11.2 Adaptive immune responses against viruses
4 min read•august 1, 2024
Adaptive immune responses against viruses are crucial for fighting infections. and work together, with T cells killing infected cells and B cells producing antibodies. This teamwork creates a powerful defense system that remembers past invaders.
The immune system's memory is key to long-term protection. After an infection, memory cells stick around, ready to jump into action if the same virus shows up again. This quick response is why vaccines work so well, giving us a head start against dangerous viruses.
T cells and B cells in viral immunity
Lymphocyte functions and differentiation
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- T cells and B cells function as central components of adaptive immunity against viral infections
- T cells responsible for differentiate into CD4+ helper T cells and CD8+ cytotoxic T cells
- CD4+ helper T cells activate and regulate other immune cells (B cells, CD8+ T cells, macrophages)
- CD8+ cytotoxic T cells directly kill virus-infected cells by releasing cytotoxic granules (perforin, granzymes) and inducing apoptosis
- B cells produce antibodies for neutralizing viruses and marking them for destruction
- Clonal expansion occurs when T and B cells recognize viral antigens resulting in rapid increase of virus-specific lymphocytes
Collaborative mechanisms and memory formation
- T cells and B cells collaborate through cytokine signaling and direct cell-to-cell interactions for effective antiviral response
- Example: CD4+ T cells provide co-stimulatory signals to B cells in germinal centers
- Example: Cytokines like IL-2 from CD4+ T cells promote CD8+ T cell proliferation
- Memory T cells and B cells persist after initial infection providing long-term protection
- Memory cells can quickly reactivate upon subsequent viral exposure
- Memory B cells can rapidly differentiate into antibody-producing plasma cells
- Germinal center reactions in lymphoid tissues facilitate affinity maturation of B cells and generation of long-lived plasma cells
Antibody vs Cell-mediated immunity
Antibody-mediated immunity mechanisms
- Antibody-mediated (humoral) immunity involves production of virus-specific antibodies by B cells and plasma cells
- Antibodies contribute to viral clearance through multiple mechanisms
- Neutralization binds viral surface proteins preventing entry into host cells (hemagglutinin in influenza viruses)
- Opsonization coats viruses facilitating phagocytosis by immune cells (macrophages, neutrophils)
- Complement activation triggers complement cascade enhancing viral destruction
- Antibodies can act at multiple stages of viral life cycle
- Prevent viral attachment to host cell receptors
- Inhibit viral fusion with host cell membranes
- Aggregate viruses reducing infectivity
Cell-mediated immunity mechanisms
- Cell-mediated immunity primarily involves T cells particularly CD8+ cytotoxic T lymphocytes (CTLs)
- CTLs recognize viral peptides presented on MHC class I molecules on infected cell surfaces
- CTLs induce apoptosis in virus-infected cells through release of perforin and granzymes
- Perforin forms pores in target cell membrane
- Granzymes enter through pores and activate caspase-mediated apoptosis
- CD4+ helper T cells support both antibody-mediated and cell-mediated responses
- Activate B cells through CD40-CD40L interactions and cytokine production
- Enhance function of CD8+ T cells and other immune cells (macrophages, NK cells)
- Coordination between antibody-mediated and cell-mediated immunity crucial for effective viral clearance and prevention of chronic infections
Immunological memory for viral protection
Characteristics of immunological memory
- allows adaptive immune system to recognize and mount rapid enhanced response to previously encountered pathogens
- Memory B cells and memory T cells persist as long-lived lymphocytes after initial infection clearance
- Upon re-exposure to same virus memory cells quickly proliferate and differentiate into effector cells
- Secondary immune response mediated by memory cells characterized by
- Shorter lag time before response initiation (hours instead of days)
- More rapid production of high-affinity antibodies
- Greater magnitude and duration of overall immune response
Applications and variations in immunological memory
- Immunological memory forms basis for exposing immune system to attenuated or inactivated viruses or viral components
- Example: Measles vaccine induces long-lasting immunity preventing disease outbreaks
- Example: Influenza vaccines require annual updates due to viral antigenic drift
- Duration of immunological memory varies depending on virus and individual
- Some memories last lifetime (measles, mumps)
- Others may wane over time requiring booster vaccinations (tetanus, pertussis)
- Factors affecting memory longevity include
