There are three main cellular zones within each lymph node:n The outer cortex, a dense cellular area, and within the cortex is the paracortex.
n The innermost layer is the medulla, a central area consisting mostly of a sinus containing
the larger vessels.
n Trabeculae — extension of the outer capsule — extend into the node.
The cortex contains germinal centres, where the activated B lymphocytes (B cells)
multiply, and follicles (primary lymphoid follicles are composed mainly of B cells).
B cells transform into either B memory cells or plasma cells which migrate toward the
medullary region. The paracortex is mainly a T cell region. Antigen transported to the
spleen by the antigen-presenting cells remains within the paracortex and cortex.
n 5.6 THE MAIN CELL TYPES INVOLVED WITH
THE IMMUNE RESPONSE
Leukocytes are the nucleated cells found in the body fluids, including blood, are classed
as either granulocytes or agranulocytes depending upon whether or not they have gran-
ules in their cytoplasm that become visible under the light microscope after staining.
n The granulocytes, neutrophils, eosinophils and basophils all arise from the same type
of stem cells.
n The agranulocytes are monocytes and lymphocytes and are mainly derived from
myeloid stem cells (see Fig. 5.8).n 5.6.1 THE GRANULOCYTES
5.6.1.1 Neutrophils
Neutrophils (size about 14 μm), also known as polymorphonuclear leukocytes, have
multilobed nuclei with up to five lobes, linked by a chromatin strand. When stained
with Romanovsky stains the cytoplasmic granules bind to both acidic and basic dyes and
cancel out the staining effect. They then do not show up — hence the name neutrophils.
There are two types of granules — primary and secondary — depending upon the
time they appear during cell development; and they contain lyosozyme, hydrolases,
myeloperoxidases and collegenases.
Neutrophils make up approximately 60–80% of all circulating leukocytes and can
penetrate into tissues in response to an infection (the early inflammatory stages). The
primary function of neutrophils is phagocytic, that is they digest and destroy pathogenic
material, dead host cells and immune complexes by both phagocytosis and pinocytosis
and are stimulated by secretions of pathogens, and one of the components of comple-
ment. They have Fc receptors binding sites located on their cell membranes which
are activated during infections. Neutrophils can release reactive oxygen intermediates
(O 2 −radicals).
5.6.1.2 Eosinophils
Eosinophils (size about 16 μm) have a bilobed nucleus and acidophilic granules with a high
affinity for the dye eosin (stains pink). Within the cytoplasm are major basic proteins located
in ribosomes, mitochondria and granules that contain basic protein substances that,
when released, are toxic to pathogens. They constitute 3–5% of the total number of cir-
culating white blood cells. Their numbers tend to increase during allergic reactions, par-
asitic infections and asthma attacks. The majority of eosinophils are tissue-dwelling cells.
INTRODUCTION TO HOST RESPONSE