Final answer:
Infection occurs when pathogens breach the body's defenses, such as skin or mucosal barriers, and initiate a multiplication process within the host's tissues. The immune system responds to these invaders with a combination of cellular and humoral responses in an attempt to eliminate them.
Step-by-step explanation:
Infection occurs when pathogens penetrate host defenses, enter tissues, and multiply. Pathogens are the harmful microorganisms such as viruses, bacteria, and fungi that can overcome the body's protective barriers and immune responses. When pathogens invade the body through various points of entry, such as breaks in the skin or mucosa lining the oral cavity or other orifices, the immune system responds with a series of mechanisms to eliminate these invaders. Macrophages play a crucial role as they can invade host tissues and combat foreign cells. However, pathogens have various strategies to evade the immune defenses, such as antigenic variation or expressing virulence factors that help them bind to host cells, escape immune recognition, or cause direct damage to host tissues.
An infection also triggers an immediate inflammatory response. The immune system employs both the cellular response, involving phagocytes that engulf and destroy pathogens, and the humoral response, where antibodies specific to the invader are produced. Despite these sophisticated immune mechanisms, pathogens may still successfully establish infection if they can bypass these defenses.