Question

A man is exposed to a virus. The virus breaks through the body's first and second line of defense. Outline the sequence of events leading to the destruction of the virus in the body's third line of defense.

Answer 1

The immune system has three lines of defense against pathogens.

1. The first line of defense is the physical and chemical barrier. (skin, mucous membrane, lysozyme etc.)
2. The second line of defense is non specific carried out by phagocytic cells or by increasing the body temperature (fever)
3. The third line of defense is very specific. It depends on foreign partial called the antigens(Antibody generating substance) Most of these antigens are proteins and are a trigger for specific immune response. These responses are carried out by specialised lymphocytes (T cells) or by antibodies. When the response is mediated by T cells it is called as cell-mediated immunity and when it is done by the antibodies present on the B cells then it is called Humoral immunity. This kind of specific response is called adaptive immunity.
4. Please refer the image for an outline for third line of defense: