Antibodies or immunoglobulins are proteins that help in fighting foreign substances known as antigens. The immune system produces them as a response to prevent invaders from hurting the body. When any foreign body invades the body, the body's immune system activates. The antigens can be in the form of bacteria, viruses, or other harmful disease-causing organisms. The immune system creates antibodies, which lock themselves with the antigens to destroy them from the body. Based on the structural and immunological properties, there are five types of antibodies- IgM, IgA, IgE, IgG, and IgD.
When an antibody binds with an antigen, many other events co-occur. Here are the five primary functions of the antibodies.
Antibodies activate NK Cells:
NK cells or natural killer cells are lymphocytes that fall in the family of T cells that are primary cells of the body's immune system. These cells have a unique ability to kill virally infected cells and even destroy tumor cells. Antibodies can trigger NK cells, especially in cases where normal antigen-antibody complexes do not work. For virus-infected cells, the antigen-antibody complexes many times do not attract a compliment like the IgA, IgD, or IgE antibodies, or the foreign body cells may be impenetrable by antibody-antigen complements. As per this immunology article, IgG antibodies can bind to the NK cell receptors. Once this chemical reaction occurs, the NK cells inject a protein called perforin in the infected cell, resulting in the swelling and bursting of the bad cells. Thus antibodies indirectly aid in controlling the growth of cancer and killing virally infected cells by triggering killer cells.
Antibodies cause Opsonization:
Opsonization is the process by which antibodies mark a pathogen for destruction by pathogen-killing cells called phagocytes. Cells like macrophages and neutrophils are excellent phagocytes that ingest bacteria and then destroy it. The tail region of an antibody carries a protein called the Fc receptor. The Fc receptor combines with the Fc receptor of a phagocyte like macrophage cells that can eat the bacteria. On the other hand, the antigen-binding fragment (Fab receptor) of an antibody (IgG) binds with the antigen on the bacteria. Thus, the antibody forces the bacteria to move along with the antibody-antigen complex and then, delivers the pathogen to the phagocytes for self-destruction. The pathogen inside the phagocyte undergoes several destructive processes like enzymatic digestion, oxidative destruction, etc. Sometimes, the bacteria may not need to be killed by phagocytosis. The bound pathogen exposes toxic chemicals released by the cell surfaces of the macrophages and neutrophils, also known as lysis products, which eventually kill the bacteria cells without ingestion. This process is also known as antibody-dependent cell-mediated cytotoxicity.
Antibodies cause Agglutination on bacteria-infected cells:
Agglutination is the natural process of clumping of cells when antibodies combine with specific antigens on the cell's surface. The Agglutination by antibodies serves dual functions in the body. Firstly, the antibodies bind with multiple bacteria cells and create a large complex. During the microbial elimination process in the body, it becomes easy for the immune system cells to engulf the large bacterial complexes and kill them through a process called phagocytosis. Thus, the agglutination function of antibodies eliminates harmful bacteria from the body more efficiently. The second function of the Agglutination by antibodies is preventive. The blood cells have antigens and antibodies. When the antigen of a wrong blood group combines with antibodies of another blood group, the red blood cells or erythrocytes clump together and precipitate out. This behavior of antibodies in our blood helps doctors and pathologists to prevent wrong blood transfusions and thus save lives. Doctors can immediately detect the right blood group if the blood does not agglutinate with the test samples in the lab.
4. Antibodies cause Neutralization of pathogens
One of the essential functions of antibodies is that it helps in neutralizing the viral infection. Neutralization is a process where an antibody decreases the replication of viruses. There are different ways of doing it. The neutralizing antibodies can block the attachment of a virus to a host cell. They can prevent the virus from penetrating the cell membrane of the host cell. Even when the virus penetrates a host cell, the neutralizing antibodies can prevent the virus from uncoating its genome. These antibodies work by recognizing specific proteins or antigens on the surface of the virus and then bind to the virus if the antigens complement the antibodies. The binding process activates the classic complement pathway. It leads to the development of holes in the microbes cell wall. Thus, causing the death of the microbe. When creating immunization techniques for viruses, a custom antibody can supplement the body's immune system. Sometimes, the body may be deficient in certain types of proteins, which in turn may cause a depletion in antibody reserve. In such cases, scientists can produce custom antibodies using lab-generated peptides or proteins and then inject them into patients to neutralize viruses.
5. Antibodies cause Precipitation of antigens
Similar to Agglutination, antibodies can remove pathogens from the body by merely inducing a chemical reaction between the soluble antigens and antibodies and forming a solid, insoluble mass which isolates efficiently from the bodily fluids. The precipitated mass can be either excreted or ingested and destroyed by phagocytes. The precipitation action of antibodies is also useful in determining whether a patient has an infection or not. Doctors perform the precipitation test to find out the number of immunoglobulins in the blood. The antigen-antibody reaction creates a solid, insoluble precipitate that indicates the concentration of immunoglobulins in the body. If the precipitate is large, then the number of immunoglobulins released by the body is more, which also shows a highly active immune system that is fighting an infection in the body. Thus antibodies help in detecting infection levels in the body. Based on the precipitation tests, the doctors can recommend a dose of medication to treat the infection.
Thus, antibodies play a significant role in protecting the body from infections, foreign body attacks, and cancer. The functions mentioned above show different ways in which antibodies can attack pathogens and get rid of toxic pathogenic cells from the body. In addition to the above, the immune system has memory cells that rapidly create the correct antibody if the body is under attack by a previous pathogen carrying an old antigen. That is why we do not suffer long from a disease that affected us long back as the body already knows how to develop the correct antibody to complement with the antigen.