Innate immunity and acquired immunity are two important and different segments of the immune system that act together to defend the body against infection and disease. The key difference between these two segments is that, innate immunity is present from the point of birth while acquired immunity develops over growth. In this article, both systems are approached independently to highlight their differences.
What is Innate Immunity?
Innate immunity is the form of immunity that is inborn or, in other words, found naturally in an organism. It is the form of immunity that is activated immediately in response to an invading microorganism. It is nonspecific in nature i.e. despite the varied types of microorganisms invading the body at any given time, the means of the response of the innate immune system remains the same. Innate immunity is found in all types of organisms irrespective of them being unicellular, multi cellular, vertebrates or invertebrates, etc. and the mechanisms by which they confer immunity are more or less the same.
The innate immune system consists of a number of mechanisms by which it enforces immunity to the body, these include;
- Mechanical barriers of the body that prevent entry of microbes. These barriers may be physical or chemical in nature. A few of these barriers are the skin, epithelial tissue, mucous membranes, gut flora, stomach acid, the flushing action of saliva and tears,
- Chemotaxis; i.e. attraction of phagocytic cells to the site of infection by cytokines or chemokines produced by the infected tissue or cells.
- Opsonisation; i.e. the coating of the surface of the invading pathogen for easy recognition by phagocytic cells.
- Phagocytosis; i.e.engulfing and digestion of the invading pathogens by varied leukocytes (phagocytes) of the blood such as neutrophils, macrophages, natural killer (NK) cells, eosinophils and basophils.
- Inflammation; i.e. swelling, pain, redness and heat production at the site of infection.
Phagocytosis
What is Acquired Immunity?
Acquired immunity is also referred to as adaptive immunity or specific immunity. It is the type of immunity that comes into action if innate immune mechanisms are somehow breached by the invading pathogen. It is the type of immunity that is adapted by the body in such circumstances in order to defend the body against the invading pathogen. Due to the process of adaptation, the acquired immune system responds comparatively slowly than the innate immune system. The acquired immune system is highly specific in nature i.e. it responds specifically to every pathogen it encounters. The acquired immune system is only found in vertebrates. It is composed of two important components that bring about the specific mechanisms necessary for the defense of the body against invading pathogens. These are: the humoral immune system and the cell-mediated immune system.
Humoral Immune System
Humoral immunity (antibody mediated response) comprises immunity that is conferred with the help of specific antibodies. These specific antibodies are produced in response to the presence of a pathogen and are highly specific towards that pathogen. Antibodies are macromolecules that are produced by activated B cells (also referred to as ‘plasma cells’) of the acquired immune system in recognition of antigens (also macromolecules) on the surface of pathogens. In addition to being specific to each other antigens and antibodies are also complementary to each other. Antibodies bring about immunity by neutralizing the invading pathogen. The antibodies tether to the corresponding antigen and prevent further invasion and damage by the pathogen it may also aid in opsonization of the pathogen.
Another very important phenomenon achieved by antibody production in acquired immunity is ‘immunological memory’ i.e. if ever a pathogen is encountered for the very first time by the body (primary infection) the acquired immune system activates and produce antibodies. However, even after elimination of infection and a few B cells that produce antibodies against this pathogen remain available throughout life, even after the immediate infection is resolved. Theses B cells are called ‘memory cells’, so if ever the same pathogen were to be encountered again (secondary infection) these memory B cells would reactivate to produce the specific antibodies to combat the pathogen. This phenomenon is called ‘immunological memory’.
Cell-mediated Immune System
Cell mediated immunity (cell-mediated response) is conferred majorly with the help of T cells. In the course of infection, two different types of T cells can be activated, either the helper T cells or cytotoxic T cells. Helper T cells are activated when antigens from pathogens are expressed on phagocytic cells or antigen presenting cells (APCs) of the immune system. The helper T cell produces cytokines that in turn activate other immune pathways that exhibit defense against the pathogen. Cytotoxic T cells are activated in the presence of tumor cells or virus infected cells; they cause apoptosis or cell lysis of the infected cell.
For ease of understanding and means of simplicity, acquired immunity may also be divided into two other types of immunity i.e. passive and active immunity. Both of these forms of immunity can be acquired either naturally or artificially.
Passive Immunity
Passive immunity is the type of immunity that is acquired by a baby from its mother during the period of gestation. Antibodies from the mother’s system tend to cross the placenta and hence confer immunity in the baby’s system. This immunity usually lasts three months after birth and wanes thereafter. This is the natural means of acquiring passive immunity. The artificial means would be by immunizations, or in other words obtaining immunizing injections for infection or disease.
Active Immunity
Active immunity is the type of immunity acquired when one is exposed to a pathogen, and the body actively engages in combating the pathogen as in a primary infection (briefly explained above). This is the means by which active immunity is acquired The artificial means by which one receives active immunization would be via vaccinations.
What is the Difference Between Innate Immunity and Acquired Immunity?
Definition of Innate Immunity and Acquired Immunity
Innate Immunity: Innate immunity is the form of immunity that is inborn in an organism and is activated immediately in response to an invading microorganism.
Acquired Immunity: Acquired immunity, also referred to as adaptive immunity or specific immunity, is the type of immunity that is adapted by the body to defend the body against the invading pathogen.
Characteristics of Innate Immunity and Acquired Immunity
Nature
Innate Immunity: Innate immunity is generic or non-specific in nature
Acquired Immunity: Acquired immunity is specific in nature.
Acquisition
Innate Immunity: Innate immunity is present from the point of birth
Acquired Immunity: Acquired immunity develops over growth.
Inheritance
Innate Immunity: Innate immunity is inheritable
Acquired Immunity: Acquired immunity is not inheritable, with the exception of one form of passive immunity acquired by a baby from its mother during gestation.
Defense Mechanisms
Innate Immunity: Aspects of innate immunity such as mechanical barriers exert their defensive mechanics irrespective of the presence or absence of an invading pathogen
Acquired Immunity: In the case of acquired immunity, contact with a pathogen is essential to build up defensive mechanisms.
Response
Innate Immunity: Innate immunity is triggered immediately in response to infection
Acquired Immunity: Acquired immunity takes a while to develop and exert its effects.
Cells
Innate Immunity: The major immune cells involved in innate defensive mechanisms are NK cells, neutrophils, macrophages, eosinophils, basophils, etc.
Acquired Immunity: The major immune cells involved in the acquired system are majorly the lymphocytes; the B cells and T cells.
Image Courtesy:
“T cell activation” by T_cell_activation.png: Template drawing and caption text from “The Immune System”, any modifcations, made by myself are released into the public domain.derivative work: Hazmat2 (talk) – This file was derived from: T cell activation.png:. Licensed under Public Domain via Wikimedia Commons
“Phagocytosis2” by GrahamColm at English Wikipedia. ( CC BY-SA 3.0) via Wikimedia Commons