Class 12 Biology Notes Chapter 8 – Immunity And Human Immune System
Biology notes for Class 12: Immunity is the balanced state of a multi cellular organisms having adequate biological defenses to fight infection, disease, and another unwanted biological invasion, and also having adequate tolerance to avoid allergy and autoimmune diseases. Immunity is also called disease resistance. The lack of immunity is known as susceptibility.
The science dealing with the various phenomena of immunity, induced sensitivity and allergy is called immunology.
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Types of Immunity:
There is two major types of immunity: innate and acquired immunity.
(A) Innate or Natural or Nonspecific Immunity (L. innatus = inborn)
Innate immunity is inherited by an organism from the parents and protects it from birth throughout life. Innate or nonspecific immunity lacks specific responses to specific invaders. This type of immunity is provided by various barriers present in our body. Innate immunity consists of four types of barriers— physical, physiological, cellular and cytokine barriers.
(B) Acquired Immunity (= Adaptive or Specific Immunity)
The immunity that an individual acquires after the birth is called acquired or adaptive or specific immunity. It is specific and mediated by antibodies or lymphocytes or both which make the antigen harmless.
It not only relieves the victim of the infectious disease but also prevents its further attack in future. The memory cells formed by В cells and T cells are the basis of acquired immunity. Thus acquired immunity consists of specialized В and T lymphocytes and Antibodies.
Characteristics of Acquired Immunity:
It is the ability to differentiate between various foreign molecules (foreign antigens).
It can recognize a vast variety of foreign molecules (foreign antigens).
(iii) Discrimination between Self and Non-self:
It can recognize and respond to foreign molecules (non-self) and can avoid responding to those molecules that are present in the body (self) of the animal.
When the immune system encounters a specific foreign agent, (e.g., a microbe) for the first time, it generates an immune response and eliminates the invader. This is called the first encounter. The immune system retains the memory of the first encounter. As a result, a second encounter occurs more quickly and stronger than the first encounter.
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Types of Acquired Immunity:
Acquired ( Adaptive) Immunity is of two types: active immunity and passive immunity.
- Active Immunity:
In this immunity person’s own cells produce antibodies in response to infection or vaccination. It is slow and takes time in the formation of antibodies. It is long lasting and is harmless. Active immunity may be natural or artificial.
(a) A person who has recovered from an attack of smallpox or measles or mumps develops natural active immunity.
(b) Artificial active immunity is the resistance induced by vaccines. Examples of vaccines are as follows: Bacterial vaccines, (a) Live- BCG vaccine for tuberculosis, (b) Killed vaccines- TAB vaccine for enteric fever. Viral vaccines, (a) Live – Sabin vaccine for poliomyelitis, MMR vaccine for measles, mumps, rubella, (b) Killed vaccines- Salk vaccine for poliomyelitis, neural and non-neural vaccines for rabies. Bacterial products. Toxoids for Diphtheria and Tetanus.
- Passive Immunity:
When ready-made antibodies are directly injected into a person to protect the body against foreign agents, it is called passive immunity. It provides immediate relief. It is not long lasting. It may create problems. Passive immunity may be natural or artificial.
(a) Natural passive immunity is the resistance passively transferred from the mother to the foetus through the placenta. IgG antibodies can cross the placental barrier to reach the foetus. After birth, immunoglobulins are passed to the newborn through the breast milk. Human colostrum (mother’s first milk) is rich in IgA antibodies. Mother’s milk contains antibodies which protect the infant properly by the age of three months.
(b) Artificial passive immunity is the resistance passively transferred to a recipient by administration of antibodies. This is done by administration of hyperimmune sera of man or animals. Serum (plural is sera) contains antibodies. For example, anti-tetanus serum (ATS) is prepared in horses by active immunization of horses with tetanus toxoid, bleeding them and separating the serum. ATS is used for passive immunization against tetanus. Similarly, anti-diphtheric serum (ADS) and anti-gas gangrene serum (AGS) are also prepared.
The immune response involves primary immune response and secondary immune response.
(a) The primary immune response:
After an initial contact with an antigen, no antibodies are present for a period of several days. Then, a slow rise in the antibody titer (arbitrary units) occurs, first IgM and then IgG followed by a gradual decline in antibody titer. This is called the primary immune response.
(b) The secondary immune response:
Memory cells may remain in the body for decades. Every new encounter with the same antigen results in a rapid proliferation of memory cells. This is also called “booster response”. The antibody titer after subsequent encounters is far greater than during a primary response and consists mainly of IgG antibodies. This accelerated, more intense response is called the secondary immune response. Antibodies produced during a secondary response have an even higher affinity for the antigen.
The increased power and duration of the secondary immune response explain why immunization (called vaccination) is usually accomplished by injecting antigen in multiple doses.
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