Innate vs. Acquired Immunity (Definition, Types, Examples)
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinters, distinguishing them from the organism`s own healthy tissue. The immune system also helps to maintain homeostasis and regulate inflammation.
The immune system can be divided into two major branches: innate immunity and acquired (adaptive) immunity. Innate immunity is the non-specific arm of the immune system that an individual is born with. It acts as a barrier against foreign invading materials and provides immediate defense against infections. Acquired immunity is the specific arm of the immune system that is developed during an individual`s lifetime. It involves the recognition of specific antigens and the generation of immunological memory.
The immune system consists of various types of cells and molecules that are distributed throughout the body. Some of these cells and molecules are specialized for innate immunity, while others are specialized for acquired immunity. Some of them can perform both innate and acquired functions. The main components of the immune system include:
- Physiological and anatomical barriers: These include the skin, mucous membranes, gastric acidity, enzymes, antimicrobial peptides, and other factors that prevent or limit the entry of pathogens into the body.
- Phagocytes: These are cells that can engulf and destroy foreign particles or microorganisms by a process called phagocytosis. Examples of phagocytes include macrophages, neutrophils, dendritic cells, and monocytes.
- Inflammatory reactions: These are responses that occur when host tissue is damaged or infected by pathogens. They involve the release of various mediators, such as histamine, cytokines, defensins, and kinins, that cause increased blood flow, swelling, redness, pain, and heat at the site of injury or infection. Inflammation helps to recruit more immune cells and molecules to the affected area and facilitates healing.
- Complement system: This is a group of proteins that circulate in the blood and can be activated by pathogens or antibodies. The complement system enhances the ability of phagocytes to clear infections, promotes inflammation, and directly kills some pathogens by forming pores in their membranes.
- Natural killer (NK) cells: These are lymphocytes that can recognize and kill cells that are infected by viruses or transformed by cancer. NK cells do not need prior exposure to antigens to function. They use receptors that can detect abnormal or missing self-molecules on the surface of target cells.
- Antibodies: These are proteins produced by B lymphocytes that can bind to specific antigens with high affinity and specificity. Antibodies can neutralize pathogens or toxins, opsonize them for phagocytosis, activate complement, or recruit other immune cells to eliminate them.
- T lymphocytes: These are lymphocytes that can recognize antigens presented by specialized cells called antigen-presenting cells (APCs). T lymphocytes can be divided into two main types: helper T cells and cytotoxic T cells. Helper T cells secrete cytokines that regulate the activity of other immune cells. Cytotoxic T cells can kill infected or abnormal cells by releasing perforin and granzymes.
- Regulatory T cells: These are a subset of T lymphocytes that can suppress excessive or unwanted immune responses. Regulatory T cells help to prevent autoimmune diseases, allergies, chronic inflammation, and transplant rejection.
The immune system is constantly surveilling the body for signs of infection or damage. When it encounters a foreign material or a pathogen, it initiates an immune response that involves both innate and acquired components. The innate immune response is rapid and non-specific, while the acquired immune response is slower but more specific and effective. The acquired immune response also generates immunological memory, which allows for a faster and stronger response upon re-exposure to the same antigen.
The immune system is essential for maintaining health and preventing diseases. However, sometimes it can malfunction or be overwhelmed by pathogens. This can result in immunodeficiency disorders, autoimmune diseases, allergic reactions, chronic inflammation, or cancer. Therefore, it is important to understand how the immune system works and how it can be modulated by various factors, such as vaccines, drugs, nutrition, stress, and lifestyle.
Innate immunity is the type of immunity that is present in an individual at birth and lasts throughout their life. It is also called natural immunity or non-specific immunity because it does not require prior exposure to a pathogen or antigen. Innate immunity provides the first line of defense against foreign invaders by preventing their entry, recognizing their presence, and eliminating them from the body.
Innate immunity consists of various components that work together to protect the body from infection. These include:
- Physiological barriers: These are physical and chemical factors that prevent the entry of pathogens into the body. Examples are the skin, mucous membranes, tears, saliva, stomach acid, and enzymes.
- Anatomical barriers: These are cells and tissues that trap and destroy pathogens that manage to enter the body. Examples are the cilia in the respiratory tract, the tonsils in the throat, and the lymph nodes in the lymphatic system.
