Third line of Defence (the specific Defence)

 Third Line of Defense (The Specific Defense)

If a pathogen still manages to cross first and second line of defenses and get into your bloodstream, you're not beaten yet! All cell membranes have protein coats that line the outside of them. Early on, when you were still developing as a baby, your body begins learning which cells belong to you and which don't. Now, your developed body has a good idea of whether cells are pathogens or not. This is accomplished by two defense mechanisims, cell mediated and humoral.T lymphocytes or T cells, along with macrophages, oversee the Cell mediated response, while the B lymphocytes or B cells take charge of the humoral Response. Monocytes develop from stem cells in the bone marrow. They then go into blood, where they circulate and then migrate into tissues. In the tissue they further mature into macrophages. Monocytes and macrophages play important roles in the immune defence and inflammation.Lymphocytes are one of the five kinds of white blood cells or leukocytes), circulating in the blood. Although mature lymphocytes all look pretty much alike, they are extraordinarily diverse in their functions. The most abundant lymphocytes are:

**B lymphocytes ago(B cells)
**Tlymphocytes(T cells).
B cells are produced in the bone marrow. The precursors of T cells are also produced in the bone marrow but leave the bone
marrow and mature in the thymus. Each B cell and T cell is specific for a particular antigen.

Basic Types of Immunity

There are two main types of
immunity :

1. Innate or Inborn or Natural immunity:

The innate immunity system is what we are born with and it is nonspecific; all antigens are attacked pretty much equally. It is genetically based and we pass it on to
our offsprings. It is present from the birth and is inherited from the mother to offspring through the placenta. 

2. Acquired or Adaptive immunity:

It is not present from the birth but is acquired during one's own life. It is
developed by the organism in response to a disease caused by the infection of microbes
or vaccine. In this, the protective lymphocytes of body produce antibodies which not only inactivate the antigens and relieve from an infectious disease but also provide immunity against further attack.
It is so as some antibody producing cells persist as “memory cells” for long period and produce antibodies immediately after second infection to counter it. The acquired immunity so developed may be temporary (e.g., influenza) or permanent (e.g., measles, mumps, polio, smallpox) for life long...

Acquired immunity is of two types

(a) Active or Natural immunity. It is a long lasting immunity developed by antibodies

produced by an individual's own cells. It is developed in three ways:

a**By having the disease e.g., chickenpox, mumps, measles etc.

b**By having a subclinical infection e.g. live 'Sabin' vaccinc (against polio).By having killed micro-organisms or detoxified toxins e.g., killed 'Salk' vaccine (against polio), tetanus toxoid (against tetanus).

(b) Passive or Artificial immunity. Passive immunity is "borrowed" from another source and it lasts for a short time. For example, antibodies in a mother's breast milk provide a baby with temporary immunity to diseases the mother has been exposed to. This can protect the baby against infection during the early years of childhood. Everyone's immune system is different. Some people never seem to get infections, whereas others seem to be sick all the time. As people get older, they usually become immune to more germs as the immune system comes into contact with more and more of them. That's why adults and teens tend to get fewer colds than kids  their bodies have learned to recognize and immediately attack many of the viruses that cause colds

Vaccination a mean to develop active acquired immunity

Immunization is the process where body is made immune or resistant to an infection disease, typically by the administration of a vaccine. Vaccine stimulates the body's own immune system to protect the person against subsequent infection or disease. The word vaccination or vaccine had been evolved from vacca which means similar in structure. If the human body is exposed

cow pus, which contains virus for cowpoz. The cowpox and smallpox virus is very

to the cowpox virus through vaccine, the body acquired immunization against smallpox. We can define vaccine as a immuno-biological substance

which produce specific protection against a given disease. It stimulates the production of protective antibody and other immune mechanism. Vaccines may be prepared from live modified organisms, inactivated or killed organisms, extracted cellular fractions toxoids or combination of these. Prevention of disease is the need of the day.

The morbidity caused by the disease and rising costs of treating them requires us to

focus more on their prevention. Immunization is among the most successful

components of preventive medicine. It is effective public health intervention that has

had the greatest impact on health of the people. Every year millions of children around the globe are saved from illness or death because of vaccines.

 Specific defense mechanisms

Specific defense mechanisms are effective against specific pathogens. There are two

main types of specific defense mechanisms

involved in the immune system.

a. Cellmediated immune response

b. The antibody mediated immune response

a. Cell mediated Immune response

The cell-mediated immune system consists of T-cells which originate in the bone marrow, but moves to the thymus

where their development is completed. T-cells are highly-specialized cells in

the blood and lymph. They fight bacteria, viruses, fungi, protozoans, cancer, etc. within host cells and react against foreign matter such as organ transplants.

There are three kinds of T-cells. Cytotoxic T-cells directly kill invaders. Helper T-cells

aid B cells and other T-cells to do their jobs. Suppressor T-cells suppress the activities

of B- and other T-cells so they don't overreactin the cellular immune response, cells of the immune system kill cells of the body that have been infected with a virus or that are cancerous. This response relies on cytotoxic T (Te) cells, Tc cells contain molecules, called perforins, that they release onto target cells. The perforin makes holes in the target cells and thereby kills them. The cellular immune response occurs in two phases. In the first, called the activation phase, Te cells that have the appropriate T-cell receptors are activated and triggered to divide repeatedly. In the second called the

effectors phase, these activated Tc cells encounter target cells and kill them.

b. The antibody mediated immune response

The humoral immune system consists of B-cells which originate in the Bone marrow

and stay there to develop. B-cells can produce antibodies, but need exposure to foreign antigens to do so. These antigens are cell surface oligosaccharides and proteins which the cell uses as "ID tags". Antibodies are chemically proteins present in blood plasma and lymph. They help in fighting bacteria and viruses in body fluids. All daughter cells of a B- cell will be able to produce the same antibodies as the mother cell. Antibodies bind to certain parts of an antigen to mark it for destruction (by the T-cells).

 The body humoral or antibody mediated immune response begins in the same manner as the cell mediated response. But here the macrophages are joined by B cells. The pathogens activate only those B cells with matching receptors. These cells stand ready to enter the battle. Meanwhile the antigens presenting macrophages activate those AT helper T cells with matching receptors. These T cells in turn lead the battle front with activated B cells. Triggers by this meeting the helper release chemicals which make the selected B cells to go into rapid reproduction. Some B cells become memory cells ready to respond to a later invasion by the same pathogens but most become antibody producing factories called Plasma cells. Freely circulating in the body antibody dock with pathogens this neutralizes them or marks them for destruction by other weapons

in our immune arsenal.