Antibodies are glycoprotein molecules synthesized by plasma cells (mature, activated B cells) with the capacity of binding to any potential antigen epitope. For a review of lymphocytes and how they are activated, see this link where you will find more information about antibody production in response to ’challenge’.
An (IgG) antibody with basic structural features labeled
Antibodies are the natural products of these plasma cells and function in a variety of ways to effect immunity. Most basically, they bind and may interrupt the function of the target molecules or trigger a response disadvantageous to the pathogen. In addition, a number of other functions are mediated by these molecules, including recruitment of complement and of phagocytic cells that will digest and inactivate the cell / antigen.
Therapies, such as vaccines, are designed to separate and eliminate the disease-causing elements of a pathogen from those that generate an immune response, thereby initiating a normal immune response to antigens without the dangerous exposure to live pathogens. Most often, these are prophylactic vaccines that initiate the development of immune ’memory’ prior to any disease exposure.
In some cases, therapeutic vaccines do much the same job, but are used to ’jump-start’ an immune response that has failed to initiate naturally for some reason (this may be because the target of the therapy is very similar to ’self’ as is the case with cancer), or because a long-term, chronic disease has fooled the body into tolerating an unwanted condition.
Additionally, some molecular therapies provide passive immunity by administering exogenous antibody that fulfills these functions. A weakness of these therapies is that, by providing pre-made antibody, potential antigens are blocked and no endogenous antibody response will be elicited.
A final use of antibodies, to be elaborated further here, is to provide targeted delivery of toxins to pathogens or infected cells or to deliver antigens to the immune system.
Purpose: to trigger / amplify immunity to an ongoing infection or disease
Considerations:
1. Target protein or cell – what cell and what protein on that cell should be targeted to elicit the desired immune response?
2. How to get antibody to the site where target cells are present?
3. What is the desired response / activity of the target cell?
4. What, if any, molecule is being delivered to these cells?
5. Lastly, how can efficacy be measured and what are the objective endpoints that will be used to determine whether therapy is effective?
Although this antibody is not currently in use therapeutically, I will use, as an example, one that I made while working for a biotech company some years ago.
An antibody with an antigen conjugated to the Fc portion
The antibody we used specifically bound to the macrophage mannose receptor (MMR) expressed by macrophages and the similar phagocyte cells, dendritic cells. Natively, this protein binds to a sugar, mannose, that is commonly charged to protein molecules. Once bound, the MMR will direct receptor-mediated endocytosis of the bound protein and deliver it to endolysosomes for processing and presentation upon MHC class II molecules (see animation below). As explained in the link, processing and presentation lead to the activation of T Cells and the resulting immune response.
Using an antibody that targets this molecule (MMR), a target compound can be fused to the antibody (chemically or genetically) leading to the precise delivery of this compound into the cell and the generation of a response. The antibody will guide the (tumor) antigen to the phagocytic cell. In this way, the antibody serves only as a vehicle. This vehicle takes its passenger, the antigen that we would like to generate an immune response against, and inserts this antigen into the processing and presenting apparatus of these ‘professional’ antigen presenting cells.
Animation of Antibody delivering a Target Antigen to an APC:
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