A great video about intro to immunology

Immunology is the study of how the body defends itself. This video explains it very clearly! It actually ends two minutes and 30 seconds in even though the length is at 4 mins long.

So basically what happens is there is an infection from an antigen which is a foreign cell. Macrophages recognizes the pathogen and ingest it. When it does that it becomes an anti-presenting cells or APC. After this happen a white blood cell called T-helper cell shows up and reacts with the Macrophage. The Macrophage releases a chemical warning alerting T-helper of the situation. This is called interleuken 1. The T-helper cell responding by releasing interleuken 2 which causes the proliferation of cytotoxic cells or B Cells. Pathogens that enter the cell may leave some of themselves on the cell surface. This leads cytotoxic cells to bind to that cell's position and killing off the infection.
The B Cells prefer an indirect route. They release antibodies to the blood stream to fend off the infection.
The other B Cell is memory B cell but that doesn't participate initially in fighting off the infection. It just serves to remember this infection so the body can fight it off more easily next time.