Lymphocytes

Lymphocytes are found in the blood, lymph and organised lymphoid tissues such as the lymph nodes and spleen, where they function to orchestrate and effect immunity.

• here are three types of lymphocytes: T, B and natural killer (NK) cells
• all lymphocytes originate from stem cells in the bone marrow, and the majority either mature in the bone marrow to form B cells, or migrate to the thymus, to eventually become T cells
• only T and B lymphocytes exhibit memory and specificity and, as such, are responsible for the unique quality of the adaptive immune system
• resting B lymphocytes
  • able to react with free antigen directly when it binds to their cell surface immunoglobins which act as receptors
• T lymphocytes do not react with free antigen and instead make use of APCs to phagocytose the antigen and then to express its component proteins on the cell surface adjacent to special host proteins called major histocompatibility complex (MHC) class II molecules
  • antigen presenting cells which express MHC class II molecules include dendritic cells and macrophages
    • this “afferent” phase must occur in order for the T cell to recognise the antigen
      • “efferent” phase occurs when activated lymphocytes enter the tissue and meet antigen again
        • results in multiplication and secretion of cytokines or immunoglobins in order to destroy the antigen
• NK cells have lymphocyte morphology although do not exhibit memory and specificity