- Phagocytosis: This is the process of engulfing and digesting pathogens by specialized cells called phagocytes. Examples of phagocytes are neutrophils, macrophages, dendritic cells, and natural killer cells.
- Inflammatory reactions: These are responses that occur when tissues are damaged by pathogens or other factors. They involve increased blood flow, swelling, redness, heat, pain, and the release of chemical mediators such as histamine, cytokines, defensins, and kinins. These mediators attract more phagocytes to the site of infection and facilitate healing.
Innate immunity is not specific to a particular pathogen or antigen. Instead, it relies on pattern recognition receptors (PRRs) that can detect common molecular structures found on many pathogens called pathogen-associated molecular patterns (PAMPs). For example, PRRs can recognize bacterial cell wall components, viral nucleic acids, or fungal carbohydrates. When PRRs bind to PAMPs, they trigger various immune responses such as phagocytosis, inflammation, complement activation, and cytokine production.
Innate immunity is important for preventing infections and limiting their spread. It also helps to activate and modulate the adaptive immune system by presenting antigens to lymphocytes and releasing cytokines that influence their differentiation and function. However, innate immunity has some limitations, such as a lack of specificity, memory, and diversity. Therefore, it is complemented by the adaptive immune system that provides a more tailored and long-lasting protection against pathogens.
The innate immune system consists of various components that work together to protect the body from foreign invaders. These components can be classified into four main categories:
- Physiological barriers: These are the chemical and physical factors that prevent the entry or survival of pathogens in the body. For example, the skin acts as a mechanical barrier that blocks the penetration of most microbes. The mucous membranes that line the respiratory, digestive, and urogenital tracts also trap and expel pathogens. The acidity of the stomach, the enzymes in saliva and tears, and the antimicrobial peptides secreted by epithelial cells are some of the chemical barriers that inhibit microbial growth.
- Anatomical barriers: These are the specialized cells and tissues that recognize and respond to foreign molecules or antigens. The most important anatomical barriers are the pattern recognition receptors (PRRs) that are expressed on various cells of the innate immune system, such as macrophages, dendritic cells, neutrophils, and natural killer (NK) cells. PRRs can bind to specific structures on pathogens called pathogen-associated molecular patterns (PAMPs) and trigger an immune response. For example, toll-like receptors (TLRs) are a type of PRR that can recognize bacterial lipopolysaccharides (LPS), viral nucleic acids, and fungal cell wall components.
- Phagocytosis: This is the process by which certain cells of the innate immune system engulf and destroy foreign particles or cells. The main phagocytic cells are neutrophils and macrophages, which can migrate to the site of infection and ingest microbes by forming a membrane-bound vesicle called a phagosome. The phagosome then fuses with a lysosome, which contains digestive enzymes and toxic substances that kill the microbe. The phagocytic cells can also present antigens to the adaptive immune system by displaying them on their surface along with major histocompatibility complex (MHC) molecules. For example, macrophages can phagocytose Mycobacterium tuberculosis and present its antigens to T cells.
- Inflammatory reactions: These are the local responses that occur when tissues are damaged or infected by pathogens. The main features of inflammation are redness, heat, swelling, and pain, which are caused by increased blood flow, vascular permeability, and leukocyte infiltration. The inflammatory reactions are mediated by various molecules that are released by damaged cells or activated immune cells. These molecules include histamine, prostaglandins, cytokines, chemokines, complement proteins, and kinins. They act on blood vessels, nerve endings, and immune cells to coordinate the elimination of pathogens and the repair of tissues. For example, histamine causes vasodilation and increased vascular permeability, which allows more blood and leukocytes to reach the site of inflammation.
These components of innate immunity provide a rapid and non-specific defense against pathogens. They also help to activate and modulate the adaptive immune system, which is more specific and long-lasting.
Innate immunity can be viewed as comprising four types of defensive barriers: anatomic (skin and mucous membranes), physiologic (temperature, low pH, and chemical mediators), endocytic and phagocytic (cells that engulf and destroy foreign particles), and inflammatory (local response to tissue injury or infection). However, innate immunity also includes individual, racial, and species immunity, which are based on the genetic differences among individuals, races, and species that affect their susceptibility or resistance to certain infections.
