There are a lot of factors in play here.  Consider how the antigen is associated with adjuvant particles - different antigen/adjuvant pairs will associate in different ways and their relative concentrations as well as their geometries may influence the way the antigen presents top the immune system as well as the rate at which the antigen migrates away from the injection site. As as providing a depot effect for antigen, adjuvants stimulate inflammatory and other components of the immune system and thus their phyisicochemical properties can impact the type, extent and duration of the initial immune response. All of these factors in turn can impact the nature, titer and longevity of the primary immune responses (think IgM, initial IgG and mature IgG) as well the long term priming of the immune system (think T cells and B cells). So much for the inherent complexity.  Then there is some measurement complexity - as your questions notes - different studies have measured immune responses which generally means different modes of measurement - dependent of dose, timing, sample prep and detection methods (think ELISA versus functional measurement, or simply the differences between two ELISA ,methods).  I am positive that there is no 'ideal particle size' however in many cases (but not all) the nature, titer and durability of an immune response will likely be influenced by the phyisicochemical properties of the adjuvant including adjuvant particle size. Thus for experimental purposes it is often critical to carefully control the preparation and use of adjuvant (perhaps try to use the same preparation of adjuvant for a series of studies if it is too hard to understand and control lot to lot variability). On the other hand for vaccine development or simply for functional immunobiology it is certainly worthwhile looking at deliberately varying the physicochemical properties of your adjuvant to at least investigate the impact of changing the particle size distribution - after all, we would not be calling it an adjuvant unless it at least doubled the immune response (however measured) and often the response with adjuvant can be 10 times that seen with antigen alone.... thus 90% of the immune response is due to the modulating effect of the adjuvant.... that is something that you want to pay attention to.  For clinical or commerical production of vaccine, as with so many others areas of cGMP work - consistency is the key - the phyiscochemical properties of the adjuvant for ongoing and future production need to fall within the range of properties that were shown to be safe and effective in the relevant preclinical and clinical studies. 

Hi Munir;

Particles with a size range of 10–200 nm enter the initial lymphatic vessels from the interstitial space by directly diffusing through lymphatic endothelial cell junctions. Particles of this size can also enter the lymphatic vessels after being endocytosed by dendritic cells (DCs), which then enter the lumen of the vessel by intravasation. Particles that are larger than 200 nm diffuse from the interstitial space into the lumen less efficiently with increasing size. Particles that are 500 nm–1 μm are too large to pass through endothelial cell junctions and are transported into the lymph vessel following uptake by DCs. 

Different nanoparticles have different actions on the immune cell activation, it depends on their chemical constitution.

I could comment only (some) live vaccines to control viral diseases of poultry (birds). The particle size of 2-5 microns of lentogenic vaccines (water suspension - no specific diluent needed) will enter the entire respiratory system and induce long lasting immunity. Compared to all other modes of vaccination it is much better because specific refracterity is developed in approximately 24 hours. HVT can bi given that way. Positive experience we have also with mild NDV, IB, IBD, and what is of paramount importance - pox virus. The vaccination is performed alredy in hatchery. Try to immagine what is offered by using POX recombinant (+AI) or Pox at day old and later usual spray vaccination with POX+AE (at age of 2-4 months. POX vaccine is safe for all newly hatched poultry.

El peligro de emplear la vía respiratoria radica en la presencia del Mycoplasma, además precipitada, si consideramos la inmunidad materna y renovación tisular permanente del Aparato respiratorio que no provee una larga protección. para enfermedades tales como ND donde es importante la inmunidad humoral mientras en LTI es la inmunidad celular.Confío más en bioseguridad y progresiva maduración del Sistema inmune, para una mejor respuesta. Reservo la vía respiratoria para controlar epizootias.

thanks Mark and Faez for your valueable answer.

And dear Gino, can you translate your answer in English?

Hi Muhammad,

other reason to select you adjuvante to use is what kind of Ag you are intent do and route of admin, also what kind of Ab do you want stimulate, from there, then you can replanted or reformulated your question. In addition, people will talk about his experience, and other thing you most to see or evaluated if the adjuvante that you are intent to use is approved for use in animals or human by regulatory agency first of your country and then international agency and it´s not you most to demonstrate safety and not toxicity of product.

The danger of using the airway is the presence of Mycoplasma and also consider maternal immunity and permanent respiratory tissue renewal that does not provide long-term protection. for diseases such as ND where humoral immunity is important while LTI celular.Confío immunity is more biosafety and progressive maturation of the immune system, for better response. Reserve the airway to control epidemics

While size matters, it is not everything--even for immune antigens and adjuvants. Surface chemistry of the particle greatly influences the immune response. See the review: Powell et al. Clin Exp Vaccine Res. 2015 Jan;4(1):23-45, and several references therein.

For a focused example regarding the common adjuvant Alhydrogel, a simple adjustment of an adjuvanted vaccine formulation to 4mm phosphate can greatly improve its immune efficacy and longevity, as originally shown by Watkinson et al. 2013 (Clin Vaccine Immunol. 2013;20:1659–1668). Besides unawareness of this simple solution, the variable capability of adjuvants has also been confused by over-use of the term "alum" for unrelated aluminum salts having different surface chemistries.