Antibodies are also known as immunoglobulins and abbreviated as Ig. These are gamma globulin proteins that are found in blood or other bodily fluids of vertebrates and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses.
They are typically made of basic structural units—each with two large heavy chains and two small light chains.
Antibodies are produced by a kind of white blood cell called plasma cell.
There are several different kinds of antibodies which are grouped into different isotypes based on which heavy chain they possess.
Five different antibody isotypes are known in humans and other mammals, which perform different roles and help direct the appropriate immune response for each different type of foreign object they encounter.
Although the general structure of all antibodies is very similar, a small region at the tip of the protein is extremely variable, allowing millions of antibodies with slightly different tip structures to exist. These tips are known as antigen binding sites or hypervariable region.
Each of these variants can bind to a different target, known as an antigen. This huge diversity of antibodies allows the immune system to recognize an equally wide diversity of antigens.
The unique part of the antigen recognized by an antibody is called an epitope. The antigen binding site is known as paratope. These epitopes bind with their antibody in a highly specific interaction, called induced fit, that allows antibodies to identify and bind only their unique antigen in the midst of the millions of different molecules that make up an organism.
The large and diverse population of antibodies is generated by random combinations of a set of gene segments that encode different antigen binding sites, followed by random mutations in this area of the antibody gene, which create further diversity.
Antibody genes also re-organize in a process called class switching that changes the base of the heavy chain to another, creating a different isotype of the antibody that retains the antigen specific variable region. This allows a single antibody to be used by several different parts of the immune system.
Production of antibodies is the main function of the humoral immune system.
How does antibody participate in immune and inflammatory response?
There are three main ways in which antibody is immunologically active:
Antibody can coat and neutralize invading organisms, thus not allowing the organism any access to the host.
Two classes of antibody (IgM and IgG) activate complement, resulting in
- Cell chemotaxis
- Increased vascular permeability
- Target cell lysis
Antibody coats foreign particles (opsonization), increasing the efficiency of phagocytosis by cells that contain surface immunoglobulin receptors (neutrophils and macrophages).
Antibodies mediated immunity is very effective in bacterial infections especially bacteria with a polysaccharide capsule.
Patients with severe antibody deficiency (hypogammaglobulinemia) suffer from recurrent sinopulmonary infections, especially with encapsulated organisms like Pneumococcus & Haemophilus influenzae.
Types of Antibodies:
Depending on their structure and function there are five types of antibodies
IgG – It has the highest concentration in serum and has excellent penetration into tissues. It can cross the placenta by 16 weeks of pregnancy. It fixes complement.
IgA – It is the most important antibody for host defense at mucosal surfaces (sites of antigen entry). It is produced locally and is often present in a modified form in secretions such as tears and salvia (secretory IgA). Secretory IgA is more resistant to enzymatic degradation.
IgM – It is the first class of antibody made in primary response to antigen. It vigorously fixes complement and is very important in host defense against blood-borne antigens.
IgD – It functions mainly as an antigen receptor on B cells that have not been exposed to antigens. It has been shown to activate basophils and mast cells to produce antimicrobial factors.
IgE – It binds to the surface of mast cells and basophils, and when cross-linked, results in the release of granular contents (primarily histamine). It is important in allergic diseases and host defense against parasites.