The Immune System The key primary lymphoid organs of the immune system are thymus and bone marrow Secondary lymphatic tissues such as spleen, tonsils, lymph vessels, lymph nodes, adenoids, and skin.

Macrophage Macrophage: A type of white blood that ingests (takes in) foreign material. Macrophages are key players in the immune response to foreign invaders such as infectious microorganisms. Macrophages help destroy bacteria, protozoa, and tumor cells. They also release substances that stimulate other cells of the immune system. And they are involved in antigen presentation. To do this, they carry the antigen on their surface and present it to a T cells.

Neutrophis and macrophages (phagocytes) Engulf (phagocytose) and destroy micro-organisms

Eosinophils Secrete factors which kill protozoa and worms

Natural killer cells Can lyse and kill cancer and virus infected body cells They do not engulf the cells but inject chemicals called perforins that open up channels and disintegrate the nucleus.

Langerhans cells Langerhans cells are dendritic cells in the skin. Skin is an important barrier to infection and it has been generally assumed that the Langerhans cells only serve to warn the immune system of skin pathogens.

Dendritic cell There are two cell types with similar names but different functions. Cells of the dendritic cell (DC) lineage are bone marrow derived. In the skin they are known as Langerhans Cells (LC). These cells efficiently process antigen but cannot present it to T cells. LC have been shown to pick up antigen in skin and carry it via afferent lymphatic vessels to lymph nodes. Dendritic cells in lymph are known as "veiled" cells. In lymph nodes the cells, now known as tissue dendritic cells or interdigitating cells, may efficiently present antigen if they encounter the right T cell. In fact these are the best APC...............far fewer DC are required to initiate an immune response than any other APC.

Mast cells (concentrated under the body surfaces) Release factors which increase blood flow and vascular permeability bringing components of immunity to the site of infection