- Individual immunity: Some individuals of the same race and same species can have varied experiences with certain infections. For example, children are more susceptible to viral fever than adults. This may be due to differences in the expression or function of innate immune receptors, such as toll-like receptors (TLRs), that recognize common patterns in pathogens. Alternatively, it may be due to variations in the levels or activity of innate immune cells, such as natural killer (NK) cells, that can eliminate infected or abnormal cells.
- Racial immunity: Individuals of different races within the same species have varied susceptibility or resistance toward infection caused by the same etiological agent. For example, races with sickle cell anemia prevalent in Africans on the Mediterranean coast are resistant to malaria caused by Plasmodium falciparum. This is because sickle cell anemia causes an alteration of the shape of the erythrocyte, which prevents its parasitization. Another example is the higher frequency of a mutation in the CCR5 gene among people of European descent, which confers resistance to HIV infection. CCR5 is a co-receptor that HIV uses to enter human cells, and the mutation prevents its binding.
- Species immunity: Individuals from different species have different susceptibilities toward any infection. For example, humans are not affected by chicken cholera, infectious horse anemia, etc., while animals are resistant to many human diseases like syphilis, gonorrhea, measles, etc. This may be due to differences in the structure or expression of receptors or molecules that pathogens use to attach or enter host cells. For instance, humans are not susceptible to canine distemper viruses because they lack the receptor for it in their cells.
Innate immunity is not uniform in all individuals of the same species or race. It can vary depending on several factors that affect the body`s ability to resist or fight infections. Some of these factors are:
- Age: The innate immune system is not fully developed at birth and declines with aging. Newborns have lower levels of complement proteins, natural killer cells, and cytokines than adults. They also have immature epithelial barriers and phagocytic cells that make them more susceptible to infections by bacteria, viruses, and fungi. On the other hand, elderly people have reduced production and function of innate immune cells and molecules due to aging. They also have impaired wound healing and increased inflammation that predispose them to chronic infections and autoimmune diseases.
- Hormonal level: Hormones are chemical messengers that regulate various physiological processes in the body, including immunity. Some hormones can enhance or suppress the innate immune response depending on their concentration and target cells. For example, estrogen can stimulate the production of antimicrobial peptides and cytokines by epithelial cells and macrophages, while progesterone can inhibit the expression of toll-like receptors and inflammatory genes by these cells. Cortisol, a stress hormone, can suppress the activity of natural killer cells, dendritic cells, and neutrophils, as well as the production of cytokines and complement proteins. Thyroid hormones can modulate the phagocytic activity and oxidative burst of neutrophils and macrophages.
- Nutritional status: Nutrition is essential for maintaining the health and function of the innate immune system. Malnutrition can impair the development and maturation of innate immune cells and molecules, as well as their ability to respond to pathogens. Deficiencies in certain nutrients, such as protein, zinc, iron, vitamin A, vitamin C, vitamin D, vitamin E, and selenium, can compromise the integrity of epithelial barriers, the production of antimicrobial peptides and cytokines, the activation of the complement system, the phagocytosis and killing of microbes by neutrophils and macrophages, and the cytotoxicity of natural killer cells.
Therefore, innate immunity is influenced by various factors that can modulate its effectiveness and efficiency in protecting the body from infections. A balanced diet, adequate sleep, regular exercise, and stress management can help to optimize the innate immune response and prevent diseases.
Innate immunity is the first line of defense against foreign invaders and potentially harmful substances. It has several roles and functions that are essential for the protection and maintenance of the body. Some of the significance and examples of innate immunity are:
- Physical and chemical barriers prevent the entry of foreign materials by creating an impermeable or hostile environment for them. For example, the skin, mucous membranes, tears, saliva, stomach acid, and sweat are all part of the innate immune system that acts as physical or chemical barriers.
- Defense mechanisms such as secretions, phagocytosis, and inflammatory reactions help to eliminate or neutralize the foreign materials that have breached the barriers. For example, lysozyme in saliva and tears can break down bacterial cell walls, phagocytes can engulf and destroy microbes and debris, and inflammatory mediators can increase blood flow, temperature, and pain to recruit more immune cells and facilitate healing.
