Health and Diseases - part 01 - Immunity

 

 Health and Diseases - part 01 - Immunity

Health :

• Does not simply mean ‘absence of disease’ or physical fitness. 
• In fact, it is difficult to aptly define health. 
• According to the WHO,Health is defined as the state of complete physical, mental and social wellbeing and not merely the absence of disease or infirmity.
• In short, health is birth right of humans.
• Reflects the metabolic and functional efficiency of living organisms. 
• Hygiene is a science of rules of health. 
• To achieve good health, it is therefore, necessary that we have hygienic balanced diet, clean drinking water, personal and community hygiene, regular physical exercise, knowledge about diseases and their effect on body, proper disposal of waste and control of vectors.
• Everyday we are exposed to various foreign bodies, including infectious agents like bacteria, viruses, etc. 
• Despite constant exposure to variety of pathogens, most of us remain healthy. 
• This is due to fact that the human body has ability to resist almost all type of these foreign bodies. 
• The system which protects us from various infectious agents, is called immune system. 
• Resistance is the ability to prevent the damage or disease, through our defense mechanism.

Immunity:

• Term ‘immunity’ has traditionally referred to as the resistance exhibited by the host towards injury caused by pathogens and their products. 
• However, protection against infectious diseases is only one of the many consequences of immune response, which is entirely concerned with the reaction of the body against any foreign antigen.
• Immunity is in fact the ‘‘freedom’’ or ‘‘exempt’’. 
• Concept of immunity is believed to be started by Edward Jenner in England. 
• He developed cowpox vaccine for the protection against the attack of small pox (virus). 
• Immunology is a branch of science which deals with the study of immune system,immune responses to foreign substances and their role in resisting infection by pathogens.
• Most important characteristic of immune system is that it distinguishes self (body’s own cells) and non-self (foreign molecules or invading cells). 
• So, the immune system differentiates between the body cells and the invaders. 
• Any foreign substance invading body and capable of stimulating an immune response, is called an antigen. 
• The protective chemicals produced by immune cells in response to antigens are called antibodies.

Types of immunity:

1.  Innate or Inborn (inherited) immunity and 
2. Acquired or Adaptive immunity.

i. Innate immunity or Inborn immunity:

• Is the resistance to infections that an individual possesses by virtue of his or her genetic make-up. 
• Natural (inborn) defense system of the body. 
• Not affected by prior contact with microorganisms or immunization. 
• Nonspecific, when it indicates a degree of resistance to infection in general, or specific where resistance to a particular pathogen is concerned. 
• One form of innate immunity comprises the various types of barriers which prevent entry of foreign agents into the body.

a. Epithelial surface :

• The intact skin and mucous covering the body, protect it considerably against invasion by microorganism(s).
• Healthy skin possesses bactericidal activity due to the presence of high concentrations of salt in drying sweat.
• Sebaceous secretions and long chain of fatty acids have bactericidal and fungicidal properties. 
• The mucosa of the respiratory tract has several innate mechanisms of defense. 
• Nose prevents entry of microorganisms to a large extent, the inhaled particles being arrested through hair at or near the nasal orifices. 
• Those that pass beyond are held by mucus lining the epithelium and are swept back to pharynx where they tend to swallowed or coughed out.
• The cough reflex is an important defence mechanism of respiratory tract. 
• The mouth is constantly bathed in saliva which has inhibitory effect on microorganisms. 
• The acidity of gastric secretions in the stomach destroys microorganisms. 
• The flushing action of urine eliminates bacteria from the urethra. 
• Spermine and zinc present in semen are antibacterial.

Knowledge Enhancer:

• The conjunctiva is freed of foreign particles by the flushing action of lachrymal secretions. 
• Eyes become susceptible to infection when lachrymal secretion is absent. 
• Tears contain the antibacterial substance Lysozyme (Ist described by A. Fleming). 
• This is a thermolabile, low molecular weight, basic protein which acts as a muramidase. 
• Lysozyme is active only against some pathogenic Gram +ve bacteria.

b. Antimicrobial substances in blood and tissues:

• Complement system contains more than 30 serum proteins, circulating in the blood in an inactive state. 
• The presence of microbial pathogens activates the ‘Complement cascade’ to eliminate pathogens. 
• The interferons are a class of cytokines (soluble proteins) released by virally cells infected with viruses and certain white blood cells to stimulate other cells to protect themselves from viral infection.

