We are surrounded by pathogens of various diseases but we don’t get sick too often. Through the course of evolution, we have got a well-developed immune system to fight with deadly diseases. Our body can differentiate between self and non-self. The foreign agents or technically antigens can be detected and be remembered by our immune system. Let’s divide them into lines of defence.
First, we have a layer of skin with dead cells forming the physical barrier upon which most of the pathogens cannot live or grow. The mucous layer lining the gastrointestinal tract, urinogenital tract, which traps and removes microbes forms another physical barrier.
Secretions like sebum (oily secretion) from the ear, eyes with tears have lysozyme (anti-microbial) which kills the microbes trying to enter. Some parts have acidic pH like the stomach and genital tract which is inhabitable for most microbes. They form the physiological barrier for microbes. These skin, pH, and mucus lining constitute the 1st line of defense.
Although, even if they enter the body or blood, the second and third line of defense comes to rescue. Blood cells like macrophage in tissue and neutrophils, monocytes, and natural killer cells in the bloodstream, interferons secreted by virus-infected cells to protect non-infected cells from viral infection and fever constitute the second line. After them, the third line B-cells (produces antibody) and T-cells cells (a type of lymphocyte) come to play.
Types of Immunity
Innate immunity
The immunity with which we are born or present at the time of birth is innate immunity. They are non-specific means work the same against any pathogen. the physical, physiological, cellular(macrophages, neutrophils), and cytokine (interferons) barriers are included in this.
Acquired immunity
The immunity which we acquire in our life span through encounters with various pathogens. They are pathogen-specific and have memory. They can recognize a diverse variety of micro-organisms.
Active Immunity vs Passive Immunity
Passive immunity
when readymade antibody or antitoxins are given directly against a disease or pathogen. They are administered when a rapid response is required but it is short-lived and less intensified. In this case, there is no role of antigen, no antibody is produced by the body and the body is unable to fight future infection by the same pathogen. No memory of the pathogens is stored there. Used for the treatment of acute infection. The host immune system does not participate.
Active immunity
When pathogens (or antigen) enter our body either deliberately by vaccination or through infection we produce antibodies against it to fight and a memory of the pathogen is developed for future encounters with them. The first or primary response is although the slow and less intensified secondary or anamnestic response is rapid and more violent. This is used as a prophylaxis to increase resistance.
Antigens-antigens are pathogens or their parts, or foreign agents having different antigenic determination sites or epitopes.antigens are multivalent.
Components of Immune System
lymphoid organ
Lymphocytes are originated, matured, and proliferated here.
They are primary and secondary
Primary Lymphoid Organ – They include bone marrow(where all blood cells are produced) and thymus (lobed organ beneath the breast bone). Here immature lymphocytes are made antigen sensitive. They provide a micro-environment for the development and maturation of T-lymphocyte.
Secondary Lymphoid Organ – They are the site for the interaction of lymphocytes with an antigen which then proliferates to become effecter cells. They are the training points of lymphocytes. They include tonsils, Peyer’s patches (in the small intestine), appendix, lymph nodes, spleen.
Lymphoid tissue
They are organised structure which helps in immune response Mucosal associated lymphoid tissue(MALT) lines the respiratory, digestive, urogenital tracts constitute half of the lymphoid tissue.
Cells (lymphocytes)
T cells help in cell-mediated immune response or cellular immunity. There are four types of T-cells i.e. helper T-cells, killer T-cells, suppressor T-cells, and memory T-cells.B-cells plays role in antibody-mediated immunity or humoral immunity.
Antibodies
They are soluble glycoprotein molecules secreted by effecter B-cells in response to antigen invading the body. They are denoted by Ig(immunoglobulin). They are of five types.
IgM- 1st antibody generated at the time of defence and 1st antibody formed by foetus in the 5th month against congenital infection.Largest and heaviest antibody.
IgG- Main role in chronic infection and secondary immune response. They provide immune power to the embryo also.
IgD-Activation of B-lymphocyte.
IgE- Related to allergic reactions.
IgA-Protection of mucous membrane and the outer surface of the body.
How does the immune system work?
When pathogens enter the body first they encounter the macrophages which activate the T-helper cells by releasing cytokines or interleukins and the war begins. Helper T- cells stimulate B-cells to produce antibodies.
T-helper cells stimulate the killer T-cells which can destroy the infected cell or target cell. Killer T-cells secrete lymphokines (messenger molecules) that attract phagocytes at the site of infection. Killer T-cells are responsible for cell-mediated immunity. They kill the transplanted cell, tumor cells, and other foreign cells.
B and T cells after stimulation, begin to clone itself called clonal selection. T-helper cells produce lymphokines that bring WBC to the affected area.
Another cell called memory T-cells retain the memory of each encounter and converts into effecter cells on subsequent encounter by the same pathogen.
If the attack damaging own body suppressor cells suppresses the entire immune system from further harm.
What antibodies do?
Antibody molecule has a site for the antigen on the surface of the pathogen to bind (agglutination) in the lock and key fashion to make antigen-antibody complex and destroys them by breaking them (cell lysis).
The coating (opsonization) of the pathogen with antibody (IgG and IgM) facilitates the phagocytosis of them. The antibody neutralizes the toxin of the pathogen by attaching.
Vaccination and Immunisation
This is mainly based on the power of memory of the immune system. A preparation of antigenic proteins of pathogen or inactivated/weakened pathogen having antigenicity but no pathogenicity (produces no disease) is given. They stimulate antibody production and hence neutralize the pathogens on the actual infection.
Using recombinant DNA technology, antigenic polypeptides can be produced in bacteria or yeast. With this technique, large-scale production of vaccines is possible.
Types of vaccines
Live vaccines
BCG and oral polio vaccine a weakened (attenuated) vaccine and in smallpox, a natural vaccine is given.
Killed vaccine
In influenza, pneumonia, cholera, rabies, whooping cough inactivated vaccine is given.
Toxoid vaccines
In tetanus, Diphtheria, botulism bacterial exotoxin is used as a vaccine.
Recombinant antigen vaccines
This is the 2nd generation vaccine. The hepatitis-B vaccine is produced from transgenic fungus (yeast).