1. What are the key differences between innate immunity and adaptive immunity?
Answer:
Innate immunity is the body’s first line of defense and is non-specific, meaning it does not differentiate between different pathogens. It includes physical barriers such as skin and mucous membranes, and chemical defenses like stomach acid. Cells such as phagocytes (neutrophils and macrophages) and natural killer cells play an essential role in innate immunity. It responds immediately or within hours of an infection. Importantly, innate immunity does not improve with subsequent exposure to the same pathogen.
In contrast, adaptive immunity is specific to particular pathogens and involves the recognition of specific antigens. It requires activation by the innate immune response and involves specialized cells such as B cells and T cells. Adaptive immunity has two main branches: humoral immunity, mediated by B cells and the antibodies they produce, and cell-mediated immunity, involving T cells. Unlike innate immunity, adaptive immunity has memory, meaning it responds more quickly and effectively upon subsequent exposure to the same pathogen.
2. Describe the role of macrophages in innate immunity.
Answer:
Macrophages are large white blood cells that play a central role in innate immunity by recognizing and eliminating pathogens. They are part of the body’s first line of defense against infections. When pathogens enter the body, macrophages detect them using pattern recognition receptors (PRRs) that bind to pathogen-associated molecular patterns (PAMPs) found on microbial surfaces. This process triggers the phagocytosis of the pathogen.
Once engulfed, the pathogen is enclosed in a vesicle within the macrophage. The macrophage then releases enzymes and reactive oxygen species to kill the pathogen. Additionally, macrophages act as antigen-presenting cells (APCs), displaying fragments of the pathogen (antigens) on their surface to activate adaptive immune responses, specifically T cells.
3. What is the function of antibodies in adaptive immunity?
Answer:
Antibodies, also known as immunoglobulins, are proteins produced by B cells in response to specific antigens. Their primary function is to neutralize pathogens like bacteria and viruses by binding to them. Antibodies can neutralize pathogens directly by preventing their entry into host cells, agglutinating (clumping) pathogens for easier removal, or activating the complement system to lyse pathogens.
In addition to neutralization, antibodies play a key role in opsonization, where they mark pathogens for destruction by phagocytes. Antibodies also help prevent infections by recognizing specific pathogens and binding to them, marking them for later recognition by other immune cells. The diversity of antibodies produced allows the immune system to respond to a wide variety of pathogens.
4. Explain the concept of immunological memory and its significance in adaptive immunity.
Answer:
Immunological memory is the ability of the immune system to “remember” a pathogen after an initial exposure and respond more rapidly and effectively during subsequent exposures. This is a key feature of adaptive immunity. When a pathogen invades the body, the immune system produces specific B and T cells that recognize the pathogen’s antigens. Some of these cells become memory cells after the infection is cleared. Memory B cells retain the ability to produce antibodies, while memory T cells are able to recognize the same pathogen and launch a faster immune response.
The significance of immunological memory is seen in vaccinations. A vaccine introduces a harmless form of a pathogen or its components, prompting the immune system to develop memory without causing disease. Upon later exposure to the actual pathogen, the immune system responds more quickly and efficiently, often preventing illness.
5. What are the main types of T cells involved in adaptive immunity, and what are their functions?
Answer:
There are two main types of T cells involved in adaptive immunity: helper T cells (Th cells) and cytotoxic T cells (Tc cells).
- Helper T cells (CD4+ T cells): These cells are critical in activating and regulating other immune cells. They bind to antigen-presenting cells (APCs) that display foreign antigens via major histocompatibility complex (MHC) class II molecules. Once activated, helper T cells release cytokines that stimulate B cells to produce antibodies, activate cytotoxic T cells, and enhance macrophage activity.
- Cytotoxic T cells (CD8+ T cells): These cells are responsible for directly killing infected cells and cancer cells. They recognize infected cells that display foreign antigens on MHC class I molecules. Upon recognition, cytotoxic T cells release cytotoxic substances like perforin and granzymes, which induce apoptosis (programmed cell death) in the infected cells.
Together, these T cells coordinate the adaptive immune response, with helper T cells promoting immunity and cytotoxic T cells eliminating infected cells.
6. How do vaccines work to stimulate adaptive immunity?
Answer:
Vaccines work by mimicking an infection without causing disease. They introduce a harmless version of a pathogen or its components, such as proteins or inactivated viruses, to the immune system. This stimulates the body to produce an immune response, including the generation of specific antibodies and the activation of memory B and T cells.
When the immune system encounters the pathogen in the future, memory cells recognize the pathogen’s antigens and respond more quickly and effectively. Vaccines thus provide immunity by preparing the immune system for future infections, often preventing the onset of disease or reducing its severity.
