Introduction
In today’s time, people are gaining more and more knowledge about the human body and its several functions and the infections that harm the body system. When reading about it, it’s also important to understand the basics and the true meaning and function of antigens and antibodies.
So here, in today’s topic, we’ll understand the basic definition and the differentiating points between the antigens and the antibodies.
Antigens vs Antibodies
Antigens are actually big protein molecules that can be found on the pathogen's surface, such as bacteria, fungi, viruses, and other foreign particles. When these hazardous substances enter the body, they trigger an immunological response that leads to the formation of antibodies. On the other hand, Immunoglobulin (Ig) is another name for antibody (Ab) (Ig). Plasma cells create Y-shaped blood proteins that are large in size. They attach to and infiltrate foreign particles. Antigens are foreign pathogens that infiltrate the body and have the ability to elicit an immunological response from our immune system, either by forming a complex with another molecule or by attaching to antibodies for a specific immune response. As a result, antigens cause the system to produce antibodies (Difference Between Antigen and Antibody, n.d.).
Difference Between Antigens and Antibodies in Tabular Form
| Parameters of Comparison | Antigens | Antibodies |
| Basic meaning | Is a foreign substance that causes the immune system to react negatively. | Is a protein in the body that recognizes and binds foreign molecules. |
| Type of molecule | Polysaccharides, lipids, and nucleic acid. | These are Proteins. |
| Origin | Internally formed, but can also be induced externally. | Formed within the body only. |
| Its effects | Causes disease in the body. | Protect the body against foreign materials. |
| Prevalence | Exist in all types of cells. | Exist in specific types of cells (7 Crucial Differences between Antigen and Antibody with Comparison Table, n.d.). |
What is Antigen?
According to experts, Antigen (Ag) is actually a molecule or molecular structure, as well as any foreign particulate matter or pollen grain, that can attach to a specific antibody or T-cell receptor in immunology. Also, Antigens in the body can cause an immunological response. Basically, the term antigen is also referred to as a material that produces antibodies. Proteins, peptides (amino acid chains), polysaccharides (chains of monosaccharides/simple sugars), lipids, and nucleic acids are all examples of antigens.
Antigen receptors, such as antibodies and T-cell receptors, recognise antigens. Immune system cells produce a variety of antigen receptors, each of which is specific for a single antigen. Only the lymphocytes that recognise the antigen are activated and enlarged when they are exposed to it, a process known as clonal selection. Antibodies can usually only react to and attach to one antigen; nevertheless, antibodies can cross-react and bind to many antigens in some conditions.
So basically, The antigen might come from within the body ("self-protein") or from the outside world ("external antigen") ("non-self"). Because of negative selection of T cells in the thymus and B cells in the bone marrow, the immune system recognises and kills "non-self" external antigens but does not normally react to self-protein.
Vaccines are immunogenic antigens that are provided to a recipient with the goal of inducing the adaptive immune system's memory function to antigens of the pathogen invading that recipient. A common example is the seasonal influenza vaccine.
Some Terminologies
- Antigenic determinant, epitope — the unique surface characteristics of an antigen.
Surface characteristics of antigenic molecules, which are typically "large" biological polymers, might operate as locations of interaction for certain antibodies. An epitope is any feature like this. Most antigens can be bound by numerous antibodies, each of which is specific to one of the epitopes of the antigen. The antigen can be viewed as a string of keys (epitopes), each of which corresponds to a different lock in the "lock and key" metaphor (antibody). Complementarity-determining regions are produced differently in different antibody idiotypes.
- A material that can cause an allergic reaction is known as an allergen. The (harmful) reaction might occur as a result of ingestion, inhalation, injection, or skin contact.
- Superantigens are antigens that cause T-cells to get activated in a non-specific manner, resulting in polyclonal T-cell activation and enormous cytokine release.
- Tolerogen — A chemical that, according to its molecular form, causes a specific immunological non-responsiveness. A tolerogen can become an immunogen if its molecular shape is altered.
