Immune Response- Definition, Types, Factors, Examples

An immune response is a physiological reaction that occurs when the body recognizes and defends itself against foreign substances, such as bacteria, viruses, toxins, or other harmful agents. These substances are called antigens, and they can trigger an immune response by binding to specific receptors on the surface of immune cells. The immune system consists of various cells, tissues, organs, and molecules that work together to protect the body from infection and disease.

There are two main types of immune responses: innate and adaptive. Innate immunity is the first line of defense that provides immediate and nonspecific protection against any antigen. It involves physical barriers, such as skin and mucous membranes, and chemical mediators, such as enzymes, complement proteins, interferons, and cytokines. Innate immunity also includes cells such as macrophages, neutrophils, natural killer cells, and dendritic cells that can phagocytose (engulf) or destroy antigens directly or present them to adaptive immune cells.

Adaptive immunity is the second line of defense that provides specific and long-lasting protection against a particular antigen. It involves lymphocytes, such as B cells and T cells, that can recognize and remember antigens through their unique receptors. B cells produce antibodies that can bind to antigens and neutralize them or mark them for destruction by other immune cells. T cells can either help B cells produce antibodies (helper T cells) or kill infected or abnormal cells directly (cytotoxic T cells). Adaptive immunity also generates memory cells that can quickly respond to the same antigen in the future.

The immune response is regulated by various factors, such as the type of antigen, the route of antigen entry, the antigen-presenting cells, the antigen receptors, and the antigen complexity. The immune response can also be influenced by genetic factors, environmental factors, age, nutrition, stress, and diseases. The immune response can be beneficial by eliminating pathogens and preventing infections. However, it can also be harmful by causing allergies, autoimmune diseases, chronic inflammation, or immunodeficiency.

The immune response is a complex and dynamic process that involves multiple interactions between different components of the immune system. The immune response aims to maintain a balance between protection and tolerance of foreign substances in order to preserve the health and integrity of the body.

A primary immune response is the first time that the immune system recognizes and responds to a particular antigen. An antigen is any substance that can trigger an immune response, such as a virus, a bacterium, a toxin, or a foreign protein. The primary immune response involves both the innate and the adaptive branches of the immune system.

The innate immune system consists of physical barriers, such as the skin and mucous membranes, and cellular and molecular components, such as phagocytes, natural killer cells, complement proteins, and cytokines. The innate immune system provides a rapid and non-specific response to any potential pathogen, but it does not generate immunological memory.

The adaptive immune system consists of lymphocytes, such as B cells and T cells, and their products, such as antibodies and cytokines. The adaptive immune system provides a specific and tailored response to each antigen, and it generates immunological memory. This means that the adaptive immune system can remember the antigens that it has encountered before and respond more quickly and effectively to them in the future.

The primary immune response begins when an antigen enters the body and is detected by antigen-presenting cells (APCs), such as dendritic cells, macrophages, and B cells. These cells process the antigen and present it to naive T cells in the lymph nodes. Naive T cells are T cells that have not encountered their specific antigen yet. Depending on the type of antigen and the type of APC, different subsets of T cells are activated.

For example, if the antigen is a protein that is associated with a major histocompatibility complex (MHC) class II molecule on the surface of an APC, then helper T cells are activated. Helper T cells are CD4+ T cells that help other immune cells to perform their functions. They secrete cytokines that stimulate B cells to produce antibodies, cytotoxic T cells to kill infected cells, and macrophages to destroy pathogens.

If the antigen is a protein that is associated with an MHC class I molecule on the surface of an infected cell, then cytotoxic T cells are activated. Cytotoxic T cells are CD8+ T cells that directly kill infected or abnormal cells by releasing perforins and granzymes that induce apoptosis.

The activation of naive T cells requires two signals: one from the antigen-MHC complex and one from a co-stimulatory molecule on the APC. Once activated, T cells proliferate and differentiate into effector T cells and memory T cells. Effector T cells are T cells that perform their specific functions against the antigen. Memory T cells are long-lived T cells that remain in a quiescent state until they encounter the same antigen again.

