Q.Mention the chemical nature of an antibody and name the type of cells they are produced by. Write the difference between active and passive immune responses on the basis of antibodies.

Structure of an Antibody (Immunoglobulin)

An antibody (immunoglobulin) is a protein produced by plasma cells in response to an antigen. Each antibody molecule has a characteristic Y-shaped structure built from four polypeptide chains held together by inter-chain disulfide (–S–S–) bridges.

The four chains are of two kinds:

• Two long (heavy) chains, usually written H, which run from the tips of the arms down through the central stem of the Y.
• Two short (light) chains, usually written L, which lie along the outer edge of each arm.

Because of this composition, an antibody is represented as $H_2L_2$. The two arms end in identical antigen-binding sites (the clefts at the tips), which is why each antibody can bind two antigen molecules; the amino (N) terminals lie at the arm tips and the carboxyl (C) terminals lie at the base of the stem. …

Let’s start with an everyday picture. Think of your body as a country, and your immune system as its army and police force. This army is always on patrol, looking for invaders — viruses, bacteria, fungi — and destroying them before they can cause trouble. Now imagine a spy so clever that it doesn’t just attack the army; it actually enters the army’s command centre, takes over the generals, and slowly disables the entire defence system. That is exactly what the HIV virus does.

HIV stands for Human Immunodeficiency Virus. The key word is immunodeficiency — a deficiency, or shortage, of immunity. The virus does not kill you directly. Instead, it systematically destroys the very cells that coordinate your body’s defence, leaving you defenceless against even mild infections.

The precise mechanism

The immune system has many types of soldiers. The most important commander cell is the helper T-cell (also called CD4+ T-cell). This cell does not kill germs itself; it sends signals that activate other cells — the killer T-cells and antibody-producing B-cells — to attack.

HIV has a special protein on its surface that fits perfectly into a receptor on the helper T-cell, like a key into a lock. Once inside, the virus hijacks the cell’s machinery. It forces the cell to produce thousands of copies of the virus. Eventually, the cell bursts open and dies, releasing new viruses that go on to infect more helper T-cells.

Over months or years, the number of helper T-cells in the blood drops steadily. When the count falls below a critical level (around 200 cells per microlitre of blood), the immune system can no longer mount a proper defence. At this stage, the person is said to have AIDS — Acquired Immunodeficiency Syndrome.

Why it matters

A person with HIV can look and feel healthy for many years. During this asymptomatic phase, the virus is still active, but the body is producing new T-cells to replace the ones being destroyed. This is a losing battle — eventually, production cannot keep up.

The virus is transmitted only through specific routes:

• Unprotected sexual contact
• Sharing of infected needles (common among drug users)
• From an infected mother to her child during pregnancy, childbirth, or breastfeeding
• Transfusion of infected blood (rare now due to screening)

It is not transmitted by casual contact — shaking hands, sharing food, mosquito bites, or living in the same room. …