mysinia

Based on my experience, I'm going to disagree some of these responses.
 
Last year when I applied to PhD programs, I had 2 years of research experience studying bacterial infection and mammalian innate immune responses, including a few solid publications in the field. I looked at a number of environmental microbiology programs which had labs that studied ecological systems of bacterial infection and immune responses in invertebrates, and since I'd done summer research internships in environmental science and ecology I figured this was a perfect integration of my 2 interests. I spoke with prospective advisors at each of my schools to see if they thought this was too much of a stretch, and they all gave me positive feedback about my research background serving as a good, unique viewpoint that they typically don't get from applicants who are usually trained as ecologists. 
 
I was rejected from every single environmental micro program I applied to, and it's not like my GPA, GRE, recommendations, or personal statements were prohibitively terrible. I figured that the jump from biomedical micro & immuno to environmental micro & immuno wouldn't be too drastic, but admissions committees at these programs did. After I received all of my rejections, I was informed that I'd been awarded an NSF-GRFP (apparently my profile was good enough for them), so I contacted schools and many of them were happy to re-evaluate my application. When I went to meet with department members at places that agreed to admit me given my fellowship, they all said the exact same thing: I had a strong applicant profile overall, but because I didn't have research experience in their specific field of study, they weren't fully convinced that I fit into their programs properly.
 
One big caveat to this is that I was trying to switch from a biomedical field to environmental programs, which are smaller, have less funding, etc. so they are typically pickier about finding applicants who will be directly admitted to specific labs. That's not exactly your case with switching from plant bio to immuno/pharmacology Biomedical programs allow for rotations, so there is some more flexibility there, which could help you out.
 
bak3rme - what have you done that makes you want to pursue pharmacology or immuno programs? Classes? Is there any way to relate your previous experiences to where you'd like to go? I think that if you have absolutely nothing relating to either of those disciplines in your background, you could have a difficult time making a case for why those programs are a proper fit for you, especially since your competing applicants will most likely have at least some exposure to those fields.
 
Anyways, that's just my two cents based on my story. Everyone's got their personal opinions based on what they've seen and experienced.

Based on my experience, I'm going to disagree some of these responses.
 
Last year when I applied to PhD programs, I had 2 years of research experience studying bacterial infection and mammalian innate immune responses, including a few solid publications in the field. I looked at a number of environmental microbiology programs which had labs that studied ecological systems of bacterial infection and immune responses in invertebrates, and since I'd done summer research internships in environmental science and ecology I figured this was a perfect integration of my 2 interests. I spoke with prospective advisors at each of my schools to see if they thought this was too much of a stretch, and they all gave me positive feedback about my research background serving as a good, unique viewpoint that they typically don't get from applicants who are usually trained as ecologists. 
 
I was rejected from every single environmental micro program I applied to, and it's not like my GPA, GRE, recommendations, or personal statements were prohibitively terrible. I figured that the jump from biomedical micro & immuno to environmental micro & immuno wouldn't be too drastic, but admissions committees at these programs did. After I received all of my rejections, I was informed that I'd been awarded an NSF-GRFP (apparently my profile was good enough for them), so I contacted schools and many of them were happy to re-evaluate my application. When I went to meet with department members at places that agreed to admit me given my fellowship, they all said the exact same thing: I had a strong applicant profile overall, but because I didn't have research experience in their specific field of study, they weren't fully convinced that I fit into their programs properly.
 
One big caveat to this is that I was trying to switch from a biomedical field to environmental programs, which are smaller, have less funding, etc. so they are typically pickier about finding applicants who will be directly admitted to specific labs. That's not exactly your case with switching from plant bio to immuno/pharmacology Biomedical programs allow for rotations, so there is some more flexibility there, which could help you out.
 
bak3rme - what have you done that makes you want to pursue pharmacology or immuno programs? Classes? Is there any way to relate your previous experiences to where you'd like to go? I think that if you have absolutely nothing relating to either of those disciplines in your background, you could have a difficult time making a case for why those programs are a proper fit for you, especially since your competing applicants will most likely have at least some exposure to those fields.
 
Anyways, that's just my two cents based on my story. Everyone's got their personal opinions based on what they've seen and experienced.