- Immunity is derived from Latin word “immunis” which means free from burden. In this case burden refers to disease caused by microorganisms or their toxic products.
- Therefore Immunity is defined as the state of resistance or in susceptibility to disease caused by particular microorganisms or their toxic products.
Types of immunity:
Broadly there are two types of immunity.
- Innate or natural immunity-Immunity with which an individual is born is called innate or natural immunity.
- Acquired immunity/Adaptive Immunity-Immunity which is developed later in life after microbial infection in host is called as Acquired or developed immunity.
Innate or Natural immunity:
- Immunity with which an individual is born is called innate or natural immunity.
- Innate immunity is provided by various components such as Skin, mucus membrane, Phagocytic cells etc
- Innate immunity acts as first line of defense to particular microorganisms.
Mechanism of innate immunity:
Skin and mucus membrane:
- Skin consists of two distinct layer; a thin outer layer called epidermis and thick inner layer called dermis.
- Epidermis consists of mostly dead cell filled with keratin. Dermis is composed of connective tissue, hair follicle, sebaceous gland and sweat gland.
- Skin provides first line of defense by preventing entry of microorganisms. However skin may be penetrated by injury or insects.
- Below skin, the mucus membrane prevents the entry of microorganism to the body. And also it secrets mucus that entraps microorganisms.
Anatomical barriers provide immunity by following ways.
- At first skin and mucus membrane prevent entry of microorganism into host body by mechanical separation. For example, Skin surrounds the host body from external and mucus membrane surrounds the body tracts.
- They also have mechanism to kill the pathogen before entry to body. For example; lysozyme, acidic pH, sebum, high salt concentration in sweat are antimicrobial agents found in skin and mucus membrane.
- Skin and mucus membrane provides first line of defense against microorganism as they are first component to encounter with microorganism.
- Physicochemical barrier includes physiological barrier and chemical barrier.
- Physiological conditions of body such as normal body temperature, normal body pH etc provides immunity.
- Some species are resistant to certain disease simply because of their higher body temperature. For example, mammals are susceptible to anthrax but birds are resistant to anthrax. It is because Bacillus anthracis are killed by higher body temperature of birds (39°C).
- Similarly, body pH also provides immunity. For example acidity of stomach kills most of the ingested bacteria and provides immunity. In infants stomach is less acidic. This is the reason why infants suffer more from gastrointestinal disturbance than adults.
- Chemical barriers include various antimicrobial chemicals found in body fluids. For examples, Lysozyme found in tear and mucus kills many Gram +ve bacteria.
- Interferon found in blood and lymph kills viruses. Other antimicrobial chemicals found in body fluids include complement protein, collectins, etc.
3.Phagocytosis or Phagocytic barrier of immune system
- Phagocytosis is an important defense mechanism of host to provide immunity. Most of the bacteria that enter into host are killed by phagocytic cells such as Neutrophils, monocytes and macrophages.
4.Inflammatory barrier or Inflammation
- Inflammation is an important defense mechanism of host to prevent infection. It is induced in response to tissue damage caused by microorganism, toxins or by mechanical means.
- The inflammation may be acute; for eg. in response to tissue damage or chromic; for eg. Arthritis, cancer etc.
- Main aim of inflammation is to prevent spread of injected microorganism or toxin from site of injection and kill them on spot by phagocytosis.
Characteristics of inflammation:
- Rubor: redness
- Tumor: swelling
- Calor: heat
- Dolor: pain
- Functio laesa: loss of function
Acquired or Developed immunity:
- Immunity which is developed later in life after microbial infection in host is called as Acquired or developed.. For example, If an individual is infected with chicken pox virus, he/she become resistant to same virus in later life.
- Acquired immunity is provided by Antibodies and certain T-lymphocytes.
- Components of acquired immunity such as Antibodies and T- cells are specific to particular microorganism. Therefore acquired immunity is also known as Specific immunity.
Characteristics of Acquired immunity:
- Self/non-self recognition
- Immunological memory
Types of acquired immunity:
- Active immunity
- Passive immunity
1. Active immunity:
- If host itself produces antibodies, it is called active immunity.
- It is of two types; artificial active immunity and natural active immunity.
Artificial active immunity: Immunity provided by vaccination.
Natural active immunity: immunity provided by natural infection.
2. Passive immunity:
- If host does not produce antibodies itself but antibodies produced in other host provides immunity, than it is known as Passive immunity.
- It is of two types; natural passive immunity and Artificial passive immunity
- Natural passive immunity: IgG antibody produced in mother cross placenta and protects fetus up to 6 month old age.
- Artificial passive immunity: if preformed antibody are injected into host for immunity. Eg. Anti-venom, Rabies vaccine (* it is not a vaccine, it is preformed anti rabies antibody)
Adaptive Immunity – Humoral and Cellular Immunity
There are two main mechanisms of immunity within the adaptive immune system – humoral/antibody mediated and cellular.
Humoral immunity is also called antibody-mediated immunity. With assistance from helper T cells, B cells will differentiate into plasma B cells that can produce antibodies against a specific antigen. The humoral immune system deals with antigens from pathogens that are freely circulating, or outside the infected cells. Antibodies produced by the B cells will bind to antigens, neutralizing them, or causing lysis (dissolution or destruction of cells by a lysin) or phagocytosis.
Cellular immunity occurs inside infected cells and is mediated by T lymphocytes. The pathogen’s antigens are expressed on the cell surface or on an antigen-presenting cell. Helper T cells release cytokines that help activated T cells bind to the infected cells’ MHC-antigen complex and differentiate the T cell into a cytotoxic T cell. The infected cell then undergoes lysis.