Transport of antigens

The intravenous injection of a dose of antigen may be experimentally convenient but it is rather unnatural. Apart from some arthropod-borne parasitic and viral diseases, secondary septicemia, etc., the presence in the blood vascular system of significant amounts of microbial antigens is fortunately a rare event. Most naturally occurring infections begin with the penetration into the tissue fluid, via the skin or mucosae, of invading microorganisms. In such cases, the exogenous material is swept up into the lymphatics and conveyed to the regional node. This mechanism is mimicked by injecting antigens either intradermally or subcutaneously.

A foreign soluble protein that finds its way into the tissue fluid will be conveyed almost quantitatively to the regional nodes. Unless pre-existing specific antibodies convert it into immune complexes, or unless it is a lectin or toxin that binds with high affinity to mammalian cells, very little of it will be retained in the node. Most of it will pass through the lymph nodes and enter the central lymph, and thus the blood. For this reason, many ordinary soluble proteins tend to be poorly immunogenic unless they are first aggregated and/or mixed with adjuvant. In the presence of specific antibody, the antigen will accumulate in germinal centers of lymphoid tissues, where it will be presented to B lymphocytes in the form of antigen-antibody complexes on follicular dendritic cells.

Particulate material such as bacteria and viruses tend to be filtered out of the lymph stream more efficiently by the phagocytes of the regional node. This is partly because particles are more susceptible to mechanical filtration, and partly because of the presence of 'natural' antibodies that can react with such materials. Of course, there are exceptions, such as certain pathogenic microorganisms which, almost by definition, are able to elude host defenses unless a state of specific immunity has been induced previously.

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