LYMPHOCYTES

     * The cells with ability to carry out immense responses, when stimulated, are known as lymphocytes. These are of two types - 'T' lymphocytes and 'B' lymphocytes. they both develop in primary lymphatic organs (red bone marrow and the thymus) from multi-potent cells.

"T' lymphocytes develop from pre-"T' cells that migrate from bone marrow into thymus, where they mature. They are of four types: (a) Cytotoxic 'T' cells or killer cells (b) Helper 'T' cells (c) Memory cells and (d) Suppressor Cells.

'B' lymphocytes develop in the bone marrow. They are also of two types (a) Plasma 'B' cells and (b) Memory cells.

Immunity

Immunity is resistance to foreign proteins and invading pathogens. It can be either: (a) Active immunity or (b) Passive immunity.

(a)Active immunity: In active immunity the lymphocytes are activated by antigen present on the surface of the pathogen. It is produced either naturally or artificially. In natural immunity the activation takes place during the natural course of an infection, whereas, in artificial  immunity, the activation occurs by injection of antigens into the body. It is also known as vaccination.

(b) Passive immunity : In this type of immunity the individual becomes of exposure to preformed antibodies. It is also of two types- natural and artificial. In natural passive immunity the antibodies pass naturally from  mother to baby through placenta or breast milk, and in artificial passive immunity the per-formed antibodies extracted from one-individual are injected into another as 'servum', eg., snake bite.