Chapter 21 - Section 21.5 - Humoral Immunity - Before You Go On - Page 837: 22

Antibodies, also known as immunoglobulins (Igs), are versatile molecules that play a key role in the immune response. They can act against antigens through various mechanisms, contributing to the neutralization and elimination of pathogens. Here are four ways in which antibodies act against antigens: 1. **Neutralization:** Antibodies can neutralize pathogens by binding to their surface and blocking their ability to infect host cells. This can prevent the pathogen from attaching to or entering target cells, effectively rendering the pathogen harmless. Neutralization is particularly effective against viruses and bacterial toxins. By binding to the active sites of toxins, antibodies can prevent the toxins from interacting with their cellular targets. 2. **Opsonization:** Antibodies can act as opsonins, molecules that mark pathogens for phagocytosis by immune cells such as macrophages and neutrophils. Antibodies bind to antigens on the surface of pathogens, creating a "handle" that makes it easier for phagocytes to recognize and engulf the pathogen. Opsonization enhances the efficiency of phagocytosis and the clearance of pathogens. 3. **Complement Activation:** Antibodies can trigger the activation of the complement system, a group of proteins that enhance the immune response. When antibodies bind to antigens on the surface of pathogens, they can initiate the classical pathway of complement activation. This leads to the formation of membrane attack complexes (MACs) that create pores in the pathogen's membrane, causing it to lyse (burst) and die. 4. **Antibody-Dependent Cellular Cytotoxicity (ADCC):** Antibodies can mediate the destruction of target cells by engaging immune cells that possess Fc receptors, such as natural killer (NK) cells. NK cells recognize the Fc portion of antibodies bound to target cells and release cytotoxic substances, such as perforin and granzymes, which induce apoptosis in the target cell. ADCC is effective against cells that are coated with antibodies, such as virus-infected cells and cancer cells. 5. **Cross-linking and Agglutination:** Antibodies can bind to multiple antigens on the surface of pathogens, cross-linking them and causing them to clump together in a process called agglutination. This aggregation makes it easier for immune cells to recognize and eliminate the clustered pathogens. These mechanisms collectively enable antibodies to neutralize, target, and eliminate a wide range of pathogens, contributing to the body's defense against infections. The specific mechanism used by antibodies depends on the nature of the antigen, the type of pathogen, and the context of the immune response.

Antibodies, also known as immunoglobulins (Igs), are versatile molecules that play a key role in the immune response. They can act against antigens through various mechanisms, contributing to the neutralization and elimination of pathogens. Here are four ways in which antibodies act against antigens: 1. **Neutralization:** Antibodies can neutralize pathogens by binding to their surface and blocking their ability to infect host cells. This can prevent the pathogen from attaching to or entering target cells, effectively rendering the pathogen harmless. Neutralization is particularly effective against viruses and bacterial toxins. By binding to the active sites of toxins, antibodies can prevent the toxins from interacting with their cellular targets. 2. **Opsonization:** Antibodies can act as opsonins, molecules that mark pathogens for phagocytosis by immune cells such as macrophages and neutrophils. Antibodies bind to antigens on the surface of pathogens, creating a "handle" that makes it easier for phagocytes to recognize and engulf the pathogen. Opsonization enhances the efficiency of phagocytosis and the clearance of pathogens. 3. **Complement Activation:** Antibodies can trigger the activation of the complement system, a group of proteins that enhance the immune response. When antibodies bind to antigens on the surface of pathogens, they can initiate the classical pathway of complement activation. This leads to the formation of membrane attack complexes (MACs) that create pores in the pathogen's membrane, causing it to lyse (burst) and die. 4. **Antibody-Dependent Cellular Cytotoxicity (ADCC):** Antibodies can mediate the destruction of target cells by engaging immune cells that possess Fc receptors, such as natural killer (NK) cells. NK cells recognize the Fc portion of antibodies bound to target cells and release cytotoxic substances, such as perforin and granzymes, which induce apoptosis in the target cell. ADCC is effective against cells that are coated with antibodies, such as virus-infected cells and cancer cells. 5. **Cross-linking and Agglutination:** Antibodies can bind to multiple antigens on the surface of pathogens, cross-linking them and causing them to clump together in a process called agglutination. This aggregation makes it easier for immune cells to recognize and eliminate the clustered pathogens. These mechanisms collectively enable antibodies to neutralize, target, and eliminate a wide range of pathogens, contributing to the body's defense against infections. The specific mechanism used by antibodies depends on the nature of the antigen, the type of pathogen, and the context of the immune response.