dyes. Basophils make up less than 1% of the normal leukocyte component of circulating
blood.
The granules contain the precursors for histamine, heparin and other pharmacologic-
ally active compounds (they are very similar in function to tissue mast cells). Basophils
can be linked to allergens such as pollen grains by an antibody bridge which stimulates
them to degranulate.
n 5.6.2 THE AGRANULOCYTES
5.6.2.1 Monocytes
Monocytes (size 15–30 μm) have a characteristic ‘kidney’-shaped nucleus with clear
cytoplasm and make up at least 6% of the circulating leukocytes. They are the largest of
the white blood cells and can survive for 1–2 months. Monocytes migrate out of the blood
vessels into the tissue to become tissue histiocytes (usually at a site of infection). They
become either fixed or motile phagocytic cells acting as scavenger cells or antigen-
presenting cells (APCs). Monocytes have receptors on the surface of their cell membranes
for IgG2a, IgG2b(in mice), IgG and IgE (in humans), cytokines and the C3b component
of complement. Activated monocytes produce complement components, interferons, inter-
leukins (mainly IL-1) and prostoglandins.
5.6.2.2 Lymphocytes
Lymphocytes (size 8–12 μm) are the smallest of the white blood cells with a large
nucleus to cytoplasm ratio. The have clear cytoplasm with no granules visible under a
light microscope after staining. Lymphocytes comprise about 20–30% of the circulating
leukocytes and accumulate within the thymus, spleen and lymph nodes. They circulate
within the blood vessels and lymphatic ducts and migrate out into tissues when there is
an infection.
Lymphocytes originate from haemopoietic stem cells located in the bone marrow. Once
the lymphocytes have been differentiated from stem cells in the bone marrow, they
migrate to the thymus, lymph nodes and spleen where they divide by mitosis when stimu-
lated. Lymphocytes develop into several different phenotypes depending upon what
organ and where in the organ they settle. Each phenotype has a specific function in an
immune response.
Those lymphocytes that migrate from the bone marrow directly to the thymus and are
‘processed’ there, form the ‘seed’ T lymphocytes (T cells). Lymphocytes that migrate
directly to the lymph nodes or spleen from the bone marrow develop into the B lym-
phocytes (B cells). Within the T cell phenotype there are further subsets — the T helper
(Th) cells, T cytotoxic (Tc) cells and T suppressor (Ts) cells.
n B cells after stimulation develop into either plasma (antibody-secreting) cells or B
memory cells.
n Both B and T cells have antigen receptors on their surface membranes.
n The B cell antigen receptors are surface (or membrane) immunoglobulins (sIg).
n The T cell the antigen receptor is a molecular complex on the surface membrane
known as a CD3 molecule.Both types of receptor have a ‘variable’ end that binds to the epitope of the antigen.
The structure of the variable end molecule of each individual lymphocyte is unique and
hence there are numerous variations.
INTRODUCTION TO HOST RESPONSE