Immunology is concept based, and many other subjects are about the same. You need to have a good grasp of the basics of immuno and take it further by being repetitive and avoid memorizing..but hold the concepts:
Start with the concepts of Immunity (innate, adaptive, acquired and so on), then move on to the role of interlukins and complement cascade and what active them and their roles. If you can understand this alone, you will be able to answer many questions on the exam.
One thing, stick with one book, such as Immuno make incredibly simple or kaplan note. The books mostly explain the same thing, and you need to find one that does not give too many confusing details. The board will not try to trick you with long and hard questions on immuno. They may give you a long question, but the answer should be quite simple.
Take your time with immuno nemonics and listen to at least pass program Immuno for a quick understanding on what you need for the exam. You must not create confusion within your very self. I am sure you already know more than you thought. You just need to do questions and integrate the concepts. You will be amazed to see how much you know!
Trust yourself and stop being too negative on yourself also. Study is a process that take time and patience, and for the board, as long as you are the smartest person on earth, it is even harder. So, you should be patient and do not take no for an answer when it come to what you can do.
If you understand pathology, immuno should also be part of what you already learned. hang in there and start with the basic.
God bless!