Innate Immunity | Biology for MCAT PDF Download

Introduction

The human body encounters numerous potentially harmful pathogens on a daily basis, including bacteria and viruses. Despite this constant exposure, most individuals are able to function normally without falling ill. This is thanks to the human immune system, which consists of a complex network of defenses. The immune system can be broadly divided into two main classes: the innate immune system and the adaptive immune system. This article will focus on the innate immune system, which serves as the body's first line of defense against pathogens.

Self vs. Non-self: How does the body know?

To effectively protect the body, the immune system must be able to distinguish between particles that belong to the body (self) and those that are foreign (non-self). Self particles refer to molecules, such as proteins, that are produced by the body and are not targeted for destruction by the immune system. On the other hand, non-self particles are recognized as potentially harmful and include bacteria, viruses, parasites, pollen, dust, and toxic chemicals.
Non-self particles, especially infectious or pathogenic ones, produce proteins called antigens. Antigens act as identifiers, signaling the presence of pathogens to the immune system. They can be either whole pathogens or smaller proteins expressed by the pathogens. Different antigens can be general, indicating a potential threat, or specific, providing detailed information about the pathogen's characteristics.

The Innate Immune System

The innate immune system is comprised of various mechanisms that are immediately activated upon pathogen invasion. Its primary goal is to prevent the entry and spread of viruses, bacteria, parasites, and other foreign particles throughout the body. The key components of the innate immune system include:

• Physical Barriers: These barriers, such as the skin, gastrointestinal tract, respiratory tract, nasopharynx, cilia, and body hair, serve as the body's first line of defense by preventing the entry of pathogens.
• Defense Mechanisms: Secretions, such as mucous, bile, gastric acid, saliva, tears, and sweat, possess antimicrobial properties that help neutralize pathogens.
• General Immune Responses: Inflammation, complement activation, and non-specific cellular responses are part of the innate immune system's arsenal. Inflammation involves increasing blood flow to the site of infection, while complement proteins mark pathogens for destruction and create holes in their cell membranes.

Cells of the Innate Immune System

The innate immune system relies on specialized white blood cells, known as leukocytes, to defend the body. These cells circulate throughout the body via the circulatory system. Here are some key leukocytes involved in innate immunity:

• Phagocytes: Phagocytes are cells that engulf and destroy potential threats like bacteria and viruses. They act as the body's security guards, constantly patrolling for pathogens.
  
• Macrophages: Macrophages are efficient phagocytes that can exit the circulatory system and move across capillary walls. This ability allows them to hunt pathogens more effectively and recruit other cells to the site of infection through cytokine release.
  
• Mast Cells: Found in mucous membranes and connective tissues, mast cells play a crucial role in wound healing and defense against pathogens. When activated, mast cells release cytokines and granules that initiate an inflammatory response and attract other immune cells to the infection site.
• Neutrophils: Neutrophils are highly abundant phagocytes that are among the first cells to arrive at the site of infection. They contain granules that are toxic to bacteria and fungi, effectively stopping their growth or killing them upon contact.
• Eosinophils: Eosinophils target multicellular parasites and secrete toxic proteins and free radicals to eliminate them. While they make up a small portion of white blood cells, eosinophils are found in various locations throughout the body.
• Basophils: Similar to mast cells, basophils release histamine and participate in allergic responses. They are also involved in combating multicellular parasites.
• Natural Killer Cells: Natural killer cells destroy infected host cells to prevent the spread of infection. They respond to specific signals from compromised cells and antigen presentation.
• Dendritic Cells: Dendritic cells, located in tissues, act as antigen-presenting cells. They play a vital role in identifying threats, relaying information to the immune system, and bridging the gap between the innate and adaptive immune systems.

The Complement System

The complement system is an integral part of the innate immune response. It consists of proteins circulating in the blood that enhance the immune response. When activated, these proteins initiate a cascade of events, including:

• Opsonization: Marking foreign particles for phagocytosis by tagging infected cells and identifying circulating pathogens expressing the same antigens.
• Chemotaxis: Attracting macrophages and neutrophils to the site of infection through chemical signals (cytokines and chemokines).
• Cell Lysis: Breaking down the membrane of foreign cells, impairing their ability to proliferate and spread.
• Agglutination: Clustering and binding pathogens together, facilitating their elimination by other immune cells.

The complement system complements other aspects of the immune response by aiding in the detection and removal of antigens, promoting efficient immune surveillance.

Conclusion

The innate immune system acts as the body's initial defense against pathogens, using physical barriers, defense mechanisms, and general immune responses to protect against infection. It relies on a variety of specialized leukocytes and the complement system to detect and eliminate foreign particles. While the innate immune response is effective in many cases, it can also collaborate with the adaptive immune system when necessary. By understanding the intricacies of innate immunity, we can better appreciate the remarkable complexity of the human immune system's defense mechanisms.