- General immune responses such as complement activation, cytokine release, and antigen presentation help to identify and mark foreign materials for clearance or destruction by other immune cells. For example, complement proteins can coat bacteria and make them more susceptible to phagocytosis or lysis, cytokines can signal other immune cells to migrate to the site of infection or inflammation, and antigen-presenting cells can display fragments of foreign materials on their surface to activate the adaptive immune system.
- Types of innate immunity such as individual, racial, and species immunity reflect the variation in susceptibility or resistance to certain infections among different individuals, races, or species. For example, some individuals may have genetic mutations that confer resistance to HIV infection, some races may have inherited traits that protect them from malaria or tuberculosis infection, and some species may be naturally immune to certain diseases that affect other species.
- Factors influencing innate immunity such as age, hormonal level, and nutritional status affect the strength and function of the innate immune system. For example, very young or old individuals may have weaker innate immunity than adults due to immature or declining immune cells and organs, hormonal imbalances may impair immune cell production or activity, and nutritional deficiencies may compromise immune cell function or integrity.
Acquired immunity, also called adaptive immunity, is the type of immunity that develops after exposure to a specific antigen. An antigen is a foreign substance that triggers an immune response in the body. Acquired immunity involves the production of antibodies and immune cells that can recognize and eliminate the antigen in the future.
Acquired immunity is different from innate immunity, which is the natural and non-specific defense system that we are born with. Innate immunity does not have memory and cannot distinguish between different antigens. Acquired immunity, on the other hand, is highly specific and has memory, meaning that it can remember the antigens that it has encountered before and mount a faster and stronger response upon re-exposure.
Acquired immunity can be divided into two types: active and passive. Active immunity is when the body produces its own antibodies and immune cells after being exposed to an antigen, either naturally (through infection) or artificially (through vaccination). Passive immunity is when the body receives preformed antibodies or immune cells from another source, such as from the mother (through the placenta or breast milk) or from an injection (such as immunoglobulin therapy). Active immunity is long-lasting and can provide protection for years or even a lifetime. Passive immunity is temporary and lasts only as long as the antibodies or immune cells are present in the body.
Acquired immunity can also be divided into two types based on the mediators of the immune response: humoral and cellular. Humoral immunity is mediated by antibodies, which are proteins produced by a type of immune cell called B lymphocytes (or B cells). Antibodies bind to antigens and neutralize them or mark them for destruction by other immune cells. Humoral immunity is effective against extracellular pathogens, such as bacteria, viruses, fungi, and parasites that are outside of the host cells. Cellular immunity is mediated by another type of immune cell called T lymphocytes (or T cells). T cells can directly kill infected or abnormal host cells or activate other immune cells by releasing cytokines (chemical messengers). Cellular immunity is effective against intracellular pathogens, such as viruses, bacteria, and parasites that are inside of the host cells, as well as against cancer cells and transplanted tissues.
Acquired immunity plays a vital role in protecting the body from various diseases and infections. It also enables the development of immunological memory, which allows for a faster and more potent response upon re-exposure to the same antigen. Furthermore, acquired immunity can be enhanced by vaccination, which introduces weakened or killed forms of antigens into the body to stimulate an immune response without causing disease. Vaccination can prevent or reduce the severity of many infectious diseases, such as measles, polio, tetanus, influenza, and COVID-19.
Acquired immunity is the type of immunity that is specific to a particular antigen and is developed during the lifetime of an individual. The mediators of acquired immunity are the cells and molecules that recognize and respond to foreign antigens. There are two main types of mediators of acquired immunity: humoral and cellular immunity.
Humoral immunity is the type of acquired immunity that is mediated by antibodies, which are proteins produced by a specialized type of white blood cells called B lymphocytes or B cells. Antibodies bind to the antigens and neutralize them or mark them for destruction by other immune cells. Humoral immunity protects the body against extracellular pathogens, such as bacteria, toxins, and viruses that are outside the cells.
The process of humoral immunity involves the following steps:
- Antigen recognition: When a B cell encounters an antigen that matches its specific receptor, it binds to it and becomes activated. The B cell may also receive signals from helper T cells or other molecules that enhance its activation.