B cells are also involved in the primary immune response. B cells have B cell receptors (BCRs) on their surface that can bind to specific antigens. Some B cells can act as APCs and present antigens to helper T cells. Other B cells can be activated by antigens directly or by helper T cell-derived cytokines. Once activated, B cells proliferate and differentiate into plasma cells and memory B cells. Plasma cells are antibody-secreting cells that produce large amounts of antibodies against the antigen. Memory B cells are long-lived B cells that remain in a quiescent state until they encounter the same antigen again.

The primary immune response takes several days to develop and reaches its peak after about 10 to 17 days. The antibodies produced during this response are mainly of the IgM class, which have low affinity but high avidity for the antigen. The IgM antibodies can activate the complement system and opsonize pathogens for phagocytosis. Some IgM antibodies can also switch to IgG antibodies, which have higher affinity but lower avidity for the antigen. The IgG antibodies can neutralize toxins and viruses, opsonize pathogens for phagocytosis, activate antibody-dependent cellular cytotoxicity (ADCC), and cross the placenta to provide passive immunity to the fetus.

The primary immune response is important for clearing the initial infection and generating immunological memory. However, it is not very effective at preventing reinfection by the same pathogen. This is because the primary immune response is slow, weak, and short-lived. Therefore, a secondary immune response is needed to provide long-lasting protection against repeated exposures to the same antigen.

The primary immune response is the first time that the immune system encounters a foreign antigen and produces antibodies against it. This response takes about 14 days to resolve and involves the activation of naive B cells and T cells.

The primary immune response can be divided into four phases: lag, exponential, plateau and decline.

Lag phase

• The lag phase is also known as the latent phase.
• It is the phase of the initial exposure to the antigen.
• This leads to the activation of naive B cells that produce antibodies to counter the antigen.
• This phase takes a week, activating the specialized B and T cells that come into contact with the antigen.

Exponential phase

• The exponential phase is the phase of a rapid increase in antibody production by the differentiated plasma cells.
• The increase in antibody production is because of the large number of plasma cells.
• The initial antibodies are detected in low-affinities of IgM and in high-affinity IgG, especially when the antigen is persistent.

Plateau phase

• The plateau phase is a steady phase where the antibody level remains constant to maintain the levels of antibody replenishing and production.
• This means that the antibodies that get used up equal the antibodies that are produced.

Decline phase

• The decline phase involves a decrease in antibody levels due to the decline in plasma cell numbers which are dying out of exhaustion of antibody production.
• During this phase, there are no new plasma cells being produced because the antigen or immunogen has been eliminated from the system.
• Without a continued antigenic challenge, the antibody levels decline subsequently.

A secondary immune response is the subsequent immune response after the primary immune response, also known as anamnestic immune response. The response that occurs the second or third or fourth etc time the body encounters the same antigen it encountered during the primary response. This response is mediated by the memory lymphocytes that were produced during the primary response.

Immediately after the same antigen is encountered, the memory lymphocytes induce the production of antibodies. This response has a very short sharp lag phase which means that both lag and exponential phases take place at ago. The antibody production levels increase rapidly within a short period, normally within a few days. This is because of the antigen-specific memory T and B-cells produced during the primary response.

Because of the rapidity of the secondary response, the antigen gets eliminated as soon as it encounters the memory cells and before it can cause disease. The antibodies produced during this response remain circulating freely to ensure complete elimination of the antigen.

The secondary immune response is characterized by several features that distinguish it from the primary immune response:

• It has a shorter lag phase and a faster antibody production rate .
• It produces higher levels and frequency of antibodies, especially IgG or IgA depending on the site of antigen response .
• It generates antibodies with higher affinity and specificity for the antigen due to somatic hypermutation and affinity maturation .
• It occurs mainly in the secondary lymphoid organs such as spleen and lymph nodes, where memory cells reside .
• It provides stronger and better immune protection against reinfection by the same antigen .

The secondary immune response also involves a secondary T-cell response that occurs as a result of re-exposure to antigens, and therefore it elicits a 10,000-fold increase in the respective T-cell specific cytotoxic T-cells. These T-cells can directly kill infected cells or secrete cytokines that activate other immune cells. The secondary T-cell response also produces memory T-cells that can quickly respond to future encounters with the same antigen.

