● `color{Violet}"Memory-based"` `color{Violet}"acquired immunity"` evolved in higher vertebrates based on the ability to `color{Violet}"differentiate"` foreign organisms (e.g., pathogens) from self cells.

● While we still do not understand the basis of this, `color{Violet}"two corollaries"` of this ability have to be understood.

● One, higher vertebrates can distinguish `color{Violet}"foreign molecules"` as well as `color{Violet}"foreign organisms"`.

● Most of the `color{Violet}"experimental immunology"` deals with this aspect.

● Two, sometimes, due to `color{Violet}"genetic"` and other unknown reasons, the body `color{Violet}"attacks self-cells"`.

● This results in damage to the body and is called `color{Brown}"auto-immune disease"`.

● `color{Violet}"Rheumatoid arthritis"` which affects many people in our society is an `color{Violet}"auto-immune disease"`.

● The human immune system consists of `color{Violet}"lymphoid organs"`, `color{Violet}"tissues"`, `color{Violet}"cells"` and soluble molecules like `color{Violet}"antibodies"`.

● `color{Violet}"Immune system"` is unique in the sense that it `color{Violet}"recognises foreign antigens"`, responds to these and remembers them.

● The immune system also plays an important role in `color{Violet}"allergic reactions"`, `color{Violet}"auto-immune"` diseases and `color{Violet}"organ transplantation"`.

`color{Brown}"Lymphoid organs"`:

● These are the organs where `color{Violet}"origin"` and/or `color{Violet}"maturation and proliferation"` of lymphocytes occur.

● The `color{Brown}"primary lymphoid organs"` are `color{Violet}"bone marrow"` and `color{Violet}"thymus"` where `color{Violet}"immature"` lymphocytes differentiate into `color{Violet}"antigen-sensitive"` lymphocytes.

● After `color{Violet}"maturation"` the lymphocytes migrate to `color{Brown}"secondary lymphoid organs"` like `color{Violet}"spleen"`, `color{Violet}"lymph nodes"`, `color{Violet}"tonsils"`, `color{Violet}"Peyer’s patches"` of small intestine and appendix.

● The secondary lymphoid organs provide the `color{Violet}"sites for interaction"` of lymphocytes with the antigen, which then proliferate to become `color{Violet}"effector cells"`.

● The `color{Violet}"bone marrow"` is the main `color{Violet}"lymphoid organ"` where all `color{Violet}"blood cells"` including lymphocytes are produced.

● The `color{Violet}"thymus"` is a `color{Violet}"lobed organ"` located near the heart and beneath the breastbone.

● The thymus is quite large at the time of birth but keeps `color{Violet}"reducing in size"` with age and by the time puberty is attained it reduces to a very small size.

● Both bone-marrow and thymus provide `color{Violet}"micro-environments"` for the development and maturation of `color{Violet}"T-lymphocytes"`.

`color{Brown}"Spleen"`:

● The spleen is a large `color{Violet}"bean shaped organ"`.

● It mainly contains `color{Violet}"lymphocytes and phagocytes"`.

● It acts as a `color{Violet}"filter"` of the blood by `color{Violet}"trapping"` blood-borne `color{Violet}"microorganisms"`.

● Spleen also has a large `color{Violet}"reservoir of erythrocytes"`.

`color{Brown}"Lymph Nodes"`:

● The lymph nodes are `color{Violet}"small solid structures"` located at different points along the `color{Violet}"lymphatic system"`.

● Lymph nodes serve to `color{Violet}"trap the micro-organisms"` or other antigens, which happen to get into the lymph and tissue fluid.

● Antigens trapped in the lymph nodes are responsible for the `color{Violet}"activation of lymphocytes"` present there and cause the immune response.

`color{Brown}"MALT"`:

● There is `color{Violet}"lymphoid tissue"` also located within the `color{Violet}"lining"` of the `color{Violet}"major tracts"` (respiratory, digestive and urogenital tracts) called `color{Brown}"mucosal associated lymphoid tissue"` (`color{Violet}"MALT"`).

● It constitutes about `color{Violet}"50 per cent"` of the lymphoid tissue in human body.