- Proteins like protein A, protein G, and protein L that can attach to antibodies outside of the antigen-binding site are known as immunoglobulin-binding proteins. Antibodies "attack" antibodies, while immunoglobulin-binding proteins "target" antigens.
- Antigens that require the aid of T cells to promote the production of specific antibodies are known as T-dependent antigens.
- T-independent antigens are antigens that directly excite B lymphocytes.
- Immunodominant antigens - Antigens that outperform all other pathogen antigens in terms of eliciting an immune response. T cell responses are normally directed against a small number of immunodominant epitopes, while they can be diffused over a large number of parasite antigens in some situations (e.g., infection with the malaria pathogen Plasmodium spp.).
Different Sources
Exogenous Antigens
Those Antigens that have entered the body from the outside, such as through inhalation, ingestion, or injection, are known as exogenous antigens. Exogenous antigens often elicit a subclinical immune response. Exogenous antigens are absorbed into antigen-presenting cells (APCs) and digested into pieces via endocytosis or phagocytosis. T helper cells (CD4+) are then presented with the fragments by APCs using class II histocompatibility molecules on their surface. The peptide:MHC complex is specific for some T lymphocytes. They get activated and begin to generate cytokines, which activate CTL, antibody-secreting B cells, macrophages, and other particles.
Endogenous Antigens
Endogenous antigens are produced by normal cell metabolism or by viral or intracellular bacterial infection within normal cells. The fragments are subsequently combined with MHC class I molecules and displayed on the cell surface. When activated cytotoxic CD8+ T cells recognise them, they produce poisons that cause the infected cell to lyse or apoptose. To prevent cytotoxic cells (self-reactive T cells) from killing cells just for presenting self-proteins, tolerance causes the cytotoxic cells (self-reactive T cells) to be destroyed (negative selection). Xenogenic (heterologous), autologous, idiotypic, or allogenic (homologous) antigens are examples of endogenous antigens. Antigens can sometimes be found in the host of an autoimmune illness.
Autoantigens
An autoantigen is a self-protein or protein complex (and occasionally DNA or RNA) that the immune system of people with autoimmune diseases recognises. These self-proteins should not be immune system targets under normal circumstances, but in autoimmune disorders, their linked T cells are not destroyed and instead assault.
Neoantigens
Neoantigens are genes that aren't found in the human genome at all. Neoantigens are more relevant to tumour control than nonmutated self-proteins because the quality of the T cell pool accessible for these antigens is unaffected by central T cell tolerance. Only lately has technology been accessible to evaluate T cell response to neoantigens in a systematic manner. Neoantigens can be discovered and quantified directly using a technology called MANA-SRM developed by Complete Omics Inc., a molecular diagnostics firm, in collaboration with a team at Johns Hopkins University School of Medicine.
Viral antigens
Epitopes produced from viral open reading frames contribute to the pool of neoantigens in virus-associated tumours, such as cervical cancer and a subset of head and neck cancers.
Tumor antigens
Tumor antigens are antigens that are displayed on the surface of tumour cells by MHC class I or MHC class II molecules. Tumor-specific antigens (TSAs) are antigens found only on these cells and are usually the product of a tumor-specific mutation. Antigens that are expressed by both tumour cells and normal cells, known as tumor-associated antigens, are more common (TAAs). Antigen-recognizing cytotoxic T lymphocytes may be able to kill tumour cells.
Tumor antigens can emerge on the tumor's surface in the form of a mutant receptor, for example, and be identified by B cells.
The specificity of antigens
Antigenic specificity refers to a host cell's ability to recognise an antigen as a distinct molecular entity and distinguish it from other antigens with extreme accuracy. The antigen's side-chain conformations are principally responsible for antigen specificity. It does not have to be linear or a rate-limited step or equation to be quantifiable. T cells and B cells are both cells that make up adaptive immunity (Antigen, n.d.).
