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 number down below 1 through 6; we have a few of the possible outcomes of antibody binding to antigen.
Now, the term serology refers to the scientific study of blood serum, especially the blood serum's antigen-antibody reactions. Once again, number down below, we have 6 of the possible antigen-antibody reactions. Notice the very first one that we have here is a recall because we've discussed this before in some of our previous lesson videos. That is that antigen-antibody reactions can lead to opsonization, which recall is a process that makes microbes significantly easier to bind and engulf during phagocytosis, allowing phagocytes to bind to the microbes and engulf them much more easily.
Each of these numbers that you see in the text above corresponds with the numbers that we have down below in our image. 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 opsonization, improved phagocytosis by phagocytes such as macrophages. Notice that this macrophage is able to easily phagocytose this microbe and digest and eliminate that microbe, thanks to the antigen-antibody reaction.
The second possible outcome that we have is that antibodies can prevent the adherence of microbes to host cells, and antibodies can also immobilize microbes. When antibodies bind to antigens, again, they can block those antigens from interacting with the host cells and causing them harm. Or, the antibodies can bind to pathogens and immobilize them, meaning that they can prevent them from moving in their environments. Ultimately, that will help prevent the infection of host cells. If we take a look at our image down below at number 2, notice that we're showing you the immobilization, as well as the prevention of adherence that antibodies can provide. When these antibodies, these Y-shaped proteins here, bind to microbes like this blue one here, they can block that microbe's interaction with host cells. In some cases, that can prevent those pathogens from causing us harm. Also, antibodies can bind to structures on the surface of pathogens that can 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 that we have here is also a recall because we have also talked about this in our previous lesson videos. That is that antibodies binding to antigens can lead to the activation of the complement system or complement system activation. Recall that the activation of the complement system is going to occur via the classical pathway, whenever antibodies are involved. If we take a look at our image down below at number 3, notice that we're showing you complement system activation again via the classical pathway. Notice that here we have these antibodies binding to these specific antigens on these microbes and that can lead to the activation of the complement system, activating several complement proteins, and ultimately that will lead to some innate effector 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 that we have is that antibodies can neutralize toxins and viruses. So they can lead to toxin or virus neutralization. When antibodies bind to toxins and, again, viruses, they can make those toxins and viruses ineffective and harmless again, helping to protect us and provide immunity. If we take a look at number 4, notice that we're showing you toxin and virus neutralization here. Notice that we have the toxin in red, and we have the virus over here in the green. Notice that we have antibodies binding to the toxin, again, neutralizing that toxin, making it ineffective and harmless. Over here we have a virus and notice that the antibodies are binding to the virus, potentially making this virus ineffective and harmless.
The fifth possible outcome that we have of antigen to 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, multiple different antigens, and can lead to what is known as cross-linking. This can cross-link separate antigens essentially clumping those antigens together so that those antigens are all relatively in the same location. That makes them a lot easier to phagocytose in large groups and eliminate pathogens in larger groups. If we take a look at our image down below notice, at number 5 over here, we're showing you agglutination which again is just a fancy scientific word for clumping. You can see the cross-linking of microbes for Phagocytosis and again notice that a single antibody is capable of binding to multiple different antigens. Notice that this single antibody is cross-linking this microbe with this other microbe over here because it's bound to both of them. It will help clump these microbes together, so that these foreign invaders can all be targeted for immunity and protection more effectively.
And last but not least the sixth possible outcome that we have listed here of antibodies binding to antigens is that, antibodies can lead to what is known as antibody-dependent cell-mediated cytotoxicity, which is commonly abbreviated as ADCC. ADCC is a process that allows natural killer cells or NK cells to target and kill antibody-bound infected host cells. We'll be able to talk more details about this process here, ADCC, later in our course as we move forward. We'll also get to talk more about natural killer cells as well as we move forward. But if we take a look at our image down below at number 6, notice that we're showing you antibody-dependent cell-mediated cytotoxicity or ADCC. Notice that we have an infected host cell over here in green, and this infected host cell is going to be bound by antibodies. When these antibodies 구d 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 immunity. This 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.