- Clonal expansion: The activated B cell divides rapidly and produces many identical copies of itself, called clones. Some of these clones become plasma cells, which secrete large amounts of antibodies into the blood and lymph. Other clones become memory B cells, which remain in the body for a long time and can quickly respond to the same antigen in the future.
- Effector functions: The antibodies produced by the plasma cells have various functions that help to eliminate the antigen. For example, they can block the binding of the antigen to its target cell, opsonize the antigen for phagocytosis by macrophages or neutrophils, activate the complement system to form pores in the membrane of the antigen or trigger antibody-dependent cellular cytotoxicity (ADCC) by natural killer (NK) cells or eosinophils.
Cellular immunity is the type of acquired immunity that is mediated by T lymphocytes or T cells, which are another type of white blood cells. T cells have receptors that recognize antigens that are presented by specialized molecules called major histocompatibility complex (MHC) on the surface of other cells. T cells can directly kill infected or abnormal cells or secrete cytokines that regulate the activity of other immune cells. Cellular immunity protects the body against intracellular pathogens, such as viruses, bacteria, fungi, and parasites that are inside the cells.
The process of cellular immunity involves the following steps:
- Antigen presentation: When a cell is infected or damaged by a pathogen, it displays fragments of the pathogen`s antigens on its surface along with MHC molecules. These antigens can be recognized by T cells that have matching receptors. There are two types of MHC molecules: MHC class I and MHC class II. MHC class I molecules present antigens to cytotoxic T cells (also called CD8+ T cells), which can kill infected or abnormal cells. MHC class II molecules present antigens to helper T cells (also called CD4+ T cells), which can activate other immune cells.
- Clonal expansion: When a T cell recognizes an antigen-MHC complex on a cell, it becomes activated and undergoes rapid division and differentiation. Some of these clones become effector T cells, which perform various functions depending on their subtype. For example, cytotoxic T cells release perforins and granzymes that induce apoptosis in the target cell, helper T cells secrete cytokines that stimulate B cells, macrophages, NK cells, or other T cells, and regulatory T cells suppress excessive or unwanted immune responses. Other clones become memory T cells, which persist in the body for a long time and can quickly respond to the same antigen in the future.
- Effector functions: The effector T cells carry out their functions according to their subtype and target cell. For example, cytotoxic T cells can recognize and kill virus-infected cells, tumor cells, or transplanted cells; helper T cells can enhance humoral immunity by activating B cells or cellular immunity by activating macrophages or other T cells, and regulatory T cells can prevent autoimmune diseases by suppressing self-reactive immune responses.
Acquired immunity is the type of immunity that develops after exposure to a specific pathogen or antigen. It involves the production of antibodies and T cells that can recognize and eliminate the pathogen. There are two types of acquired immunity: active and passive immunity.
Active immunity is the type of immunity that results from the activation of the immune system by a pathogen or a vaccine. It involves the generation of memory cells that can provide long-lasting protection against future infections by the same pathogen. Active immunity can be natural or artificial.
Natural active immunity occurs when a person gets infected by a pathogen and develops an immune response that prevents re-infections-infection by the same pathogen. For example, a person who recovers from chickenpox will have natural active immunity against the varicella-zoster virus.
Artificial active immunity occurs when a person receives a vaccine that contains a weakened or killed form of a pathogen or its antigen. The vaccine stimulates the immune system to produce antibodies and memory cells that can protect against future infections by the same pathogen. For example, a person who receives the measles vaccine will have artificial active immunity against the measles virus.
Passive immunity is the type of immunity that results from the transfer of antibodies or immune cells from another person or animal to a person who does not have immunity against a specific pathogen. It provides immediate but temporary protection against infection. Passive immunity can also be natural or artificial.
Natural passive immunity occurs when a mother passes antibodies to her baby through the placenta or breast milk. These antibodies can protect the baby from some infections for a few months after birth. For example, a baby who receives maternal antibodies against tetanus will have natural passive immunity against tetanus.
Artificial passive immunity occurs when a person receives an injection of antibodies or immune cells that are produced by another person or animal. These antibodies or immune cells can neutralize or destroy the pathogen in the body. For example, a person who receives an antivenom after a snakebite will have artificial passive immunity against the venom.