The secondary immune response is the basis of immunological memory and vaccination. By exposing the body to a weakened or inactivated form of an antigen, vaccination induces a primary immune response that generates memory cells. When the body encounters the same antigen again in its natural form, it can mount a secondary immune response that can prevent or reduce disease symptoms .

• Antigens are substances that can trigger an immune response by activating B cells and T cells.
• Antigens can be classified into two types based on their ability to stimulate B cells: thymus-dependent (TD) antigens and thymus-independent (TI) antigens .
• TD antigens are usually proteins that require the help of T cells (especially T helper cells) to activate B cells .
• TD antigens can induce the production of different classes of antibodies (IgM, IgG, IgA, IgE), the generation of memory B cells and the affinity maturation of antibodies .
• TD antigens are more complex and diverse than TI antigens and are present in most pathogenic microbes.
• TI antigens are usually polysaccharides or lipopolysaccharides that can activate B cells directly without the help of T cells .
• TI antigens can only induce the production of IgM antibodies and do not generate memory B cells or affinity maturation .
• TI antigens have repetitive molecular structures (epitopes) that bind to B cell receptors (BCR) or toll-like receptors (TLR) on B cells .
• TI antigens are less common and less immunogenic than TD antigens and are often found in bacterial cell walls.
• TI antigens can be further divided into two types: TI-1 and TI-2 antigens .
  • TI-1 antigens are mitogens that can stimulate the proliferation of B cells nonspecifically at high concentrations. An example of TI-1 antigen is lipopolysaccharide (LPS) from Gram-negative bacteria .
  • TI-2 antigens are multivalent antigens that can cross-link BCRs and induce specific antibody production. An example of TI-2 antigen is pneumococcal polysaccharide from Streptococcus pneumoniae .

Summary

• TD antigens need T cell help to activate B cells, while TI antigens do not.
• TD antigens induce various antibody classes, memory B cells and affinity maturation, while TI antigens only induce IgM antibodies.
• TD antigens are complex proteins, while TI antigens are simple polysaccharides or lipopolysaccharides.
• TD antigens are common in pathogens, while TI antigens are rare and mostly found in bacterial cell walls.

• TI antigens can be either mitogens (TI-1) or multivalent (TI-2) antigens.

  Factors that Influence the Type of Immune Responses

The immune system is a complex network of cells, molecules, and organs that protect the body from various pathogens and diseases. The immune system can be divided into two main types: innate and adaptive. Innate immunity is the first line of defense that responds quickly and broadly to any foreign substance or danger. Adaptive immunity is the second line of defense that responds more slowly and specifically to a particular antigen, and generates immunological memory for future encounters.

The type of immune response that is elicited by an antigen depends on several factors, such as:

• Type of antigen: Antigens are substances that can be recognized by the immune system and trigger an immune response. Antigens can be classified into two types: thymus-dependent (TD) or thymus-independent (TI). TD antigens are usually proteins that require the help of T cells to activate B cells and produce antibodies. TI antigens are usually polysaccharides or lipopolysaccharides that can directly activate B cells without T cell help. TD antigens induce a stronger and more diverse antibody response than TI antigens, and also generate immunological memory .

• Route of antigen entry: The route of antigen entry into the body can influence the type and location of the immune response. Antigens can enter the body through different routes, such as the bloodstream, the skin, or the mucosal surfaces. The bloodstream route triggers an immune response in the spleen, where blood-borne antigens are filtered and presented to immune cells. The skin route triggers an immune response in the regional lymph nodes, where antigens that penetrate the skin are captured and transported by dendritic cells. The mucosal route triggers an immune response in the submucosal lymphoid tissues, such as the tonsils, Peyer`s patches, or appendix, where antigens that enter through the gastrointestinal, respiratory, or reproductive tract are encountered by specialized immune cells.

• Antigen-presenting cells: Antigen-presenting cells (APCs) are innate immune cells that process and present antigens to adaptive immune cells, such as T cells and B cells. APCs include dendritic cells, macrophages, and B cells. Dendritic cells are the most effective APCs, as they can capture antigens from various sources and migrate to lymphoid tissues to activate naive T cells. Macrophages are phagocytic cells that ingest and degrade antigens, and present them to T cells in inflamed tissues or chronic infections. B cells are also capable of presenting antigens to T cells, especially in secondary immune responses when they encounter antigens that match their B cell receptors.