7. Describe the role of dendritic cells in initiating adaptive immunity.
Answer:
Dendritic cells are essential for the initiation of adaptive immunity, acting as key antigen-presenting cells (APCs). They are located at the interface between the body and the environment, such as the skin and mucosal surfaces. Dendritic cells capture pathogens through phagocytosis or receptor-mediated endocytosis and process these pathogens into smaller peptides.
After processing, dendritic cells migrate to lymph nodes, where they present the pathogen-derived antigens on their surface via MHC molecules to T cells. This activates T cells, particularly helper T cells, which in turn help activate B cells and cytotoxic T cells. Dendritic cells are crucial for bridging the innate and adaptive immune systems.
8. What is the complement system, and how does it contribute to innate immunity?
Answer:
The complement system is a group of proteins in the blood and tissue fluids that assist in immune responses, particularly in innate immunity. These proteins work together to enhance the ability of antibodies and phagocytes to clear pathogens from the body. The complement system can be activated through three pathways: the classical pathway, the alternative pathway, and the lectin pathway.
Once activated, complement proteins contribute to pathogen elimination in several ways. They can directly lyse pathogens by forming membrane attack complexes, enhance phagocytosis through opsonization, and promote inflammation by recruiting immune cells to the site of infection. The complement system acts as a bridge between innate and adaptive immunity by enhancing both the recognition and elimination of pathogens.
9. What is the role of the major histocompatibility complex (MHC) in adaptive immunity?
Answer:
The major histocompatibility complex (MHC) plays a crucial role in adaptive immunity by helping the immune system recognize foreign pathogens. There are two main classes of MHC molecules: MHC class I and MHC class II.
- MHC class I molecules are found on the surface of all nucleated cells and present antigens derived from intracellular pathogens (e.g., viruses) to cytotoxic T cells (CD8+). This helps T cells recognize and destroy infected cells.
- MHC class II molecules are expressed on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. These molecules present antigens from extracellular pathogens (e.g., bacteria) to helper T cells (CD4+), initiating a cascade of immune responses.
MHC molecules are essential for antigen presentation and the activation of both helper and cytotoxic T cells in adaptive immunity.
10. Explain the concept of clonal selection in adaptive immunity.
Answer:
Clonal selection is the process by which a specific B or T cell is activated by an antigen. When a pathogen enters the body, it carries unique antigens that are recognized by specific receptors on B and T cells. Each B or T cell receptor is unique, and only the cell with the receptor that matches the pathogen’s antigen will be activated. This cell then undergoes clonal expansion, rapidly dividing to produce a large number of identical cells (clones). Some of these clones become effector cells that fight the pathogen, while others become memory cells that provide long-term immunity.
Clonal selection ensures that only the immune cells capable of recognizing and responding to the pathogen are activated, leading to an efficient and specific immune response.
11. What is the difference between active and passive immunity?
Answer:
Active immunity occurs when the body produces its own antibodies or activates T cells in response to an infection or vaccination. This type of immunity is long-lasting because the immune system creates memory cells that can recognize the pathogen if it is encountered again. Vaccines are an example of active immunity, where a weakened or inactivated pathogen prompts the immune system to create a defense without causing disease.
Passive immunity, on the other hand, occurs when antibodies or immune cells are transferred from another individual. For example, maternal antibodies passed through the placenta or breast milk provide temporary immunity to a newborn. Passive immunity is short-lived because the recipient does not produce memory cells or antibodies on their own.
12. What are the various types of pathogens that the immune system defends against?
Answer:
The immune system defends against a wide variety of pathogens, including:
- Bacteria: These are single-celled microorganisms that can cause infections such as tuberculosis, pneumonia, and urinary tract infections. The immune system uses phagocytosis and antibodies to target bacterial infections.
- Viruses: Viruses are intracellular pathogens that infect host cells. The immune system uses cytotoxic T cells to destroy infected cells and neutralizing antibodies to block viral replication.
- Fungi: Fungal infections, such as candidiasis and athlete’s foot, are defended against by phagocytes and antibodies. In some cases, T cells also help protect against fungi.
- Parasites: Protozoa, helminths, and arthropods can cause parasitic infections. The immune system combats parasites through immune cells like eosinophils and antibodies, which target the outer membranes of these pathogens.
13. What are the factors that can impair the immune system’s function?
Answer:
Several factors can impair the immune system’s function, including:
- Genetic disorders: Conditions like severe combined immunodeficiency (SCID) or X-linked agammaglobulinemia lead to compromised immune function.
- Age: As people age, their immune systems tend to become less efficient, leading to increased susceptibility to infections.
- Nutritional deficiencies: A lack of essential nutrients, such as vitamins and minerals, can weaken immune responses. For instance, vitamin D deficiency can impair T cell function.
- Chronic diseases: Conditions such as diabetes, HIV/AIDS, and cancer can impair immune function, making individuals more vulnerable to infections.
