Chapter 21 - Section 21.4 - Cellular Immunity - Before You Go On - Page 829: 18

The immune response can be broadly divided into three phases: the recognition phase, the activation phase, and the effector phase. These phases represent the sequence of events that occur when the immune system encounters a foreign antigen or pathogen. Here's an overview of each phase: 1. **Recognition Phase:** - In this phase, immune cells recognize the presence of foreign antigens or pathogens. This recognition is achieved through interactions between antigen-presenting cells (APCs) and immune receptors on T cells and B cells. - Antigen-presenting cells (such as dendritic cells) capture and process antigens. They then present antigen fragments on their surface using major histocompatibility complex (MHC) molecules. - T cells and B cells have receptors (TCRs for T cells, B cell receptors for B cells) that can specifically bind to these antigen-MHC complexes. This recognition is the first step in initiating an immune response. 2. **Activation Phase:** - Once recognition occurs, the activated immune cells enter the activation phase, during which they undergo clonal expansion and differentiation. - Clonal expansion involves the rapid proliferation of antigen-specific T and B cells, resulting in a larger population of cells with identical receptors. - Differentiation leads to the formation of various effector cells, each with specialized functions. For example, activated T cells can differentiate into cytotoxic T cells, helper T cells, and regulatory T cells. B cells can differentiate into plasma cells that produce antibodies. 3. **Effector Phase:** - The effector phase is characterized by the actions of the differentiated effector cells to eliminate the antigen or pathogen. - Cytotoxic T cells kill infected or abnormal cells, helper T cells assist in the activation of other immune cells, and regulatory T cells modulate the immune response to prevent excessive reactions. - Plasma cells produce antibodies that target the antigen, neutralize pathogens, and promote their clearance by other immune mechanisms. - Other immune cells, such as macrophages and neutrophils, contribute to the effector phase by engulfing and destroying pathogens through phagocytosis. These three phases are interconnected and represent a coordinated sequence of events that collectively make up the immune response. The immune system's ability to recognize, activate, and effectuate responses against foreign antigens is crucial for maintaining health and protecting the body from infections.

The immune response can be broadly divided into three phases: the recognition phase, the activation phase, and the effector phase. These phases represent the sequence of events that occur when the immune system encounters a foreign antigen or pathogen. Here's an overview of each phase: 1. **Recognition Phase:** - In this phase, immune cells recognize the presence of foreign antigens or pathogens. This recognition is achieved through interactions between antigen-presenting cells (APCs) and immune receptors on T cells and B cells. - Antigen-presenting cells (such as dendritic cells) capture and process antigens. They then present antigen fragments on their surface using major histocompatibility complex (MHC) molecules. - T cells and B cells have receptors (TCRs for T cells, B cell receptors for B cells) that can specifically bind to these antigen-MHC complexes. This recognition is the first step in initiating an immune response. 2. **Activation Phase:** - Once recognition occurs, the activated immune cells enter the activation phase, during which they undergo clonal expansion and differentiation. - Clonal expansion involves the rapid proliferation of antigen-specific T and B cells, resulting in a larger population of cells with identical receptors. - Differentiation leads to the formation of various effector cells, each with specialized functions. For example, activated T cells can differentiate into cytotoxic T cells, helper T cells, and regulatory T cells. B cells can differentiate into plasma cells that produce antibodies. 3. **Effector Phase:** - The effector phase is characterized by the actions of the differentiated effector cells to eliminate the antigen or pathogen. - Cytotoxic T cells kill infected or abnormal cells, helper T cells assist in the activation of other immune cells, and regulatory T cells modulate the immune response to prevent excessive reactions. - Plasma cells produce antibodies that target the antigen, neutralize pathogens, and promote their clearance by other immune mechanisms. - Other immune cells, such as macrophages and neutrophils, contribute to the effector phase by engulfing and destroying pathogens through phagocytosis. These three phases are interconnected and represent a coordinated sequence of events that collectively make up the immune response. The immune system's ability to recognize, activate, and effectuate responses against foreign antigens is crucial for maintaining health and protecting the body from infections.