• Antigen receptors: Antigen receptors are molecules on the surface of adaptive immune cells that recognize and bind to specific antigens. B cells have B cell receptors (BCRs), which are membrane-bound antibodies that can bind to soluble or membrane-bound antigens. T cells have T cell receptors (TCRs), which can only bind to processed antigens that are presented by MHC molecules on APCs or infected cells. The specificity and diversity of antigen receptors are generated by random rearrangement of gene segments during lymphocyte development.

• Antigen complexity: Antigen complexity refers to the number and variety of epitopes on an antigen. Epitopes are the parts of an antigen that are recognized by antigen receptors or antibodies. Antigens with more epitopes can stimulate more immune cells and elicit a more robust immune response than antigens with fewer epitopes. Antigens with different types of epitopes can also induce different types of immune responses, such as humoral or cellular immunity.

Other factors that influence the type of immune response include clonal expansion, affinity maturation, class switching, and memory cells. Clonal expansion is the process by which activated lymphocytes proliferate and differentiate into effector or memory cells. Affinity maturation is the process by which B cells undergo somatic hypermutation and selection to produce antibodies with higher affinity for the antigen. Class switching is the process by which B cells switch from producing IgM antibodies to producing other classes of antibodies, such as IgG, IgA, or IgE, depending on the cytokine signals they receive from T cells or other sources. Memory cells are long-lived lymphocytes that retain their antigen specificity and can quickly respond to subsequent encounters with the same antigen.

Examples of Immune Response

The immune response is the way the body recognizes and defends itself against foreign substances, such as bacteria, viruses, parasites, toxins, and transplanted organs. The immune response can be divided into two types: innate and adaptive. Innate immunity is the first line of defense that is present at birth and responds to all antigens in a nonspecific manner. Adaptive immunity is the second line of defense that develops with exposure to specific antigens and involves the production of antibodies and memory cells. Here are some examples of immune response in different situations:

Immune Response in Type I Diabetes

Type I diabetes is an autoimmune disease, which means that the immune system attacks the body`s own cells by mistake. In this case, the immune system targets the beta cells of the pancreas, which produce insulin, a hormone that regulates blood sugar levels. The destruction of beta cells leads to a lack of insulin and high blood sugar levels, which can cause serious complications.

The exact cause of type I diabetes is unknown, but it is thought to involve a combination of genetic and environmental factors. Some possible triggers of the autoimmune attack are viral infections, dietary factors, or exposure to chemicals. The immune response involves both innate and adaptive immunity. The innate immune system recognizes the beta cells as foreign and activates inflammation, which attracts other immune cells to the site. The adaptive immune system produces antibodies against the beta cell antigens, as well as cytotoxic T cells that directly kill the beta cells. The immune response also generates memory cells that persist for a long time and can reactivate the attack if the beta cells regenerate.

Acute Immune Responses to Coronaviruses

Coronaviruses are a family of viruses that cause respiratory infections in humans and animals. Some examples are SARS-CoV-2 (the virus that causes COVID-19), SARS-CoV (the virus that causes severe acute respiratory syndrome), and MERS-CoV (the virus that causes Middle East respiratory syndrome). These viruses enter the body through the nose or mouth and attach to receptors on the surface of epithelial cells in the respiratory tract. They then inject their genetic material into the host cell and use its machinery to replicate.

The immune response to coronaviruses involves both innate and adaptive immunity. The innate immune system detects the viral infection through pattern recognition receptors (PRRs) that recognize viral components, such as RNA or proteins. The PRRs activate signaling pathways that induce the production of cytokines, which are molecules that regulate inflammation and immune cell activation. The cytokines recruit and activate other innate immune cells, such as natural killer (NK) cells, macrophages, dendritic cells, and neutrophils, that can directly kill infected cells or present viral antigens to adaptive immune cells. The adaptive immune system produces antibodies against viral antigens, as well as helper T cells that stimulate B cell and cytotoxic T cell responses. The cytotoxic T cells can directly kill infected cells or release cytokines that enhance inflammation and antiviral activity. The adaptive immune system also generates memory cells that can provide long-term protection against reinfection.

