variable domains are the constant regions (C_H and C_L) that define the antibody’s isotype and mediate interactions with other immune molecules. The constant region of the heavy chain forms the Fc fragment, which binds Fc receptors on cells and activates complement.³

Functional Regions: Fab and Fc

The antibody is traditionally divided into two functional fragments:

• Fab (Fragment antigen‑binding) – comprises the entire light chain plus the V_H and the first constant domain of the heavy chain (C_H1). Each Fab contains one antigen‑binding site; the two Fab arms are identical, allowing bivalent binding to the same epitope on a multivalent antigen.⁴
• Fc (Fragment crystallizable) – consists of the remaining constant domains of the heavy chains (C_H2 and C_H3 in IgG). The Fc does not bind antigen but interacts with Fc receptors (FcγR, FcεR, etc.) and complement component C1q, triggering downstream immune responses.³

How the Antigen‑Binding Site Is Formed

The paratope is a groove formed by the complementary‑determining regions (CDRs) of the V_H and V_L domains. The antigen presents a specific surface patch called an epitope. When the shapes and charge distributions of the paratope and epitope match, non‑covalent forces (hydrogen bonds, van der Waals contacts, electrostatic interactions) hold them together with high affinity.¹

Example: An IgG Antibody Targeting Influenza Hemagglutinin

Consider a human IgG antibody raised against the hemagglutinin (HA) protein of the influenza virus. The two Fab arms each recognize the same epitope on HA, allowing the antibody to cross‑link two HA molecules on the viral surface. This multivalent binding neutralizes the virus by blocking receptor attachment. Meanwhile, the Fc region engages Fcγ receptors on macrophages, promoting phagocytosis of the virus‑antibody complex.

Diagram (Textual Description)

Figure 1 – Labeled Antibody Diagram

Imagine the Y‑shaped antibody viewed from the side. Starting from the top of each arm and moving downwards:

1. Heavy chain (red) – long polypeptide.
2. Light chain (yellow) – shorter, attached to the heavy chain.
3. Variable region (V) – at the tip of each arm; together they form the antigen‑binding site (paratope).
4. Constant region (C) – below the variable region; part of Fab.
5. Fab region – the entire arm (V + C of one heavy and one light chain).
6. Hinge region – flexible connector between Fab arms and Fc stem.
7. Fc region – lower stem composed of the constant domains of the two heavy chains; binds Fc receptors.
8. Disulfide bonds – shown as short lines linking heavy–heavy and heavy–light chains.
9. Carbohydrate side‑chains – small “Y” symbols attached to the Fc region (glycosylation).
10. Antigen (green sphere) – bound to the paratope.
11. Epitope – highlighted patch on the antigen surface that contacts the paratope.

This textual map mirrors the labeled diagram provided by educational resources.²

Self‑Check

Question: Which part of the antibody determines its class (IgG, IgM, etc.) and where is it located?

Answer: The constant region of the heavy chains (the Fc fragment) determines the antibody class. It is located in the stem of the Y, below the hinge.

Understanding the modular architecture of antibodies makes it easier to grasp how they can be engineered (e.g., monoclonal antibodies, antibody‑drug conjugates) and why different isotypes have distinct biological activities.