c. Cellular factors in innate immunity :

• Natural defence against the invasion of blood and tissues by microorganisms and other foreign particles, is mediated to a large extent by phagocytic cells which ingest and destroy them. 
• Phagocytic cells (discovered by Metchnikoff in 1882) are grouped as microphages and macrophages. 
• They remove foreign particles that enter the body. 
• A class of lymphocytes called Natural killer (NK) cells is important in nonspecific defence against viral infections and tumors.

d. Fever :

• Increase in the body temperature following the infection is a natural defense mechanism.
• Helps to accelerate the physiological processes to destroy the invading pathogens.
• Stimulates the production of interferon and helps in recovery from viral infections.

e. Acute phase proteins (APPs):

• Infection on injury leads to a sudden increase in concentration of certain plasma proteins, collectively called acute phase proteins. 
• These include -

1. C Reactive Protein (CRP)
2. Mannose binding protein
3. Alpha-1- acid glycoprotein
4. Serum Amyloid P, etc.

• APPs are believed to enhance host resistance, prevent tissue injury and promote repair of inflammatory lesions.

ii. Acquired immunity:

• Resistance that an individual acquires during life is known as “Acquired immunity”.
• Also known as Adaptive or Specific immunity”. 
• Involves the formation of antibodies in the body, which neutralize the antigens. 

Features :

a. Specificity: 

• It can produce specific antibody or T-lymphocyte against a particular antigen/pathogen.

b. Diversity: 

• It can recognize a vast variety of diverse pathogens or foreign molecules.

c. Discrimination between self and nonself:

• It differentiates between own body cells (self) and foreign (non-self) molecules.

d. Memory: 

• When the immune system encounters a specific foreign agent for the first time, it generates an immune response and eliminates the invader. 
• This is called first encounter. 
• The immune system retains the memory of the first encounter. 
• As a result, a second encounter with same pathogen brings about quicker and stronger immune response.

Types of Acquired Immunity:

1. Active immunity
2. Passive immunity

a. Active immunity:

• It is the resistance developed by individuals as a result of an antigenic stimulus.
• Also known as “Adaptive immunity”. 
• Active immunity may be  - 

1. Natural or 
2. Artificial.

1. Natural Acquired Active immunity:

• Immunity acquired due to infection. 
• Developed after entry of pathogens in the body. 
• Long-lasting immunity. 
• e.g. person who has recovered from attack of measles develops natural acquired active immunity to measles, for the life time.

2. Artificial Acquired Active immunity:

• It is the resistance induced by vaccines. 
• Vaccine is introduced into the body to stimulate the formation of antibodies by the immune system. 
• e.g. Polio vaccine, BCG vaccine etc.
• Such immunity may be temporary or permanent.

b. Passive immunity:

• Acquired when readymade antibodies are received by the body cells. 
• Body cells do not take any active part in the production of immunity. 
• Can be acquired either -

1. Naturally or 
2. Artificially.

1. Natural Acquired Passive immunity:

• Before birth maternal antibodies are transferred from mother to foetus through placenta. 
• After birth, antibodies are transferred from mother to infant through colostrum (first milk of mother) and continue throughout the period of breast feeding. 
• The antibodies received by baby from mother remain in the body for a short time. 
• Therefore, natural acquired passive immunity is short lived.

2. Artificially Acquired Passive immunity: 

• This immunity is developed by injecting previously prepared antibodies using serum from humans or animals. 
• For e.g. Antibodies obtained from hyper immunised horses are injected to humans against rabies pathogens.
• It is short lived.

B. Cells of Immune System:

• There are two main types of cells involved in the working of Immune system. They are -

1.  Lymphocytes and
2. Antigen Presenting cells.

a. Lymphocytes:

• Main cells of the immune system. 
• Arise from the stem cells, the haemocytoblasts.
• Present in liver of the foetus and in the bone marrow in adult. 
• Some of them undergo differentiation in the gut – associated bursal lymphoid tissues (Tonsils, Peyer’s patches) and are called Bursal or B-lymphocytes.
• Others are differentiated in the thymus gland and are termed as T-lymphocytes.
• The mature lymphocytes pass into body fluids (blood and lymph) and circulate in the body.
• Many of them stay in the lymph nodes. 
• The B-lymphocytes  form humoral or antibody-mediated immune system (AMIS) .
• T-lymphocytes form cell-mediated immune system (CMIS). 
• Both the immune systems need antigens to come into action, but they respond in different ways.

