Adaptive Immunity

Secondary lymphoid tissues are also where lymphocytes are activated. These include: lymph nodes, tonsils, spleen, Peyer's patches and mucosa associated lymphoid tissue (MALT).

Primary lymphoid organs are the thymus and bone marrow. Both T-cell and B-cells are 'born' in the bone marrow.

However, whereas B cells also mature in the bone marrow, T-cells have to migrate to the thymus, which is where they mature

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Lymphocyte recirculation:

·         Mature naive lymphocytes leave the primary lymphoid organs to travel to the sites of activation: the secondary lymphoid tissues. They do this via "homing"

·         They either get activated after recognising an antigen and differentiate to effector cells or they leave via the efferent lymphatic vessels

·         The lymph is collected by lymph nodes; the lymphocytes pass many of them, and in every node they may get activated

·         Eventually all the lymph flows into the thoracic duct and gets back to the circulation, so the lymphocytes can reach another secondary lymphoid organ/tissue

·         They do their recirculation until they find an antigen or until the end of their lifespan

(Antigens from sites of infection reach lymph nodes via lymphatics)

THE IMMUNE RESPONSE

A progenitor lymphoid cell can give rise to either naive T cells or naive B cells

T-cell receptors can only bind to antigens when they are presented to them via antigen-presenting cells, e.g. phagocytes

T cells:

·         (If a T cell recognises a self-antigen this naive T cell will be destroyed before it matures

Otherwise:)

·         When naive T cells mature (by proliferating and differentiating), they will either become a naive CD4 T-cell or a naive CD8 T-cell

·         Naive CD8 T-cell:

Ø  Infected phagocytes present a foreign antigen to the naive CD8 T-cell via the MHCI receptor. This will activate the naive CD8 T-cell to become a CD8 cytotoxic T-cell

Ø  Cytotoxic T-cells destroy infected cells that display the same foreign antigen via the release of chemicals

·         Naive CD4 T-cell:

Ø  Will become activated when a non-infected phagocyte presents a foreign antigen on an MHCII receptor. It will become activated to a T-helper cell

Ø  T-helper cells enhance the immune response by activating B cells as well as activating NK cells and macrophages (or other phagocytes)

B-cells:

·         Naive B-cells have membrane-bound antibodies,…