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Immune system and lymphatic 8

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organs (organa Iymphopoetica)

Immune system
The immune system is a host defence mechanism that
serves to protect the integrity of the organism. In addition
to innate protective mechanisms (e.g. physical barriers,
chemical factors, phagocytes), the immune system incor-
porates specific or adaptive defences that target particular
antigens. Adaptive immune responses are carried out by
the cells of the lymphatic system, via cell-mediated or
humoral pathways.
Lymphocytes (Figure 8.1) play a key role in the adaptive
immune response. During their differentiation, these cells
attain the capacity to distinguish between ‘self’ and ‘non-
self’. With the aid of receptors on their cell membrane,
lymphocytes can ‘recognise’ foreign substances (antigens)
and initiate an immune response. This endogenous system
of defence relies upon complex interactions between cell
populations and the complement system.

Principles of adaptive immunity 8.1 Lymphocytes as seen with the scanning electron
The cells of the adaptive immune system are comprised microscope (x2000).
mainly of two groups (Figure 8.2) that arise from haemo-
poietic stem cells in the bone marrow: between a specific lymphocyte and corresponding antigen
triggers the replication (cloning) of the lymphocyte over a
· lymphocytes period of just a few days. The number of different types of
− T lymphocytes (T cells), B-cell clones that can be produced is estimated at around
− B lymphocytes (B cells), 10 .
8
· antigen-presenting cells, including The antigen receptors of the T cells (TCR) are mem-
− macrophages, brane-associated immunoglobulin-like heterodimers
− non-follicular dendritic cells and consisting of αβ- or γδ-chains and an associated protein
− follicular dendritic cells (FDCs). complex, CD3. Recognition of antigens by the TCR
occurs in the presence of MHC molecules (see below).
An important characteristic of the immune response is the The antigen receptors found on B cells take the form of
ability of the body to recognise the extremely large number membrane-bound immunoglobulins (B-cell receptors).
of antigens to which an organism may be exposed during Another key feature of T and B cells is their ability, once
its lifetime. This function is performed by two groups of activated, to develop into memory cells that enable the
molecules, the T-cell receptors (TCR) and surface immu- body to detect a previously encountered antigen, and to
noglobulins on B cells. mount a more rapid response.
During its differentiation, each lymphocyte develops a
unique specificity (theory of clonal selection) that enables
the cell to recognise one of the myriad possible invading
antigens. Due to this selective differentiation, contact

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