So we know that antibodies play a huge role in adaptive immunity, but how exactly do antibodies protect us from pathogens? Well, in this video, we're going to talk about some of the possible outcomes of antibody binding to antigen. Once antibodies are secreted from plasma cells and bind to their very specific antigens, those antibodies can result in many possible outcomes that protect us from pathogens and provide immunity. Notice the numbers below from 1 through 6; we have a few of the possible outcomes of antibody binding to antigen. The term serology refers to the scientific study of blood serum, especially the blood serums antigen-antibody reactions. Once again, we have 6 of the possible antigen-antibody reactions. Notice the very first one is a recall because we've discussed this before in some of our previous lesson videos, and that is antigen-antibody reactions can lead to opsonization, making microbes significantly easier to bind and engulf during phagocytosis. This allows phagocytes to bind to the microbes and engulf them more easily. Each number you see in the text above corresponds with the numbers in our image below. If we take a look at number 1 in our image, notice that we're showing you opsonization. When antibodies bind to antigens on microbes, for example, those antibodies can allow for improved phagocytosis by phagocytes such as macrophages. This macrophage is able to easily phagocytose this microbe and digest and eliminate it, thanks to the antigen-antibody reaction.
The second possible outcome is that antibodies can prevent the adherence of microbes to host cells and immobilize microbes. When antibodies bind to antigens, they can block those antigens from interacting with host cells and causing harm, or the antibodies can bind to pathogens and immobilize them, preventing them from moving in their environments, which ultimately helps prevent the infection of host cells. Looking at our image at number 2, we're showing you the immobilization and prevention of adherence that antibodies can provide. When these antibodies, these Y-shaped proteins, bind to microbes like the blue one here, they can block that microbe's interaction with host cells. In some cases, this can prevent those pathogens from causing us harm. Also, antibodies can bind to structures on the surface of pathogens that immobilize them, such as binding to their flagella and preventing those microbes from moving about in their environments, which can ultimately help prevent those microbes from causing us harm.
The third possible outcome is a recall because we've also talked about this in our previous lesson videos, and that is that antibodies binding to antigens can lead to activation of the complement system. The activation of the complement system occurs via the classical pathway whenever antibodies are involved. Looking at our image at number 3, notice that we're showing you complement system activation again via the classical pathway. Here we have these antibodies binding to these specific antigens on these microbes, leading to the activation of the complement system, activating several complement proteins, and ultimately leading to actions such as inflammation and cell lysis. If you don't remember much about the complement system or the classical pathway, be sure to go back to our older lesson videos to check those out.
The fourth possible outcome is that antibodies can neutralize toxins and viruses. They can lead to toxin or virus neutralization. When antibodies bind to toxins and viruses, they can make those toxins and viruses ineffective and harmless, again helping to protect us and provide immunity. Looking at number 4, notice we're showing you toxin and virus neutralization. The toxin is in red, and we have a virus over here in green. Notice that antibodies are binding to the toxin, neutralizing it, making it ineffective and harmless. Over here, we have a virus, and notice that antibodies are binding to the virus, potentially making this virus ineffective and harmless.
The fifth possible outcome of antibodies binding to antigens is agglutination. Agglutination is just a fancy scientific word for clumping. Antibodies actually have multiple antigen-binding sites. Occasionally, these antibodies can bind to multiple antigens, leading to what is known as cross-linking and can cross-link separate antigens, essentially clumping those antigens together so that they are all relatively in the same location. This makes them much easier to phagocytose in large groups and eliminate pathogens in larger groups. Looking at number 5, notice we're showing you agglutination, which is just clumping. You can see the cross-linking of microbes for phagocytosis, and notice that a single antibody is capable of binding to multiple different antigens, helping to clump these microbes together, so these foreign invaders can all be targeted for immunity and protection more effectively.
Last but not least, the sixth possible outcome of antibodies binding to antigens is that antibodies can lead to what is known as antibody-dependent cell-mediated cytotoxicity, commonly abbreviated as ADCC. ADCC is a process that allows natural killer cells (NK cells) to target and kill antibody-bound infected host cells. We'll be able to talk more about this process, ADCC, later in our course as we move forward. We'll also get to talk more about natural killer cells as well. But if we take a look at our image at number 6, notice we're showing you antibody-dependent cell-mediated cytotoxicity or ADCC. Notice that we have an infected host cell over here in green, this infected host cell is bound by antibodies. When these antibodies bind to the infected host cell, natural killer cells can detect that and release death signals to trigger the infected cell to undergo apoptosis, which ultimately helps to eliminate the pathogen and eliminate the infected cell to help provide us with immunity.
This conclusion here concludes our brief lesson on some of the possible outcomes of antibody binding to antigen. We'll be able to get some practice applying these concepts as we move forward in our course. So, I'll see you all in our next video.
