When a pathogen comes in contact with your body, it has to breach the first line of defense to get inside. Your skin and mucus membranes are the main barrier here. Mucus traps the pathogens, and then is forced out of your body when you cough or blow your nose. Your skin also secretes chemicals that have antiviral properties, killing viruses on contact. If the pathogens get through that defense, the next line is non-specific immunity cells that patrol your tissues engulfing pathogens. There are other cells that do this, like macrophages, but the dendritic cells are most important for activating the third line of defense in your body.
Dendritic cells reside in your tissues, waiting for an invader to arrive. When they do find one, they engulf it and digest it. After they do this, they select pieces of the invader called antigens and put them on their surfaces. The dendritic cells migrate back to lymph nodes, key locations in your body filled with immune cells. There, they show the antigens, called antigen presentation, to two types of lymphocytes, T-cells and B-cells, activating them for a full immune response.
