PARASITOLOGY

INTRODUCTION TO HOST RESPONSE

The progenitor cells differentiate into lymphoblasts (and these develop into B and T
lymphocytes), megakaryocytes (which give rise to platelets/megakaryocytes), monoblasts
(which develop into monocytes), and myeloblasts (which give rise to granulocytes).

5.5.1.1 The thymus
The thymus is a primary lymphoid organ where the virgin lymphocytes are processed into
mature T lymphocytes (T cells) (antigen independent maturation). It is a bi-lobed organ
located between the heart and the sternum, composed of a reticular network of fibrous
connective tissue cells surrounded by a capsular membrane. Each lobe is divided into
lobules and each lobule has an outer cortex and inner medulla comprised of a reticulum
filled with lymphocytes and some epithelial cells.
The virgin lymphocytes congregate in the medulla. Epithelial cells within the thymus
produce polypetide hormones that may help the lymphocytes to differentiate (antigen inde-
pendent differentiation). The lymphocytes accumulate in the cortex and once activated
the cells divide by mitosis producing more T lymphocytes (T cells). Only about 5% of
T cells exit from the thymus as viable cells to become antigen-reactive cells.
Maturation of T lymphocytes (T cells) begins in the subcapsular region. Cells from
the bone marrow, under the influence of epithelial ‘nurse cells,’ develop into lym-
phoblasts. Those lymphoblasts in continuous contact with non-lymphoid cells migrate
through the cortex into the medulla and mature into T cells (see Fig. 5.7).

5.5.1.2 The spleen
The spleen is the largest secondary lymphoid organ and is located below the stomach and
pancreas in the upper left-hand-side of the abdominal cavity. It is a flat dark red organ
which can swell to twice its size (splenomegaly) during the course of an infection. The
outermost layer consists of a membrane — the outer capsular membrane which surrounds
a reticulum of fibroblasts.

B cell zones of lymph nodes and spleen

MHC class II

molecule

Pre B cell Virgin B cell

Surface immunoglobulin

B cell precursor

Bone marrow

• Figure 5.6A precursor
  B cell matures within the
  B cell zones in a lymph
  node or spleen first into a
  pre B cell and then into
  a virgin B cell. On the
  surface of the B cell are
  membrane/surface
  immunoglobulin (Ig)
  molecules and MHC class
  II molecules. The lg
  molecules act as antigen
  receptors.