PARASITOLOGY

PARASITOLOGY

The lymphatic system is linked up via the blood circulatory system and a series of

lymphatic vessels which circulate lymph fluid. The lymphocyte is the main cell associated

with the lymphatic system and has a dominant role in the development of the adaptive

immune response. Nevertheless all white blood cells (leukocytes), phagocytic cells,

mast cells and keratinocytes have significant roles in the immune system.

n 5.5.1 THE LYMPHOID ORGANS

Bone marrow is a primary lymphoid organ where all the cells of the immune system ori-

ginate (see Fig. 5.5) and where the B lymphocytes (B cells) are processed (see Fig. 5.6).

The cells of the immune system are derived from basic all purpose (pluripotent) stem

cells located within the red bone marrow in the central cavity of bones. In normal

healthy adults the most active red bone marrow is in the ribs and sternum.

The red bone marrow has two zones: the haemopoietic (blood forming — consisting

of sinusoids surrounding a central vein) and vascular zones. The sinusoids contain the pro-

genitor or blast cells such as myeloblasts, erythroblasts and pluripotent stem cells.

(Myeloblasts give rise to granulocytes; erythroblasts give rise to red blood cells and

pluripotent stem cells (PSCs) develop into lymphoid cells and platelets.)

Myeloid precursor stem cells in the bone marrow

Lymphocytes released into circulation

Circulation

Lymphocytes migrate to thymus

Thymus

Processed into Circulation

T cells

T cell zones

contact with antigen

via APC

Transform into

lymphoblasts

Activated T cells cloned

B cell zones

contact with antigen

via APC

Transform into

plasmablasts

Activated B cells cloned

Activity within spleen or a lymph node

Site of infection in tissue

Activated T-cytotoxic cells Plasma cells

Lymphocytes migrate to spleen

or lymph nodes

B cells

• Figure 5.5All the white
  blood cells (leukocytes)
  including the lymphocytes
  originate from bone
  marrow stem cells. Once
  the lymphocytes are
  released into the
  circulation they are
  processed into T or B
  cells. If they proceed
  directly to the thymus they
  become T cells. If the
  lymphocytes migrate first
  to the spleen or lymph
  nodes they become
  B cells.