Antibodies

Structure

An antibody molecule consists of two identical pairs of polypeptide chains (a light chain and a heavy chain) held together by disulfide bonds with each paired unit containing an antigen binding site.

Each polypeptide chain consists of discrete functional units called domains; heavy chains consists of 4 or 5 domains whilst light chains consist of 2 domains.

A) by Tokenzero ( CC BY-SA 4.0) via Wikimedia commons B) work has been released into the public domain by its author, Je at uwo at English Wikipedia. C) work has been released into the public domain by its author D) by Tokenzero ( CC BY-SA 4.0) via Wikimedia commons

Fig 1
A) the constant and variable domains of the heavy and light chains; B) Antibody Light chains and heavy chains held together with disulfide bonds; C) Fab and Fc regions and functions; D) 3D illustration of antibody structure.

Variable and Constant Domains

Both heavy and light chains have a variable domain (V_H and V_L respectively) at their N-termini, where the amino acid sequence varies greatly from antibody to antibody. These regions fold together to form a single antigen-binding site. Thus, a single antibody, consisting of two identical heavy chain-light chain pairs, has two identical antigen-binding sites.

The remaining domains are constant (C_H and C_L respectively) since their amino acid sequence is relatively conserved among different antibodies. The constant domains of the heavy chain are numbered C_H1 to C_H3 (though μ and ε heavy chains also have a C_H4 domain).

Heavy and Light Chains

There are five types of heavy chain, as determined by their constant regions: γ (Gamma), α (Alpha), μ (Mu), δ (Delta) and ε (Epsilon). The heavy chain denotes the antibody class (IgG, IgA, IgM, IgD and IgE respectively).

There are two types of light chain types with no functional difference between them: κ (kappa) and λ (lambda). Each antibody contains either two κ or two λ chains but not one of each and they can occur with any of the five heavy chain types (ratio of κ:λ = ~2:1).

Fc and Fab regions

The antibody molecule can be functionally divided into the Fab (Fragment antigen-binding) region and the Fc (Fragment crystallizable) region.

The Fab region is responsible for binding to antigens. It consists of the entire light chain (both V_L and C_L) and the V_H and C_H1 domains of the heavy chain. The variable regions confer antigen specificity.

The Fc region is responsible for mediating effector functions by interacting with cell surface receptors (Fc receptors) and complement proteins. It consists of the remaining constant domains of the heavy chains.

The γ, α and δ heavy chains (IgG, IgA and IgD) have a hinge region between the Fab and Fc regions which can be targeted by pathogens and in pharmacology.

Classification

IgG (the most abundant)

IgG is the most abundant antibody isotype in serum. It is the main antibody produced in a secondary immune response. There are four subclasses of IgG (IgG1, IgG2, IgG3 and IgG4 in order of serum concentration).

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Fig 2
IgG is the main antibody produced in a secondary immune response. IgM is the first antibody produced in an immune response

IgG is the only antibody to cross the placenta and provides infants with passive immunity in the first 3-6 months of life due to high levels of IgG.

IgG is associated with warm autoimmune haemolytic anaemia.

IgM (the first one)

IgM is the first antibody isotype produced during a primary immune response. It is found either as the BCR on naïve B cells or as a pentamer in serum, which has 10 antigen-binding sites and is stabilised by a J chain.

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Fig 4
IgM pentamer structure

IgM is said to have

• low affinity – individually has weak antibody-antigen interaction as it is produced early in an immune response before affinity maturation
• high avidity – strong combined strength of all antibody-antigen interactions in pentameric form, due to its many antigen-binding sites, allowing it to efficiently agglutinate pathogens in the early stages of an infection.

IgM is associated with cold autoimmune haemolytic anaemia.

IgA (the secreted one)

IgA is found in two forms: as a monomer in serum, and as a dimer called secretory IgA. It is the most abundant antibody isotype in secretions.

Secretory IgA:

• forms a dimer connected by a joining (J) chain
• has 4 antigen-binding sites (instead of 2)
• has a secretory component which protects it from enzymatic digestion
• Is found in saliva, mucus, breast milk and colostrum (to protect the aerodigestive tract of breast-fed babies)

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Fig 3
Secretory IgA dimer structure

IgD

IgD is found primarily as the BCR on naïve B cells, co-expressed with IgM. It found in low concentrations in serum, but its function there is not well understood.

IgE (the T1 hypersensitivity one)

IgE is mainly found on mast cells but is also present at low levels in the blood and extracellular fluid. It is associated with allergy (particularly type I hypersensitivity reactions such as atopic disease and anaphylaxis) and the immune response to parasitic infections. It triggers histamine release from mast cells and basophils.

Functions

Antibodies have various functions mediated by the Fc region interacting with cell surface receptors (e.g. on phagocytes) and complement proteins.

Opsonisation

Opsonins are able to bind both the pathogen and a receptor on a phagocytic cell, thus “tagging” the pathogen for phagocytic destruction.

Antibodies (particularly IgG1 and IgG3) are effective opsonins because they bind to specific antigens on pathogen surfaces and Fc receptors on phagocytic cells.

IgG antibodies can also bind to infected or malignant cells alongside Fc receptors of natural killer cells, leading to release of cytotoxic granules causing antibody-dependent cellular cytotoxicity (ADCC) and release of interferons attracting phagocytes.

by Maher33 [CC BY-SA 4.0] via Wikimedia commons.

Fig 5
Pathogen opsonised by multiple antibodies; Phagocytes bind to these antibodies via their Fc receptors and initiate phagocytosis

Neutralisation

Antibodies can directly neutralise pathogens and toxins without the help of the immune system by binding to them and blocking their ability to interact with host cells. For example, antibodies can bind to viral surface proteins that are necessary for the virus to attach and enter host cells, thereby preventing infection.

Neutralising antibodies must have high affinity to be effective. IgG and IgA antibodies have the greatest effect.

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Fig 6
Neutralisation of a virus by antibodies

Complement activation

IgM and IgG can activate the classical complement pathway. Once bound to antigen, the antigen-antibody complex interacts with C1q of the complement system, triggering a cleavage cascade and downstream effects.

C3 convertase eventually produces C3b, an opsonin which also forms part of the membrane attack complex (MAC), promoting phagocytosis and cell lysis.

IgM is more efficient than IgG at activating the classical complement pathway, as a single IgM pentamer can activate complement, whereas multiple IgG molecules are required.

Summary of antibody functions