An antibody, also known as an immunoglobulin, is a Y-shaped molecule made up of four polypeptide chains: two heavy chains and two light chains.

Antibodies are divided into five major classes, each with distinct functions:

IgG

Function: Provides long-term immunity and memory; most abundant in blood and extracellular fluid.

Features: Crosses the placenta, offering passive immunity to the fetus.

IgA

Function: Protects mucosal surfaces (e.g., respiratory and gastrointestinal tracts).

Features: Found in secretions like saliva, tears, and breast milk.

IgM

Function: First antibody produced in response to an infection; effective in forming complexes.

Features: Exists as a pentamer, enhancing its ability to bind antigens.

IgE

Function: Involved in allergic reactions and defense against parasitic infections.

Features: Binds to allergens and triggers histamine release from mast cells.

IgD

Function: Functions mainly as a receptor on B cells; role in initiating immune response.

Features: Present in small amounts in the blood.

The structure of an antibody is divided into several distinct regions, each with specific functions:

Fab Region (Fragment, antigen-binding)

Structure: The Fab region consists of the variable domains of both the heavy and light chains of the antibody. Each antibody has two Fab regions, one on each arm of the Y-shaped structure.

Function: The Fab region is responsible for binding to specific antigens. It contains the antigen-binding site, which is formed by the variable regions of both the heavy and light chains. This region recognizes and binds to the specific epitope (the part of the antigen that is recognized) with high specificity.

Composition: The Fab region includes the light chain's variable region (VL) and the heavy chain's variable region (VH), as well as part of the constant region of the heavy chain (CH1).

Specificity: Each Fab region is unique to the specific antigen it binds.

Fc Region (Fragment)

Structure: The Fc region is located in the stem of the Y-shaped antibody, formed by the constant regions of the heavy chains (CH2 and CH3). It is common to all antibodies of the same isotype (IgG, IgM, IgA, IgE, and IgD).

Function: The Fc region is crucial for mediating various effector functions of the antibody. It interacts with cell surface receptors (Fc receptors) on immune cells and with components of the complement system. This interaction helps in processes like:

Antibody-dependent Cellular Cytotoxicity (ADCC): Immune cells, such as natural killer (NK) cells, recognize and destroy target cells coated with antibodies.

Complement Activation: The Fc region can initiate the complement cascade, leading to the destruction of pathogens.

Phagocytosis: The Fc region can bind to receptors on phagocytic cells, enhancing the uptake and destruction of pathogens.

Key Points

Fab Region: Contains the antigen-binding sites and is specific to the antigen.

Fc Region: Mediates immune system interactions and contributes to the antibody's effector functions.

The combination of the Fab and Fc regions allows antibodies to not only recognize and bind to specific antigens but also to recruit and activate other components of the immune system to eliminate the antigen.