Mechanism of response of T-lymphocytes to antigens: 

• On coming in contact with an antigen, a T-lymphocyte forms clones of T-cells which are similar but they perform different functions.
• The clone has four types of T-lymphocytes:

1. Helper T-cells
2. Killer T-cells 
3. Suppressor T-cells
4. Memory T-cells

1. Helper T-cells: 

• Sensitized helper T-cells produce lymphokines for performing several types of functions like -

1. proliferation of other T-cells
2. stimulation of B-lymphocytes
3. macrophages, etc.

2. Killer T-cells or Cytotoxic T-cells:

• Directly attack and destroy invading microbes, infected body cells and cancer cells. 
• Killer T-cells bind to infected cell and secrete perforins. 
• Then perforins form a hole in infected cell. 
• It also releases substances that kill the cell, hence the name cytotoxic T-cell.

3. Suppressor T-cells: 

• suppress entire immune system against attack on the own body cells.

4. Memory T-cells: 

• Previously sensitized cells which retain the sensitization memory for long time in the future.

Mechanism of action of B-lymphocytes to antigens:

• B–lymphocytes are sensitized directly by both antigens as well as by helper T-cells.
• Activated B-lymphocyte multiplies very fast to produce clone of plasma cells and memory B-cells. 
• The plasma cells produce specialized glycoproteins, called antibodies which are circulated through body fluids (humor) like blood and lymph. 
• The antibody molecules may bind to a cell membrane or they remain free.

The free antibodies have main functions:

1. Agglutination of particulate matter, including bacteria and viruses. The immobilized mass is then engulfed by phagocytes.
2. Opsonisation or coating of bacteria to facilitate their subsequent phagocytosis by macrophages.
3. Neutralization of toxins released by bacteria e.g. tetanus toxin.
4. Each antibody is specific for a particular antigen.

b. Antigen Presenting cells:

• Antigen presenting cells engulf invading pathogens and process the antigens. 
• Then the processed antigens are presented on their own surface. 
• These cells are able to deliver a stimulatory signal that is necessary for activation of helper T-cell.

C. Vaccination :

• Administration of vaccine (i.e. inactivated pathogen or antigenic protection of particular pathogen) to protect against a particular pathogen, is called vaccination.
• The body’s immune system helps to protect against pathogens that cause infection.
• It’s an efficient system, most of the time,. 
• It either keeps microorganisms out or tracks them down and gets rid of them. 
• However, some pathogens can overwhelm the immune system.
• When this happens, it can cause serious illness.
• The pathogens most likely to cause problems, are the ones the body doesn’t recognize.
• Vaccination is a way to “teach” the immune system as to how to recognize and eliminate pathogenic organism. 
• That way, body is always prepared if you are ever exposed.
• Vaccination is an important form of primary prevention, that can protect people from getting sick. 
• Vaccination has allowed us to control diseases like measles, polio, tetanus and whooping cough that once threatened many lives. 
• It’s important that as many people as possible get vaccinated. 
• Vaccinations don’t just protect individuals, when enough people are vaccinated. It also helps to protect the society.

Do u know ?
How does vaccination work?

• Healthy immune system defends against invaders. Immune system consists of several types of cells. 
• These cells defend against harmful pathogens. 
• However, they have to recognize an invader. 
• Vaccination teaches the body to recognize new pathogens causing diseases. 
• It stimulates the body to make antibodies against antigens of pathogens. 
• Also primes immune cells to remember the types of antigens. This allows a faster response to the pathogen in future.
• Vaccines work by exposing you to a safe version of pathogen. 
• Vaccines may be in the form of:

1. A protein or sugar from the pathogen.
2. A dead or inactivated form of a pathogen.
3. A toxoid containing toxin produced by a pathogen.
4. A weakened (attenuated) pathogen.

• When the body responds to the vaccine, it builds an adaptive immune response. This helps to equip the body to fight off an actual infection.

Vaccinations are safe:

• Vaccines are considered to be safe. 
• Rigorously tested and go through many rounds of study, examination, and research before they are used for the general public.
• Extensive research and evidence shows that vaccines are safe their side effects are rare and typically mild.