What is Antibody?
So according to the experts, Antibodies (Ab), that are also known as immunoglobulins (Ig), are big, Y-shaped proteins that provide help to the immune system to identify and kill foreign substances like bacteria and viruses. An antigen, which is a special and unique molecule of the pathogen, is identified by the antibody. Basically, every point in an antibody's "Y" has a paratope (like to a lock) that is actually specific for one epitope (similar to a key) on an antigen, allowing these two structures to bind with precision. So, an antibody can either tag a microbe or an infected cell for attack by other sections of the immune system or neutralise it directly via this binding mechanism.
Now, the antigen-binding sites at both tips of the antibody come in a vast range to allow the immune system to recognise millions of different antigens. The rest of the antibody, on the contrary, remains largely stable. IgA, IgD, IgE, IgG, and IgM are the only varieties that identify the antibody's class or isotype. The Sites that are involved 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 different classes are also released 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 out as the first type, linked to the surface of a B cell — these are known as B-cell receptors (BCR). When an antigen binds 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. Antibodies that are soluble are released into the blood and tissues.
Antibody-mediated immunity is sometimes 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.
Different classes- Antibodies can be classified into distinct isotypes or classes. IgA, IgD, IgE, IgG, and IgM are five antibody classes found in placental mammals, 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 (alpha), (gamma), (delta), (epsilon), and (mu).
- IgA- Prevents pathogen colonisation in mucosal regions such as the stomach, respiratory tract, and urogenital tract. Saliva, tears, and breast milk all contain the substance. The section of the heavy chain within the hinge and Fc area determines the unique characteristics of each class.
- IgD- On the B cells that have not been exposed to antigens, it primarily serves as an antigen receptor. The Basophils and mast cells have been demonstrated to produce antimicrobial substances after being activated by it.
- IgE- Binds to allergens and causes mast cells and basophils to produce histamine, which is important in allergies. IgE was originally developed to protect humans and other animals from parasitic worms, but it is now mostly associated with allergies and asthma.
- IgG- It offers the majority of antibody-based protection against invading pathogens in all four forms. The only antibody that can cross the placenta and give the foetus passive immunity.
- IgM- Extremely avidly expressed on the surface of B cells (monomer) and in a secretory form (pentamer). Pathogens are eliminated in the early stages of B cell-mediated (humoral) protection, before sufficient IgG is present (Antibody, n.d.).
Main Differences Between Antigen and Antibody In Points
- Antigens basically cause the immune system to produce antibodies, which aid in the binding of antigens in the body.
- In nature, antibodies are proteins, whereas antigens are proteins, lipids, or carbohydrates.
- Antibodies protect the body by immobilising antigen materials, which causes illnesses or allergic reactions.
- Exogenous, Endogenous, and Autoantigens are the three types of antigens, while Immunoglobulins M, G, E, D, and A are the five types of antibodies.
- The Bacteria, viruses, and fungi are examples of antigens, whereas B lymphocytes are examples of antibodies (7 Crucial Differences between Antigen and Antibody with Comparison Table, n.d.).
Conclusion
Thus, to conclude, we can say that now we have sufficient knowledge about what is an antigen and what is an antibody. Its very crucial and vital to understand their differences as well. In studying science, especially studying the human body, its very important to go in details of the given topic. That only results in a fruitful learning experience.
References
- 7 Crucial Differences between Antigen and Antibody with Comparison Table. (n.d.). Retrieved from Core Differences: https://coredifferences.com/differences-between-antigen-and-antibody/#:~:text=Core%20Differences%20between%20Antibody%20and%20Antigen%201%20Antigen,body%20by%20immobilizing%20antigen%20materials.%20More%20items...%20
- Antibody. (n.d.). Retrieved from WIKIPEDIA: https://en.wikipedia.org/wiki/Antibody
- Antigen. (n.d.). Retrieved from WIKIPEDIA: https://en.wikipedia.org/wiki/Antigen
