From the Guidelines
The immune system's primary and secondary lines of defense are crucial for protecting the body against pathogens, with primary defenses including physical barriers and innate mechanisms, and secondary defenses involving inflammation, phagocytosis, and adaptive immune responses. The primary defenses, such as skin and mucous membranes, prevent pathogens from entering the body, while secondary defenses, including inflammation and phagocytosis by white blood cells, activate when primary defenses are breached. Various factors can interfere with these mechanisms, including poor nutrition, stress, inadequate sleep, and certain medications, which can weaken both primary and secondary defenses 1.
Some specific conditions can also compromise these defense mechanisms, such as burns that damage skin barriers, and smoking that impairs ciliary action in respiratory passages. Age significantly impacts immunity, with children having immature immune systems that develop through exposure to pathogens, and elderly individuals experiencing immunosenescence, with declining T-cell function and antibody production, as seen in the reduced effectiveness of influenza vaccines in this population 1.
People with chronic diseases, like diabetes or autoimmune disorders, often have compromised immune function due to the disease itself or treatments like immunosuppressants. Innate immunity, which includes physical barriers and inflammatory responses, provides immediate but general protection, while acquired immunity, which develops through exposure to specific pathogens, offers pathogen-specific defense that can last for years or even a lifetime. Acquired immunity can be further divided into active and passive forms, with active immunity occurring when a person's own immune system produces antibodies after exposure to pathogens, either through natural infection or vaccination, providing long-lasting protection. Passive immunity involves receiving antibodies from another source, such as maternal antibodies crossing the placenta to protect newborns or immunoglobulin injections for immediate but temporary protection.
Key factors that affect the levels of immunity in different populations include:
- Age: Children have immature immune systems, while elderly individuals experience immunosenescence
- Chronic diseases: Conditions like diabetes or autoimmune disorders can compromise immune function
- Vaccination: Influenza vaccines can be effective in preventing secondary complications and reducing the risk for influenza-related hospitalization and death, especially in elderly persons and those with certain chronic diseases 1
- Lifestyle factors: Poor nutrition, stress, inadequate sleep, and certain medications can weaken immune defenses. Overall, understanding the differences between primary and secondary lines of defense, as well as the factors that interfere with these mechanisms, is crucial for developing effective strategies to prevent and treat infections, especially in vulnerable populations such as children, elderly individuals, and those with chronic diseases.
From the Research
Primary and Secondary Lines of Defense
The immune system has two primary lines of defense: innate immunity and adaptive (or acquired) immunity 2.
- Innate immunity is active at several levels, both at potential points of entry and inside the body, and includes physical barriers such as the skin, digestive enzymes, and immune cells like macrophages and lymphocytes.
- Adaptive immunity involves the activation of specific immune cells, such as T cells and B cells, to recognize and respond to pathogens.
Factors Interfering with Immune Mechanisms
Various factors can interfere with these mechanisms, including:
- Age: the immune system is less effective in children and elderly individuals, making them more susceptible to infections 3.
- Chronic diseases: certain diseases, such as HIV and influenza, can evade recognition by the immune system and cause significant harm 4, 3.
- Vaccination: vaccines can stimulate the immune system to produce antibodies and activate immune cells, providing protection against specific pathogens 5.
Active and Passive Immunity
- Active immunity involves the production of antibodies by the immune system in response to direct exposure to a pathogen or vaccination 5.
- Passive immunity involves the transfer of antibodies from one individual to another, providing temporary protection against infection.
Innate and Acquired Immunity
- Innate immunity is the first line of defense against infection and is mediated by physical barriers, immune cells, and proteins such as complement and antimicrobial peptides 2, 4.
- Acquired immunity is the second line of defense and involves the activation of specific immune cells, such as T cells and B cells, to recognize and respond to pathogens 2, 5.
Effects on Different Populations
The immune system is affected differently in various populations, including:
- Children: the immune system is still developing, making them more susceptible to infections 3.
- Elderly individuals: the immune system is less effective, making them more susceptible to infections and less responsive to vaccination 3.
- Individuals with chronic diseases: certain diseases, such as HIV and influenza, can evade recognition by the immune system and cause significant harm 4, 3.