- Persistence of antigen in lymphoid tissues
- Survival signals from stromal cells and cytokines
- Frequency of re-exposure to pathogen
Factors influencing adaptive immunity to viruses
Viral factors affecting immune responses
- Antigenic variability impacts effectiveness of adaptive immunity
- High mutation rates in viruses (influenza, HIV) enable evasion of existing antibodies and T cells
- Antigenic drift gradual accumulation of mutations in surface proteins
- Antigenic shift major changes in viral antigens (pandemic influenza strains)
- Immunosuppressive mechanisms employed by viruses
- Direct interference with immune cell function (HIV targeting CD4+ T cells)
- Inhibition of (herpes viruses downregulating MHC molecules)
- Production of immunomodulatory proteins (poxvirus-encoded cytokine receptors)
Host and environmental influences on immunity
- Host factors influencing adaptive immune response effectiveness
- Genetic background affects antigen presentation and immune cell function
- HLA types determine peptide binding and T cell recognition
- Polymorphisms in cytokine genes impact immune regulation
- Age-related changes in immunity
- Immature immune systems in newborns less effective against some pathogens
- Immunosenescence in elderly leads to decreased vaccine responsiveness
- Overall health status impacts immune function
- Malnutrition impairs lymphocyte proliferation and antibody production
- Chronic stress suppresses T cell responses and antibody production
- Genetic background affects antigen presentation and immune cell function
- Environmental factors modulating adaptive immune responses
- Exposure history shapes pre-existing immunity and cross-reactive responses
- Co-infections alter immune environment (HIV and opportunistic infections)
- Microbiome composition influences immune development and function
Key Terms to Review (18)
Antigen presentation: Antigen presentation is the process by which immune cells display foreign antigens on their surface to activate T cells, which are crucial for the adaptive immune response. This process is essential for recognizing and responding to viral infections, as it helps the immune system distinguish between self and non-self entities. The effectiveness of antigen presentation can influence the overall strength and specificity of the immune response against viruses, as well as shape the strategies that viruses may employ to evade detection.
Antigenic variation: Antigenic variation is the process by which a pathogen alters its surface proteins to evade the host's immune system. This ability to change allows viruses to escape recognition by the immune system, which poses significant challenges in controlling infections and developing effective vaccines. The continuous modification of viral antigens can lead to reinfections, complicating both adaptive immune responses and vaccine design.
B cells: B cells are a type of white blood cell essential for the adaptive immune response, primarily responsible for producing antibodies that target specific antigens, including viruses. They are critical for humoral immunity, which is the aspect of the immune response mediated by antibodies in bodily fluids. Upon encountering an antigen, B cells can differentiate into plasma cells, which secrete large amounts of antibodies, or memory B cells, which provide long-term immunity against future infections by the same pathogen.
Cell-mediated immunity: Cell-mediated immunity is a type of adaptive immune response primarily involving T cells that target and eliminate infected or abnormal cells, playing a crucial role in defending against viral infections. This immunity operates without antibodies, relying instead on direct cell-to-cell interactions to destroy pathogens or infected host cells. It is especially important in controlling intracellular pathogens like viruses and is vital for the body’s defense mechanisms during both acute and chronic infections.
Clonal Selection: Clonal selection is the process by which specific immune cells are activated and proliferate in response to a particular antigen, leading to the generation of a clone of identical cells that can effectively target that antigen. This mechanism is central to the adaptive immune response, ensuring that the body can mount a tailored defense against specific viruses or pathogens it encounters.
Cytokine Storm: A cytokine storm is an overreaction of the immune system where an excessive release of cytokines occurs, leading to widespread inflammation and tissue damage. This phenomenon is particularly critical in the context of viral infections, where the immune response can become uncontrolled, resulting in severe immunopathology and contributing to the severity of the disease. It can influence adaptive immune responses, affect viral spread in populations, and lead to complications in various organ systems.
