Class 12 Biology


The word AIDS stands for Acquired Immuno Deficiency Syndrome. This means deficiency of immune system, acquired during the lifetime of an individual. This term means that AIDS is not a congenital disease. The term ‘syndrome’ means a group of symptoms.

AIDS is caused by the Human Immuno deficiency Virus (HIV). It is a member of a group of viruses called retrovirus. Retrovirus has an envelope enclosing the RNA genome.

Transmission of HIV Infection

It happens by following means:

  1. Sexual contact with infected person,
  2. By transfusion of contaminated blood and blood products,
  3. By sharing infected needles
  4. From infected mother to her child through placenta.

Once the virus enters into the body of a person, it enters into macrophages. In macrophages, the RNA of the virus replicates to form viral DNA with the help of enzyme reverse transcriptase. The viral DNA gets incorporated into host cell’s DNA and directs the infected cells to produce virus particles. Thus, the macrophages begin to act like HIV factory.

Simultaneously, HIV enters into helper T-lymphocytes (TH), replicates and produce progeny viruses. The progeny viruses are released in the blood and begin to attack other helper T-lymphocytes. This is repeated leading to a progressive decrease in the number of helper T-lymphocytes in the body. The infected person suffers from bouts of fever, diarrhea and weight loss during this period.

Due to decrease in the number of helper T-lymphocytes, the person starts suffering from infections that could have been otherwise overcome. The person becomes so much immune-deficient that he is unable to protect himself against most of the infections.

ELISA (Enzyme Linked Immuno Sorbent Assay) is a widely used diagnostic test for AIDS. Treatment with anti-retroviral drugs is only partially effective. These drugs can only prolong the life of the patient but cannot prevent the inevitable death.

Prevention of AIDS


Cell growth and differentiation is highly controlled and regulated. There is a breakdown of these regulatory mechanisms in cancer cells. Normal cells show a property called contact inhibition. Due to this property, contact with other cells inhibits the uncontrolled growth of cells. Cancer cells apparently lose this property, and just continue to divide. Uncontrolled cell division gives rise to masses of cells called tumors. There are two types of tumors, viz. benign and malignant.

Benign Tumor

This type of tumor usually remains confined to its original location and does not spread to other parts of the body. Such tumors cause little damage.

Malignant Tumor

This type of tumor grows very rapidly and invades and damages the surrounding normal tissues. Cells of malignant tumor starve the normal cells by competing for vital nutrients. Cells sloughed from such tumors reach distant sites through blood. Wherever these cells get lodges in the body, they start a new tumor. This property is called metastasis. This is the most feared property of malignant tumors.

Causes of cancer

Normal cells may be transformed into cancerous neoplastic cells by physical, chemical or biological agents. These agents are carcinogens. Examples of physical carcinogens are; X-rays, gammas rays and UV rays. These rays damage the DNA and thus transform normal cells into cancerous cells. The chemicals present in tobacco are examples of chemical carcinogens. Some oncogenic viruses contain viral oncogenes which cause cancer. Some normal cells may also contain cellular oncogenes or proto oncogene.

Cancer detection and diagnosis

Early detection of cancer is highly helpful in treating the disease. For this, biopsy of suspected tissue needs to be done. Histological examination of extracted tissue helps in diagnosing cancer. Biopsy report is further augmented with histological tests of blood and bone marrow. Radiography, CT and MRI are also utilized for diagnosis of cancer.

Treatment of cancer

Tumor cells are killed by radiotherapy. Proper care is taken to minimize the damage to normal cells during radiotherapy. Many chemotherapeutic drugs are also used to kill cancerous cells. A combination of surgery, radiotherapy and chemotherapy is applied in most of the case.

Tumor cells have the ability to avoid detection and destruction by immune system. Hence, biological response modifiers (such as alpha-interpheron) are given. These substances activate the immune system, and help in destroying the tumor.