What is Immunity?
Did you ever notice, everyday, why even after getting exposed to a number of foreign particles we rarely fall sick? Well, this is because of immunity. Immunity is the defense mechanism of multicellular organisms that fights against pathogens. Pathogens are disease-causing microorganisms such as bacteria, viruses, and fungi. Antigens present on the surface of pathogens trigger the immune response in the body.
There are mainly two types of immunity, innate and adaptive; adaptive immunity is further classified as ‘Active’ and Passive,’ which can be obtained by ‘natural’ or ‘artificial’ means. Almost all metazoans possess innate immunity, but adaptive immunity is a characteristic of vertebrates only.
Innate immunity, also called native immunity is considered the first line of defense against pathogens. All non- vertebrates, vertebrates, and plants are blessed with innate immunity by birth. This immunity is non-specific in action. This immunity includes the following four barriers:
The main physical barrier is the skin on our body that hinders the entry of the micro-organisms inside our body. physical barriers like skin, the epithelial lining of the gut, and the respiratory tract. The other physicals barriers are the mucus coating of the epithelium lining the respiratory, gastrointestinal, and urogenital tracts, which trap microbes and prevent them from entering our body.
Body secretions like saliva, gastric juices, and vaginal secretions also play an important in providing innate immunity to an individual. They kill the foreign particle by producing a general immune response called inflammation.
Cells responsible for innate immunity are WBC’s such as basophils, eosinophils, dendritic cells, and certain phagocytic cells such as macrophages, Natural killer (NK) cells, and neutrophils. Phagocytes are a type of cells that engulf and digest foreign particles like bacteria, dust, etc.
The cells infected with the virus secrete proteins called ‘interferons,’ which prevent non-infected cells from getting infected.
Examples of Innate Immunity
- Our skin produces sebum, which is an oily substance secreted by sebaceous glands present on the body surface. Sebum locks in the moisture, thus it keeps our skin moisturized, and it also protects our skin from pathogens. Sebum is slightly acidic in nature (pH 4.5 to 6.0), which is not suitable for bacterial or fungal growth.
- The inner epithelial lining of the stomach produces acid of pH 1.5 to 3.5, which kills the pathogens ingested with food and water.
- Pathogens present in the air get trapped in the mucus and are expelled out by ciliated epithelial cells of the respiratory tract. Pathogens in the trachea get expelled out by forced exhalation (cough).
- Low pH, mucus, and lactobacilli present in the vaginal secretion act as the first line of defense against pathogens.
- Lysozyme present in tears and saliva is antibacterial in nature. They kill the bacteria by destroying their cell wall.
Adaptive or Acquired Immunity
In contrast to innate immunity that is non-specific to pathogens, adaptive or acquired immunity is pathogen-specific. The main characteristic of this type of immunity is the memory that it forms after our body encounters a pathogen for the first time and after which our body produces a low-intensity response called ‘primary response.’ When our body encounters again with the same pathogen, a highly intensified secondary or anamnestic response is produced, which confirms the presence of memory created by our body during the first encounter. These two responses are termed as the ‘primary and secondary immune responses.’ There are two special types of lymphocytes present in our blood, B-lymphocytes and T lymphocytes, which carry out the primary and secondary immune responses. When our body encounters pathogens, the B-lymphocytes produce an army of proteins into our blood to fight these pathogens. These proteins are called antibodies. Although T-cells themselves do not secrete proteins, they help B-cells produce these proteins or antibodies. Adaptive or acquired immunity is carried out with two responses, antibody-mediated and cell-mediated.
A person can become immune either actively or passively. In case the immune system starts responding in the presence of a foreign particle, the acquired immunity is said to be ‘active;’ however, when antibodies from one host are transferred to another, it is called ‘passive’ immunity.’ Both active and passive immunities can be either natural or artificial.
When pathogens, living or dead microbes or other proteins, enter our body, our own immune system recognizes them as foreign particles and produces antibodies. These antibodies are specific to the antigen molecules present on the surface of pathogens. Although active immunity is slow, it lasts long for years. Active immunity can be classified as natural or artificial.
Naturally Acquired Active Immunity
When a person falls ill after getting exposed to antigens and then recovers, it is called naturally acquired active immunity.
Artificially Acquired Active Immunity
When a person falls ill after getting exposed to antigens but recovers only after introducing dead or weakened antigens to the host’s cell through ‘vaccination,’ it is called artificially acquired active immunity.
In passive immunity, antibodies are not produced by our own immune system, instead, they are transferred to us from other hosts. In other words, we can say that a person is gaining immunity for a disease by antibodies produced in someone else’s body. Passive immunity is also classified as natural and artificial.
Naturally Acquired Passive Immunity
This immunity occurs during pregnancy when some antibodies are transferred from the maternal blood into the fetal bloodstream by crossing the woman’s placenta. This occurs mainly during the third trimester of pregnancy. Natural passive immunity can also take place through breastfeeding. Mainly IgG and IgA antibodies are transferred in this case. Breast milk contains a yellow, nutrient-rich fluid called colostrum. It is rich in antibodies and thus provides immunity to the child. This type of immunity is however not very long-lasting.
Artificially Acquired Passive Immunity
This immunity by introducing antibodies rather than antigens into our body. The antibodies obtained in this case are generally from a person or animal who has already acquired immunity to the disease. These are generally introduced by injection. It provides immunity for a few weeks or months as these antibodies break down gradually. Examples of vaccines for passive immunization are Hepatitis B, Hepatitis A, measles, etc.
Examples of Adaptive or Acquired Immunity
- A person after recovering from measles is protected against measles for a lifetime.
- A fetus becomes immune after getting antibodies from the blood of its mother through the mother’s placenta.
- An infant becomes immune after getting antibodies through breastfeeding.
Difference Between Active Immunity and Passive Immunity
- Active immunity is produced by one’s own body on exposure to an infection, whereas passive immunity is received by the host human or animal.
- Active immunity is long-lasting, whereas passive immunity lasts only for a few weeks or months.
- Active immunity is highly specific, thus it is more effective than passive immunity.
- Memory antibodies are produced only in the case of active immunity. This ensures fast and more enhanced immunological response after future infection with the same pathogen.
What are antibodies?
Antibodies are Y-protein molecules present in the blood serum and certain body secretions (such as saliva, tears, breast milk), they are also known as immunoglobulins (Ig). They are produced when our immune system is triggered by the invasion of a pathogen. They recognize a specific surface molecule of the pathogen called an antigen. They recognize and neutralize the pathogens. Antibodies are highly specific in nature. IgA, IgM, IgE, IgG are some of the antibodies produced in our body. They are different in their structure and mode of action.
What are antigens?
Antigens are the molecules that are present on the surface of a pathogen. These antigens are bound by an antigen-specific antibody or B-cell antigen receptor, and their presence in our body triggers an immune response. Our body produces a specific antibody for each antigen. Antigens are mainly proteins, peptides (amino acid chains), and polysaccharides; however, lipids and nucleic acids can also become antigens when they are combined with proteins and polysaccharides.
What are vaccines?
Vaccines are suspension of dead or inactive pathogens, one of its surface proteins or toxins produced by pathogens. Introducing inactive or dead pathogens in our body through vaccination triggers our immune system which in return produces antibodies. These antibodies circulate in the blood for a long time as memory antibodies, and if the said pathogen invades in the future, the memory antibodies will fight it off.