Makindo Medical Notes

Overview of the Immune Response

The immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful pathogens like bacteria, viruses, fungi, and parasites. The immune response can be divided into two main components: the innate immune response and the adaptive immune response. These systems collaborate to detect, destroy, and remember pathogens, ensuring both an immediate defense and long-term protection.

Innate Immune Response

The innate immune response is the first line of defense, acting immediately or within hours of encountering a pathogen. This response is non-specific, meaning it targets a broad range of pathogens rather than specific invaders. It includes physical barriers, chemical signals, and specialized immune cells.

• Physical barriers: The skin, mucous membranes, and secretions (e.g., saliva, tears) form a protective barrier that prevents pathogens from entering the body. If pathogens do breach these barriers, internal defenses are activated.
• Phagocytes: Phagocytic cells such as macrophages, neutrophils, and dendritic cells play a key role in engulfing and destroying pathogens. When they encounter a pathogen, they internalize it through a process called phagocytosis, breaking it down within the cell. Macrophages also release cytokines that recruit more immune cells to the infection site.
• Natural killer (NK) cells: NK cells are crucial in detecting and destroying virus-infected cells and tumor cells. Unlike other immune cells, NK cells do not rely on specific antigen recognition. Instead, they detect abnormal cells through changes in their surface markers and release cytotoxic substances to induce cell death.
• Inflammation: Inflammation is a hallmark of the innate immune response. When tissues are damaged or infected, chemical signals such as histamines and cytokines are released. These chemicals cause blood vessels to dilate and become more permeable, allowing immune cells like neutrophils and macrophages to move into the infected area, where they engulf and destroy pathogens.
• Complement system: The complement system is a group of proteins that circulate in the blood and become activated in response to pathogens. The complement system helps to "complement" the ability of antibodies and phagocytes to clear microbes, promotes inflammation, and can directly lyse (break down) bacterial cell membranes.
• Pattern recognition receptors (PRRs): PRRs, such as Toll-like receptors (TLRs), recognize common molecular patterns found on pathogens (pathogen-associated molecular patterns, PAMPs). This recognition triggers the activation of immune responses, including the release of cytokines and the recruitment of immune cells to the site of infection.

Adaptive Immune Response

The adaptive immune response is slower to activate but highly specific to the pathogen. It provides long-lasting protection through "immunological memory" and is divided into two main components: humoral immunity and cell-mediated immunity. The adaptive response is triggered when innate immunity is not sufficient to eliminate the pathogen.

• Antigen recognition: The adaptive immune system is activated when specialized immune cells (B cells and T cells) recognize specific molecules on the surface of the pathogen called antigens. Antigens are unique to each pathogen and are identified by receptors on B and T cells.
• B cells and antibody production (Humoral Immunity): B cells are responsible for producing antibodies. When a B cell encounters its specific antigen, it differentiates into a plasma cell that secretes large quantities of antibodies. Antibodies neutralize pathogens by binding to them, preventing them from entering host cells, and marking them for destruction by other immune cells, such as macrophages.
• Helper T cells (CD4+ T cells): Helper T cells play a central role in coordinating the immune response. After recognizing antigens presented by antigen-presenting cells (APCs), such as dendritic cells, helper T cells release cytokines that activate other immune cells, including B cells, cytotoxic T cells, and phagocytes. They are crucial in both humoral and cell-mediated immunity.
• Cytotoxic T cells (CD8+ T cells) (Cell-Mediated Immunity): Cytotoxic T cells directly attack and kill virus-infected cells and cancer cells. After recognizing infected cells via their antigen-specific receptors, cytotoxic T cells release enzymes that induce apoptosis (programmed cell death), eliminating the infected cells before the pathogen can replicate further.
• Immunological memory: After the initial infection is cleared, memory B cells and memory T cells remain in the body. These cells "remember" the pathogen and can mount a faster and stronger response if the same pathogen is encountered again, providing long-term immunity.

Types of Immunity

There are two primary types of immunity: active immunity and passive immunity.

• Active immunity: Active immunity develops when the body is exposed to a pathogen, either through infection or vaccination. In response, the immune system generates its own antibodies and memory cells. This type of immunity is long-lasting and can sometimes provide lifelong protection. Vaccines are a common way to induce active immunity without causing disease.
• Passive immunity: Passive immunity occurs when antibodies are transferred from one person to another, providing immediate but temporary protection. For example, newborns receive antibodies from their mothers through the placenta and breast milk, offering protection during the early months of life. Passive immunity can also be provided through antibody therapies, such as those used for post-exposure prophylaxis.

Examples of Immune Responses

• Response to a viral infection: When a virus enters the body, the innate immune system responds first by activating NK cells, which destroy infected cells. Simultaneously, dendritic cells present viral antigens to helper T cells, which then activate B cells to produce virus-specific antibodies. Cytotoxic T cells target and kill infected cells, and memory cells are generated to provide long-term immunity.
• Response to a bacterial infection: Phagocytes, such as macrophages and neutrophils, are quickly recruited to the site of infection, where they engulf and destroy bacteria. The complement system also becomes activated, helping to lyse bacterial cells. If the bacteria evade these defenses, B cells produce antibodies that neutralize the bacteria, while helper T cells assist in coordinating a more robust immune response.

Conclusion

The immune response to a pathogen involves a coordinated effort between the innate and adaptive immune systems. The innate immune system provides an immediate, non-specific response, while the adaptive immune system generates a targeted, long-lasting defense. Active and passive immunity are key to protecting the body from future infections, ensuring both immediate defense and lifelong protection in some cases.