Immune Response in IBD

Inflammatory bowel disease (IBD) is a chronic condition that causes inflammation and ulceration of the digestive tract. There are two main types of IBD: Crohn`s disease and ulcerative colitis. Crohn`s disease can affect any part of the digestive tract, while ulcerative colitis only affects the colon and rectum. The symptoms of IBD include abdominal pain, diarrhea, bleeding, weight loss, and fatigue.

The exact cause of IBD is unknown, but it is thought to involve a dysregulated immune response to the gut microbiota, which are the normal bacteria that live in the digestive tract. The gut microbiota plays an important role in maintaining intestinal health and immunity by producing beneficial substances, preventing pathogen invasion, and modulating inflammation. However, in some people with genetic or environmental predisposition, the gut microbiota may trigger an abnormal immune response that damages the intestinal lining.

The immune response in IBD involves both innate and adaptive immunity. The innate immune system recognizes microbial components through PRRs and produces cytokines that initiate inflammation. The inflammation attracts other innate immune cells, such as macrophages, neutrophils, eosinophils, and mast cells, that release more cytokines and inflammatory mediators, such as reactive oxygen species (ROS), nitric oxide (NO), histamine, and proteases. These substances damage the intestinal barrier and allow more microbial components to enter the tissue, perpetuating the inflammatory cycle. The adaptive immune system produces antibodies against microbial antigens, as well as helper T cells that stimulate B cell and cytotoxic T cell responses. The cytotoxic T cells can directly kill infected or damaged cells or release cytokines that enhance inflammation and tissue damage. The adaptive immune system also generates memory cells that can reactivate the inflammatory response upon re-exposure to microbial antigens.

Immune Response to Commensal vs. Pathogenic Bacteria

Bacteria are microscopic organisms that can be classified into two groups: commensal and pathogenic. Commensal bacteria are harmless or beneficial bacteria that live on or in the human body without causing disease. They help with digestion, metabolism, vitamin synthesis, immunity, and protection against pathogens. Pathogenic bacteria are harmful bacteria that cause infections or diseases in humans. They can invade tissues, produce toxins, evade or suppress immunity, or induce inflammation.

The immune response to commensal vs. pathogenic bacteria depends on several factors, such as location,

Comparison between Primary and Secondary Immune Response

Primary and secondary immune responses are two types of adaptive immune responses that occur when the body encounters a foreign antigen. The primary immune response is the first time the body meets a particular antigen, while the secondary immune response is when the same antigen is encountered again. The table below summarizes some of the main differences between primary and secondary immune responses.

The primary immune response is slower and weaker than the secondary immune response because it takes time for the naive B cells and T cells to recognize the antigen and differentiate into plasma cells or memory cells. The secondary immune response is faster and stronger because the memory B cells and T cells can quickly recognize the antigen and produce large amounts of antibodies. The secondary immune response also provides better protection against the antigen because the antibodies have higher affinity and can bind more strongly to the antigen. The secondary immune response is the basis of immunological memory and vaccination.

The immune response is the way the body recognizes and defends itself against foreign substances, such as bacteria, viruses, parasites, toxins, and transplanted organs. The immune response can be divided into two types: innate and adaptive. Innate immunity is the first line of defense that is present at birth and responds to all antigens in a nonspecific manner. Adaptive immunity is the second line of defense that develops with exposure to specific antigens and involves the production of antibodies and memory cells. Here are some examples of immune response in different situations:

Immune Response in Type I Diabetes

Type I diabetes is an autoimmune disease, which means that the immune system attacks the body`s own cells by mistake. In this case, the immune system targets the beta cells of the pancreas, which produce insulin, a hormone that regulates blood sugar levels. The destruction of beta cells leads to a lack of insulin and high blood sugar levels, which can cause serious complications.

The exact cause of type I diabetes is unknown, but it is thought to involve a combination of genetic and environmental factors. Some possible triggers of the autoimmune attack are viral infections, dietary factors, or exposure to chemicals. The immune response involves both innate and adaptive immunity. The innate immune system recognizes the beta cells as foreign and activates inflammation, which attracts other immune cells to the site. The adaptive immune system produces antibodies against the beta cell antigens, as well as cytotoxic T cells that directly kill the beta cells. The immune response also generates memory cells that persist for a long time and can reactivate the attack if the beta cells regenerate.

