Antibody structure

In one sentence

An antibody is an immune protein whose binding regions recognize a molecular feature and whose remaining structure helps determine its biological functions.

The intuition

Imagine a tool with a gripping end and a handle that can connect to other machinery. The gripping end determines what it catches. The rest affects what can happen after the catch. The common Y-shaped drawing is useful for a typical antibody, but engineered treatments can use fragments or rearranged parts.

How it works

A typical immunoglobulin G (IgG) antibody has two heavy and two light protein chains. Variable regions from a heavy and a light chain form each binding site. Each site recognizes an epitope, a particular molecular feature. The target may be a folded protein or another accessible structure; it need not be a short peptide displayed to a T cell. The antibody-structure chapter explains this architecture.

The two arms are often called Fab, for fragment antigen-binding. The base is Fc, for fragment crystallizable. Fc can interact with receptors on immune cells and with complement, a collection of immune proteins. Those interactions can help recruit other cells or clear a target. The effect depends on antibody class, molecular design and context. Some therapeutic antibodies deliberately reduce these functions.

Binding and recruitment are separate jobs. An antibody can block a receptor interaction. Another can recruit an immune cell. An antibody–drug conjugate carries an attached payload. An engineered engager can bind two different partners. These formats reuse binding parts without necessarily sharing the same Fc behavior. NCI’s antibody overview separates these uses.

Why it matters in cancer

“Antibody treatment” names a broad family, not one killing mechanism. Understanding the parts helps a reader ask whether a drug blocks signaling, recruits an effector or delivers a payload. It also explains why two drugs binding the same protein may behave differently.

Worked example

Two fictional antibodies bind the same surface target. One blocks a growth signal. The other carries a toxic payload but leaves signaling unchanged. A shared address does not make their mechanisms or safety profiles identical. Testing must evaluate each finished molecule.

Common confusions

Fab describes a structural part, not a specific tumor target.
Fc does not automatically cause killing in every format.
An antibody binding site and a conventional T-cell receptor have different recognition rules.
A Y-shaped diagram does not represent every engineered antibody-based drug.

Sources and scope

Source check: October 9, 2026. Typical antibody architecture and general therapeutic uses; product-specific function requires its own evidence. Expert and learner review remain pending.