From my experience, the scope of first AID covers all the immunology that you will need to know for the exam. Understanding the actual immunology respoonse pathway may be confusing, but I dont think you actually need to do so for the scope of the test. Focus on memorizing the facts in first aid.
There are some especially high yield things you should be very familiar with such as
-Anatomy (alot of the anatomy stuff will be referenced again in other systems of first AID. HOwever knowing which parts of the lymph node / spleen contain which cell types is testable)
-Cytokines (know their function and which cell produces them. All the ones listed in FA are high yeild. You shouldnt need to know any ones not listed)
-Immunoglobulin types and their differences (for this find a chart online
-Complement system (the pathway is low yield, but you need to know the disorders and the functions of the different complement types)
-Hypersensitivity rxns (the best way to learn this is probably through practice questions)
-Autoantibodies / HLA subtypes list (it's important, but it'll be easier to memorize if you come back to it after you've learned all the associated disorders
first
-Transplant rejection (Practice questions is a good way to go for this)
-Immunodeficiency (All of them are important. I used kaplan videos to helped understand this better)
-Pharm (immunosuppressants are important, but therapeutic antibodies I think are less high yield if youre limited in time. Its better to learn the therapeutic antibodies via practice questions as well)