The words sound almost the same, but they are completely different things. “Anti” comes from Greek, meaning against. Both antibodies and antibiotics act against pathogens, but they are very different. Antibodies are Y-shaped proteins produced by specialized white blood cells. Antibiotics are antibacterial compounds that are, at least originally, produced by microbes.  
    There are at least five types of antibodies: IgG, IgM, IgA, IgD and IgE. IgG and IgM circulate in the blood steam, while IgA is secreted into mucus and other body fluids. IgE is involved in allergic reactions, and IgD helps activate cells to produce antibodies. They all have a Y shape with variations. The upper part of the Y binds to an antigen which can be the pathogen or part of the pathogen. The lower part of the Y binds to other components of the immune system. Antibodies have a variety of roles. Some will directly kill the target organism; others will wrap the invaders in a mucus coating; others will bind to the antigen to tag it for killing by other immune cells. Antibodies circulate throughout the body or are directly attached to the surface of a B lymphocyte, which is a type of white blood cell. B lymphocytes have specialized IgM antibodies on their cell surface. Each B cell has an antibody that responds to a very specific antigen. If a B cell finds its specific antigen, the antigen binds to the IgM on the cell surface. This causes the B cell to reproduce and become a plasma cell. Plasma cells are the body’s antibody factories; each cell can produce massive amounts of antibodies. These antibodies circulate throughout the bloodstream and lymphatic system. After antibodies bind antigens, other white blood cells that have the ability to destroy the pathogen interact with the bound antigen and destroy pathogens though a variety of methods. Some activated B cells do not become plasma cells, but instead become memory cells that survive in the body and allow the immune system to respond to the same pathogen more quickly in the future.
    Antibiotics are substances originally produced by one type of microbe that destroy other microbes. An antibacterial drug is anything that kills bacteria, including antibiotics, synthetic antibacterials and antiseptics. For example, sulfonamide drugs are technically not antibiotics but are antibacterials, because they are not produced by microbes but are effective in killing bacteria. However, the term antibiotic is often commonly used to include other antibacterials such as sulfonamide drugs that did not originate by microbial production. An antimicrobial is anything that kills any type of microbe; thus, this would include antibacterial, antiviral and other drugs. Antibiotics are a specialized type of antibacterial drugs. Antibiotics are typically effective only against bacteria, though some antibiotics can have effects on certain protozoal, parasitic organisms. Antibiotics have no activity against viruses. On the other hand, antiseptics typically kill a wide variety of organisms that may include bacteria, viruses and parasitic protozoans, such as cryptosporidium. However, antiseptics differ widely in their effects on various pathogens. Some may be antibacterial and antiviral, for example, while others may be only antibacterial. Antiseptics, like iodine in teat dips, are typically used topically and not ingested.
    Antibiotics can kill bacteria, which is called bactericidal, or inhibit bacteria, which is called bacteriostatic. Just because an antibiotic is bacteriostatic does not mean it is less effective than a bactericidal one because the life of a bacterium is very short. If a drug inhibits bacterial reproduction, it may, for all practical purposes, act just as quickly as a bactericidal drug. For any antibiotic to be effective, its concentration in the target tissues must exceed the minimum inhibitory concentration, or MIC, required to kill the target organism. This is why some antibiotics are more effective for disease in certain body areas. Some drugs achieve high concentrations in the lungs and others in the gut, for example.
    Technically, both antibiotics and antibodies are produced by an organism to fight pathogens. Antibodies, though, are produced by the host body to fight invading organisms. Antibiotics, while originally produced by an organism to fight invaders, are substances given to other, more complex organisms, like humans and cows, to fight pathogens. Thus, they are entirely different things that at least in some ways accomplish the same thing.
    Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.