- Immunosuppressive treatments: Drugs such as corticosteroids, chemotherapy, and immunosuppressants can weaken the immune system to prevent organ transplant rejection or control autoimmune diseases.
14. What is an autoimmune disease, and how does it occur?
Answer:
An autoimmune disease occurs when the immune system mistakenly targets and attacks the body’s own cells and tissues as if they were foreign invaders. Normally, the immune system can differentiate between self and non-self, but in autoimmune diseases, this ability is compromised. Examples of autoimmune diseases include rheumatoid arthritis, type 1 diabetes, and multiple sclerosis.
The exact cause of autoimmune diseases is not fully understood, but they are thought to arise due to a combination of genetic predisposition and environmental triggers, such as infections or certain chemicals. The immune system’s failure to distinguish self from non-self can result in chronic inflammation and tissue damage.
15. Describe the role of neutrophils in innate immunity.
Answer:
Neutrophils are the most abundant type of white blood cells in the body and are a key component of innate immunity. They respond quickly to infections, especially bacterial ones. Neutrophils are recruited to infection sites by signaling molecules such as chemokines. Upon arrival, they engulf pathogens through phagocytosis, where they digest and destroy the pathogen using enzymes and reactive oxygen species.
Neutrophils can also release antimicrobial peptides and proteins, and they form neutrophil extracellular traps (NETs), which are networks of DNA and proteins that trap and kill pathogens. Neutrophils are essential for the early defense against infections and for orchestrating the inflammatory response.
16. What are the functions of cytokines in immune responses?
Answer:
Cytokines are small signaling molecules that play a critical role in regulating the immune response. They are produced by various immune cells, such as macrophages, T cells, and dendritic cells, in response to infection or injury. Cytokines serve as messengers that allow immune cells to communicate with each other and coordinate the immune response.
The main functions of cytokines include:
- Promoting inflammation: Cytokines like interleukins (ILs) and tumor necrosis factor (TNF) help recruit immune cells to the site of infection.
- Stimulating immune cell activity: Cytokines activate B and T cells, macrophages, and other immune cells to enhance their pathogen-fighting abilities.
- Regulating immune response: Some cytokines have anti-inflammatory effects, helping to resolve the immune response after an infection is cleared.
17. How does the skin function as a barrier in innate immunity?
Answer:
The skin is one of the most important physical barriers in innate immunity, preventing pathogens from entering the body. It is composed of multiple layers, with the outermost being the epidermis, which contains keratin, a tough protein that resists pathogen invasion. The skin also has sebaceous glands that secrete sebum, a greasy substance that inhibits the growth of certain bacteria and fungi.
Additionally, the skin produces antimicrobial peptides that directly kill or inhibit the growth of pathogens. The low pH of sweat also acts as a barrier, discouraging microbial growth. If the skin is compromised (e.g., through cuts or abrasions), the immune system’s other defenses, such as phagocytes, are activated to combat pathogens.
18. What are the characteristics of the adaptive immune response to a virus infection?
Answer:
The adaptive immune response to a viral infection involves both humoral immunity (mediated by B cells and antibodies) and cell-mediated immunity (mediated by T cells). When a virus infects a cell, viral proteins are processed and presented on the cell’s surface using MHC class I molecules. Cytotoxic T cells recognize these viral antigens and destroy infected cells.
B cells are activated by helper T cells to produce antibodies specific to the virus. These antibodies can neutralize the virus by binding to it and preventing its entry into host cells. The immune system also develops memory B and T cells, which remain in circulation and provide faster responses if the same virus is encountered again.
19. What are the functions of the lymphatic system in immunity?
Answer:
The lymphatic system plays a crucial role in immune responses by transporting lymph, a fluid that contains white blood cells, including lymphocytes. The lymphatic vessels carry lymph from tissues back to the bloodstream. Lymph nodes, which are distributed throughout the body, act as filtration stations where immune cells can detect and respond to pathogens.
Lymph nodes contain large numbers of B and T cells that become activated when they encounter pathogens. The spleen, another component of the lymphatic system, filters blood and stores immune cells that can respond to bloodborne pathogens. The lymphatic system also aids in the transport of antibodies and immune cells, ensuring a coordinated immune response.
20. How does the immune system differentiate between self and non-self?
Answer:
The immune system distinguishes between self and non-self through the recognition of molecular patterns. The body’s own cells express specific proteins called self-antigens on their surfaces, which are recognized by immune cells as “self.” This is facilitated by major histocompatibility complex (MHC) molecules, which present self-antigens to T cells.
In contrast, foreign pathogens have different molecular patterns called pathogen-associated molecular patterns (PAMPs). These patterns are detected by pattern recognition receptors (PRRs) on immune cells, triggering an immune response. The ability to differentiate between self and non-self is essential for preventing autoimmune diseases and ensuring an appropriate response to infections.