Acute Immune Responses to Coronaviruses

Coronaviruses are a family of viruses that cause respiratory infections in humans and animals. Some examples are SARS-CoV-2 (the virus that causes COVID-19), SARS-CoV (the virus that causes severe acute respiratory syndrome), and MERS-CoV (the virus that causes Middle East respiratory syndrome). These viruses enter the body through the nose or mouth and attach to receptors on the surface of epithelial cells in the respiratory tract. They then inject their genetic material into the host cell and use its machinery to replicate.

The immune response to coronaviruses involves both innate and adaptive immunity. The innate immune system detects the viral infection through pattern recognition receptors (PRRs) that recognize viral components, such as RNA or proteins. The PRRs activate signaling pathways that induce the production of cytokines, which are molecules that regulate inflammation and immune cell activation. The cytokines recruit and activate other innate immune cells, such as natural killer (NK) cells, macrophages, dendritic cells, and neutrophils, that can directly kill infected cells or present viral antigens to adaptive immune cells. The adaptive immune system produces antibodies against viral antigens, as well as helper T cells that stimulate B cell and cytotoxic T cell responses. The cytotoxic T cells can directly kill infected cells or release cytokines that enhance inflammation and antiviral activity. The adaptive immune system also generates memory cells that can provide long-term protection against reinfection.

Immune Response in IBD

Inflammatory bowel disease (IBD) is a chronic condition that causes inflammation and ulceration of the digestive tract. There are two main types of IBD: Crohn`s disease and ulcerative colitis. Crohn`s disease can affect any part of the digestive tract, while ulcerative colitis only affects the colon and rectum. The symptoms of IBD include abdominal pain, diarrhea, bleeding, weight loss, and fatigue.

The exact cause of IBD is unknown, but it is thought to involve a dysregulated immune response to the gut microbiota, which are the normal bacteria that live in the digestive tract. The gut microbiota plays an important role in maintaining intestinal health and immunity by producing beneficial substances, preventing pathogen invasion, and modulating inflammation. However, in some people with genetic or environmental predisposition, the gut microbiota may trigger an abnormal immune response that damages the intestinal lining.

The immune response in IBD involves both innate and adaptive immunity. The innate immune system recognizes microbial components through PRRs and produces cytokines that initiate inflammation. The inflammation attracts other innate immune cells, such as macrophages, neutrophils, eosinophils, and mast cells, that release more cytokines and inflammatory mediators, such as reactive oxygen species (ROS), nitric oxide (NO), histamine, and proteases. These substances damage the intestinal barrier and allow more microbial components to enter the tissue, perpetuating the inflammatory cycle. The adaptive immune system produces antibodies against microbial antigens, as well as helper T cells that stimulate B cell and cytotoxic T cell responses. The cytotoxic T cells can directly kill infected or damaged cells or release cytokines that enhance inflammation and tissue damage. The adaptive immune system also generates memory cells that can reactivate the inflammatory response upon re-exposure to microbial antigens.

Immune Response to Commensal vs. Pathogenic Bacteria

Bacteria are microscopic organisms that can be classified into two groups: commensal and pathogenic. Commensal bacteria are harmless or beneficial bacteria that live on or in the human body without causing disease. They help with digestion, metabolism, vitamin synthesis, immunity, and protection against pathogens. Pathogenic bacteria are harmful bacteria that cause infections or diseases in humans. They can invade tissues, produce toxins, evade or suppress immunity, or induce inflammation.

The immune response to commensal vs. pathogenic bacteria depends on several factors, such as location,

Primary and secondary immune responses are two types of adaptive immune responses that occur when the body encounters a foreign antigen. The primary immune response is the first time the body meets a particular antigen, while the secondary immune response is when the same antigen is encountered again. The table below summarizes some of the main differences between primary and secondary immune responses.

The primary immune response is slower and weaker than the secondary immune response because it takes time for the naive B cells and T cells to recognize the antigen and differentiate into plasma cells or memory cells. The secondary immune response is faster and stronger because the memory B cells and T cells can quickly recognize the antigen and produce large amounts of antibodies. The secondary immune response also provides better protection against the antigen because the antibodies have higher affinity and can bind more strongly to the antigen. The secondary immune response is the basis of immunological memory and vaccination.

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