Humoral immunity: Humoral immunity is a component of the adaptive immune response that involves the production of antibodies by B cells to neutralize pathogens, particularly viruses. This type of immunity plays a critical role in recognizing and eliminating viral infections through the circulation of these antibodies in the blood and lymphatic system. The effectiveness of humoral immunity can vary depending on the type of virus and the immune status of the individual.
IgG: IgG, or Immunoglobulin G, is the most abundant type of antibody found in the blood and extracellular fluid, playing a crucial role in the body's immune response against viruses. This antibody is essential for neutralizing pathogens, opsonization, and activating the complement system, making it vital for long-term immunity. IgG's ability to cross the placenta also provides passive immunity to the fetus, highlighting its importance in protecting newborns during early life.
IgM: IgM, or Immunoglobulin M, is a type of antibody that plays a crucial role in the immune response, particularly during the early stages of infection. As one of the first antibodies produced by the immune system upon exposure to pathogens, it is essential for recognizing and binding to viruses, thus helping to neutralize them and activate other components of the immune system. IgM's pentameric structure allows it to effectively target multiple antigens simultaneously, making it highly efficient in combating infections.
Immune suppression: Immune suppression refers to the reduced ability of the immune system to respond to infections and other challenges. This condition can be caused by various factors, including viral infections, medications, and underlying health conditions. Understanding immune suppression is crucial because it can significantly affect the body's ability to fight off viral pathogens and has implications for both treatment strategies and patient outcomes.
Immunological Memory: Immunological memory refers to the ability of the immune system to remember past encounters with pathogens, allowing for a quicker and more effective response upon subsequent exposures. This mechanism is vital for the adaptive immune response, as it enables the body to mount a robust defense against viruses it has previously encountered, primarily through the action of memory B cells and T cells.
Interferons: Interferons are a group of signaling proteins produced and released by host cells in response to the presence of pathogens, particularly viruses. They play a crucial role in the immune response by interfering with viral replication, activating immune cells, and enhancing the adaptive immune response. This multifaceted role makes interferons vital in the body’s defense against viral infections and also highlights their significance in understanding viral immune evasion tactics.
Memory response: Memory response refers to the ability of the adaptive immune system to remember past infections and respond more efficiently upon re-exposure to the same pathogen. This enhanced reaction is primarily mediated by memory cells, which are long-lived and can quickly proliferate and differentiate into effector cells, ensuring a faster and stronger immune response against previously encountered viruses.
MHC Class I Pathway: The MHC Class I pathway is a crucial mechanism in the immune system responsible for presenting intracellular antigens, such as viral proteins, to CD8+ T cells. This pathway allows infected cells to display pieces of pathogens on their surface, enabling cytotoxic T cells to recognize and eliminate those cells. It plays a significant role in adaptive immune responses against viruses by ensuring that infected cells are identified and targeted for destruction.
MHC Class II Pathway: The MHC Class II pathway is a cellular mechanism that presents extracellular antigens on the surface of antigen-presenting cells (APCs) to CD4+ T helper cells. This pathway is crucial for the activation of the adaptive immune response, especially against viruses, as it allows the immune system to recognize and respond to foreign proteins from pathogens that have been engulfed and processed by these cells.
T cells: T cells are a type of white blood cell that play a central role in the adaptive immune response, specifically targeting and eliminating infected cells and coordinating the immune response. They originate from bone marrow but mature in the thymus, where they gain the ability to recognize specific antigens presented by infected or abnormal cells. T cells are crucial in defending against viral infections, contributing to the body’s ability to remember past infections for quicker responses upon re-exposure.
Vaccination: Vaccination is a medical procedure that involves administering a vaccine to stimulate the body's immune response against specific pathogens, primarily viruses and bacteria. This process not only helps protect individuals from infections but also plays a vital role in controlling the spread of infectious diseases within populations.
Viral epitope: A viral epitope is a specific part of a virus that is recognized by the immune system, particularly by antibodies and T-cell receptors. These small regions are crucial for the adaptive immune response, as they help the body identify and target viral infections. Viral epitopes can be found on the surface proteins of the virus and play a key role in how the immune system distinguishes between self and non-self molecules.