Antibodies - Info

What are Antibodies?

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Definition of antibodies

Antibodies are a class of proteins that play important roles for the immune system. They are made by B-lymphocytes (a type of white blood cells) and by plasma cells in response to substances which are foreign for the organism (antigens = antibody generating). Antibodies are also called immunoglobulins and make up the main part of the specific endogenous defence.

Today, antibodies are used in medicine as well as in biology for diverse diagnostic assays like drug tests, ELISA, ELISPOT, EMSA, FACS, pregnancy tests and western blot.

How is an antibody structured?

Both, the heavy and the light chains, contain a constant (C-region) and a variable (V-region) region. While the sequence of amino acids within the C-region varies only a little between the different classes of antibodies, the sequence of the V-region at the end of the Y-arms is highly variable.

Antibodies are able to detect and bind small molecular structures at the surface of antigens (epitopes). This way the so called antigen-antibody-complex is formed (2).

The V-regions determine the specific binding sites of the antibody. These are able to detect and bind the respective antibody. Every antibody contains at least two binding sites directed against the antigen which has initiated its production.

How do antibodies function?

The antigen-antibody complex is the basis for the way antibodies work. In this process, an antibody binds to the epitope of an antigen ("antigen-antibody reaction"), causing the two to become a linked complex. The formation of such a complex leads to agglutination (agglutination or clumping) and precipitation (the clump precipitates as a solid precipitate).

Agglutination and precipitation are two processes that lead to cross-linking and clumping of antigens by antibodies, respectively. Larger antigen-antibody complexes can be better recognised and digested by phagocytotic cells.

Antigen-antibody complexes can be recognised by components of the complement system, activating it. The complement system consists of about 25 different proteins that can support the destruction of antigens by enhancing some of the reactions that occur in an immune response (opsonisation, inflammatory responses) (Murphy, K. & Weaver, C., 2018).

In most cases, antigens are proteins (e.g. from viruses and bacteria), but lipids, carbohydrates and other substances can also have an antigenic effect. Binding of antigens by antibodies has the goal of inactivating and destroying the foreign substances that have entered the body. There are various mechanisms for this:

In neutralisation, the antibody blocks a site on the antigen that it needs in order to exert its effect, such as recognition and penetration of body cells.

By attaching antibodies to the locomotor system of an invader (e.g. the flagellum of a bacterium), it can be prevented from moving (immobilised).

Some foreign substances must first be labelled as such by the body in order to be recognised and phagocytised ("eaten") by defence cells. This labelling can be done by antibodies, among other things, and is called opsonisation.

Which antibody classes exist?

Antibodies are divided into different classes according to the differences within the sequence of the C-region. In mammals exist five classes of immunoglobulins, defined by five differently structured C-regions:

• • IgA (dimer, see a) takes function as a first barrier against intruders, because of its presence in mucous membranes (mouth, nose, eyes and intestinal tract). Many foreign substances are already intercepted and eliminated by IgA. Besides mucous membranes IgA is also secreted in glands near to the maternal acromastium and transported into infants via the mother´s milk.

• • IgM (pentamer, see b) is the first antibody produced after an infection (primary response). After a few weeks the IgM titer decreases and is replaced by IgG, which is produced in the meantime.

• • IgG (monomer, see c) is the most frequent antibody. It circulates within body secretions like blood and lymph and is part of a delayed immune response (secondary response). The production of IgG starts three weeks after an infection, but it remains in the body for a long time. Based on an IgG titer past infections and vaccinations can be detected.

• • IgD (monomer, see 3c) occurs only in low concentrations in blood and lymph. At the surface of B-lymphocytes it operates as antigen receptor and supports their reproduction and differentiation. Thus, IgD supports the production of antibodies during an infection.

• • IgE (monomer, see 3c) is just a minor part of the whole antibody pool. In its free form it is nearly not present within blood and lymph. The IgE molecule is predominantly found at the surface of mast cells (cells of the immune system, which store messenger substances like histamine and heparin) and basophils (a form of white blood cells) where it stimulates the distribution of histamines and other inflammatory supporting substances in case of contact with antigens. This leads to an expansion of blood vessels which enables much easier the movement of other immune cells to the place of infection.

Difference between monoclonal- and polyclonal Antibody

Monoclonal and polyclonal antibodies are two types of antibodies that are used in medical research, diagnostics, and therapeutics. The main difference between them lies in their structure, production, and specificity.

Monoclonal- and polyclonal antibodies differ in their structure

Monoclonal antibodies are derived from a single type of immune cell, called a B cell, which produces a single type of antibody molecule that targets a specific antigen. As a result, monoclonal antibodies are highly specific and bind to a single epitope (a unique part of an antigen). On the other hand, polyclonal antibodies are a mixture of different types of antibodies produced by multiple B cells in response to an antigen. Polyclonal antibodies can recognize multiple epitopes on an antigen, resulting in a broader range of binding sites.

Monoclonal- and polyclonal Antibodies are produced differently

Monoclonal antibodies are produced through a process that involves immortalizing a single B cell to create a cell line that produces large amounts of a single type of antibody. This allows for consistent and uniform production of a specific antibody with minimal batch-to-batch variability. Polyclonal antibodies, on the other hand, are produced by injecting an animal (such as a rabbit or a goat) with an antigen, and then collecting blood from the animal to isolate the antibodies. The collected antibodies constitute a mixture of different types of antibodies, including those that bind to different epitopes of the antigen.

Monoclonal- and polyclonal antibodies differ in their specificity

Monoclonal antibodies are highly specific and typically bind to a single epitope on an antigen with high affinity. This makes them ideal for targeted therapies and diagnostics. Polyclonal antibodies, on the other hand, are less specific and can bind to multiple epitopes on an antigen. This can be advantageous in certain situations, such as when targeting complex antigens with multiple epitopes or when detecting multiple targets in a diagnostic assay.

Monoclonal- and polyclonal Antibodies have different applications

Monoclonal antibodies are widely used in various medical applications, including cancer therapy, autoimmune disease treatment, and targeted drug delivery. They are also commonly used in diagnostics, such as in immunoassays for detecting specific proteins or pathogens. Polyclonal antibodies are typically used in research and diagnostic applications where their broader reactivity to multiple epitopes can be advantageous, such as in Western blotting, immunohistochemistry, and enzyme-linked immunosorbent assays (ELISAs).

Summary: Difference between monoclonal- and polyclonal Antibody

In summary, monoclonal antibodies are derived from a single B cell and are highly specific, while polyclonal antibodies are a mixture of antibodies produced by multiple B cells and have broader reactivity. Monoclonal antibodies are widely used in targeted therapies and diagnostics, while polyclonal antibodies are commonly used in research and diagnostic applications.