Lymphoid Organs immune system of human beings consists of lymphoid organs. The organs whereas callid the maturation and proliferation of lymphocytes occurs are called lymphoid organs. Lymphoid organs are of two types. These are

Lymph nodes are small solid structures present at different points along the lymphatic system. They trap the microorganisms or other antigens that enter the lymph and tissue fluid. Antigens trapped in the lymph nodes activate the lymphocytes and produce an immune response.

Each antibody molecule has four peptide chains, two small called light chains (represented by L ) and two longer called heavy chains (represented by H ). Hence, an antibody is represented as $\mathrm{H}_2 \mathrm{~L}_2$.

When the body encounters a pathogen for the first time, it mounts an immune response by generating antibodies. This response is of low intensity. Subsequent encounter with the same pathogen elicits a highly intensified secondary response.

This is ascribed to the fact that our body appears to have memory of the first encounter. This type of secondary immune response is elicited by memory T-cells, B-cell which keep ready to mount a rapid and vigorous attack as soon as the same pathagen infects the body again.

The patient is suffering from AIDS. The causative agent is HIV virus, a member of 'retrovirus group'.

Differentiate between active immunity and passive immunity