THE IMMUNE SYSTEM

The immune system consist of structures and processes that protect the body from infections, diseases, and foreign agents. The immune system should be able to distinguish between self and non-self molecules.

There are two types of immune systems:
1. Innate immune system: This is the part of the immune system present at birth. It consist of the lymphoid system (lymph nodes, spleen, tonsils, adenoids), liver, barriers such as the skin, mucus membranes and its secretions such as tears, gastric juices, cilia and hairs in the respiratory system that trap dirt, etc. It also involves inflammation using the leucocytes (White blood cell) such as neutrophils. Macrophages use the complement system to destroy antigens and micro-organisms. Natural killer cells, a class of lymphocytes, are used to destroy cancer cells.

• Non-specific responses
• Immediate maximum response follows exposure
• Has both cellular and humoral components
• There is no immunological memory

2. Adaptive immune system: This is the part of the immune system that develops following exposure to a foreign substance (antigen). Antigen is a class of foreign molecules. It comes from the word "antibody generators". They stimulate the body to produce antibodies. Antibodies neutralize or fight off the antigens.

• Pathogen and antigen specific responses
• There is a lag phase before maximum response is seen
• Has both cellular and humoral components
• Immunological memory is present

Note:
1. Humoral immunity: This involves antibody response to the antigen. B-lymphocytes are involved here.
2. Cellular immunity: This involves the responses mounted by the T-lymphocytes.

Immunological Memory
This can be classified into:
1. Passive memory: This is a short term memory lasting from few days to months. The antibodies are injected or passed on into the individual. For example, during pregnancy, the mother's antibodies enter the fetus through the placenta to offer protection against infections. The breast milk also contains antibodies. So basically, passive immunity involve the transfer of antibody-rich serum or secretions from one individual to another.
2. Active memory: This is a long term memory acquired during infection by the activation of B-lymphocytes and T-lymphocytes. This is the principle behind vaccination. An antigen is injected into the organism to stimulate it to produce specific antibodies against it. The B-lymphocyte are changed to plasma cells which then produce the antibodies. The T-lymphocytes (called T-Helper cells) produce substances that stimulate the B-lymphocytes to produce the antibodies.

There are five classes of antibodies (immunoglobulins): Ig A, Ig G, Ig M, Ig E, Ig D

You can remember this by using the pneumonic "GAMED"