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Acquired (adaptive) immunity is important for the following reasons:
- It provides a specific and targeted response to different types of pathogens, such as bacteria, viruses, fungi, and parasites.
- It generates memory cells that can quickly recognize and eliminate previously encountered pathogens, resulting in a faster and stronger immune response upon re-exposure.
- It can adapt to new and emerging pathogens by generating new antibodies and T cells that can recognize novel antigens.
- It can regulate the immune response by activating or suppressing other immune cells, preventing excessive inflammation or autoimmunity.
Some examples of acquired (adaptive) immunity are:
- Vaccination: Vaccination is a form of artificial active immunity that involves introducing a weakened or killed form of a pathogen or its antigen into the body to stimulate the production of antibodies and memory cells. For example, vaccines against measles, mumps, rubella, tetanus, diphtheria, pertussis, polio, hepatitis B, influenza, etc., protect against these diseases by inducing acquired immunity.
- Infection: An infection is a form of natural active immunity that involves exposure to a live pathogen and its antigen, resulting in the activation of the immune system and the generation of antibodies and memory cells. For example, infection with the chickenpox virus confers lifelong immunity against chickenpox and shingles.
- Maternal antibodies: Maternal antibodies are a form of natural passive immunity that involves the transfer of antibodies from the mother to the fetus through the placenta or to the newborn through breast milk. These antibodies provide temporary protection against some infections for the first few months of life. For example, maternal antibodies can protect against measles, rubella, tetanus, etc.
- Immunoglobulin therapy: Immunoglobulin therapy is a form of artificial passive immunity that involves the administration of preformed antibodies from human or animal sources to treat or prevent certain infections or diseases. These antibodies provide immediate but short-lived protection. For example, immunoglobulin therapy can be used to treat rabies, snake bites, hepatitis A, etc.
Innate and acquired immunity are two types of immune responses that work together to protect the body from infections. However, they differ in several aspects, such as:
- Origin: Innate immunity is present at birth and does not require prior exposure to antigens. Acquired immunity is developed during the lifetime of an individual and depends on the recognition of specific antigens.
- Specificity: Innate immunity is non-specific and can respond to a broad range of pathogens. Acquired immunity is highly specific and can distinguish between different antigens and even different epitopes on the same antigen.
- Speed: Innate immunity is fast and immediate, as it does not need to activate or differentiate immune cells. Acquired immunity is slow and delayed, as it requires the activation, proliferation, and differentiation of lymphocytes.
- Memory: Innate immunity does not have memory and cannot remember previous encounters with pathogens. Acquired immunity has memory and can remember previous exposures to antigens, resulting in a faster and stronger response upon re-infection-infection.
- Components: Innate immunity involves physical and chemical barriers (such as skin, mucous membranes, acidity, enzymes, etc.), phagocytic cells (such as neutrophils, macrophages, dendritic cells, etc.), natural killer cells, complement system, inflammatory mediators (such as histamine, cytokines, etc.), and pattern recognition receptors (such as toll-like receptors, NOD-like receptors, etc.). Acquired immunity involves lymphocytes (such as B cells and T cells), antibodies (such as IgM, IgG, IgA, IgE, etc.), antigen-presenting cells (such as dendritic cells, macrophages, B cells, etc.), major histocompatibility complex (MHC) molecules, T cell receptors (TCRs), B cell receptors (BCRs), and cytokines (such as interleukins, interferons, etc.).
- Types: Innate immunity can be classified into individual, racial, and species immunity based on the level of resistance or susceptibility to infections among different individuals, races, or species. Acquired immunity can be classified into active and passive immunity based on the source of antibodies or lymphocytes.
The table below summarizes the main differences between innate and acquired immunity:
| Innate Immunity | Acquired Immunity |
| --------------- | ----------------- |
| Present at birth | Developed during lifetime |
| Non-specific | Highly specific |
| Fast and immediate | Slow and delayed |
| No memory | Has memory |
| Physical and chemical barriers, phagocytic cells, natural killer cells, complement system, inflammatory mediators, pattern recognition receptors | Lymphocytes, antibodies, antigen-presenting cells, MHC molecules, TCRs, BCRs, cytokines |
| Individual, racial, and species immunity | Active and passive immunity |
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