Antibodies

• In the recent events of the COVID pandemic, many of us must have come across the term “antibodies” especially with respect to vaccination. A curious mind, as there is, will ask what are antibodies, how are the antibodies produced, etc. Antibodies (Ab), also known as immunoglobulins (Ig), are big, Y-shaped proteins that help the immune system recognise and kill foreign substances like bacteria and viruses. The antigen, which is a unique molecule of the pathogen, is recognised by the antibody. 

• Each point of an antibody's "Y" has a paratope (like to a lock) that is specific for one epitope (similar to a key) on an antigen, allowing these two structures to bind with precision. Thus antigens and antibodies are simple lock and key like structures and the antibodies function owing to this characteristic structural behaviour. An antibody may either flag a microbe or an infected cell for assault by other sections of the immune system or neutralise it directly via this binding mechanism (for example, by blocking a part of a virus that is essential for its invasion). A simple antibody structure is given below:

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Antibodies as Part of the Immune System:

• The antigen-binding sites at both tips of the antibody come in a vast range to allow the immune system to detect millions of different antigens. The rest of the antibody, on the other hand, remains largely consistent. IgA, IgD, IgE, IgG, and IgM are the only varieties that identify the antibody's class or isotype. Sites important in interactions with other immune system components are found in the constant region at the antibody's trunk. In addition to some structural properties, the class defines the function activated by an antibody after binding to an antigen. Antibodies of various classes are also produced in different places in the body and at different stages of the immune response.

• Antibodies, together with B and T cells, are the most significant components of the adaptive immune system. They come in two types: one that is linked to a B cell and the other, which is soluble and found in extracellular fluids like blood plasma. All antibodies start off as the first type, linked to the surface of a B cell — these are known as B-cell receptors (BCR). When an antigen attaches to a BCR, the B cell activates and divides into plasma cells, which make soluble antibodies against the same paratope, or memory B cells, which persist in the body and provide long-lasting protection to the antigen. Many secretions, as well as blood and tissue fluids, include soluble antibodies. Antibody-mediated immunity is frequently referred to as, or considered a part of, humoral immunity, because these fluids were once known as humours. Individual soluble Y-shaped units can be found as monomers or in complexes of two to five.

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• Antibodies are large proteins (150 kDa), around 10 nm in size, with three globular sections organised in a Y configuration. An antibody unit is made up of four polypeptide chains: two identical heavy chains and two identical light chains joined by disulfide links in humans and most animals. A piece chain is made up of a number of domains, which are roughly comparable sequences of 110 amino acids each. In simplified schematics, these domains are commonly depicted as rectangles. Heavy chains have one variable domain V_H and three to four constant domains CH₁, CH₂,..., whereas light chains have one variable domain V_L and one constant domain CL. An antibody is divided structurally into two antigen-binding fragments (Fab), each having one VL, VH, CL, and CH₁ domain, and a crystallisable fragment (Fc), which forms the Y shape's trunk. A hinge area of the heavy chains lies between them, and its flexibility allows antibodies to attach to pairs of epitopes at different distances, form complexes (dimers, trimers, etc.), and engage effector molecules more readily. Antibodies generally migrate to the final, gamma globulin fraction, in a blood protein electrophoresis test. Most gamma-globulins, on the other hand, are antibodies, which is why the two names, as well as the symbols Ig and, were once used interchangeably. Due to inexact correlation and misunderstanding with heavy chains, which characterise the IgG class of antibodies, this alternative name went out of use.

Different Types of Antibodies:

• Antibodies can be classified into distinct isotypes or classes. IgA, IgD, IgE, IgG, and IgM are five antibody classes found in placental animals, each of which is further classified into subclasses such as IgA1, IgA2, and IgA3. IgA, IgG, IgD, IgE, and IgM are antibodies with the heavy chain types α (alpha), γ (gamma), δ (delta), ε (epsilon),  μ (mu),  The section of the heavy chain within the hinge and Fc area determines the unique characteristics of each class.

• The biological features, functional locations, and capacity to cope with diverse antigens differ across the classes, as shown in the table. IgE antibodies, an antibody example, are responsible for an allergic reaction that includes histamine release from mast cells, which contributes to asthma. The variable portion of the antibody attaches to allergy antigens, such as house dust mite particles, while the Fc region (in the heavy chains) binds to the Fc receptor on a mast cell, causing it to degranulate (release chemicals from its granules).The paratope of the antibody interacts with the epitope of the antigen. Different epitopes are frequently organised discontinuously over the surface of an antigen, and dominant epitopes on a specific antigen are referred to as determinants. By spatial complementarity, antibodies and antigens interact (lock and key). Electrostatic forces, hydrogen bonds, hydrophobic contacts, and van der Waals forces are among the weak and non-specific molecular forces involved in the Fab-epitope interaction. This indicates that antibody-antigen interaction is reversible and that the antibody's affinity for an antigen is relative rather than absolute.

Functions of Antibodies:

• The following are the primary categories of antibody action:

• Neutralization is a process in which neutralising antibodies block sections of a bacterial cell's or virion's surface, rendering the attack ineffectual. 

• Agglutination is a process in which antibodies "glue" foreign cells together forming clumps that are appealing to phagocytosis. Antibodies "glue" serum-soluble antigens together, causing them to precipitate out of solution in clumps that are appealing targets for phagocytosis. Complement activation (fixation) occurs when antibodies attach onto a foreign cell and stimulate complement to assault it with a membrane attack complex, resulting in: The foreign cell is lysed.Chemotactically recruiting inflammatory cells to promote inflammation.

• To avoid autoimmunity, an antibody can signal immune cells to send antibody fragments to T cells or downregulate other immune cells. Activated B cells can develop into either antibody-producing plasma cells that emit soluble antibodies or memory cells that stay in the body for years, allowing the immune system to remember an antigen and respond more quickly in the future.

How are Antibodies Produced?

• The existence of antibodies is given through passive immunisation from the mother during the prenatal and neonatal phases of life. Early endogenous antibody production varies by antibody type and often appears in the first few years of life. Antibodies are considered part of the humoral immune system since they circulate freely in circulation. Clonal B cells that selectively react to only one antigen create circulating antibodies (an example is a virus capsid protein fragment). Antibodies aid immunity in three ways: they prevent pathogens from entering or damaging cells by binding to them; they stimulate pathogen removal by macrophages and other cells by coating the pathogen, and they trigger pathogen destruction by stimulating other immune responses such as the complement pathway by stimulating other immune responses. Antibodies can also cause vasoactive amine degradation, which aids immunisation against certain antigens (helminths, allergens).

• In conclusion, antibodies are one of the most effective defenses of the body from foreign antigens. Although they may be slow to develop they offer long term and in many cases a lifetime of immunity against certain diseases. Plus, there are different types of antibodies not only based on their isotypes but also based on their specificity. There are certain antibodies known as monoclonal antibodies which are very specific to a particular antigen and there are polyclonal antibodies which are generalistic in nature and can attach to many different types of antigens.