9 Host Defenses and Microbial Disease

Two cooperating layers of defense

The immune system can prevent some infections, control microbial growth, or reduce disease severity, though it does not always eliminate every pathogen. Its defenses work in layers: broad, rapid responses act first, and targeted responses can develop alongside them.

Barriers and rapid innate defenses

Skin, mucus, cilia, and antimicrobial substances at body surfaces make it harder for microbes to enter or establish themselves. If microbes breach these barriers, innate immunity responds rapidly to features shared by many microbes.

Neutrophils and macrophages recognize common microbial features using pattern-recognition receptors. They can capture and destroy microbes through phagocytosis. Inflammatory signals recruit additional defenses to affected tissue. The complement system can coat microbes so they are easier to engulf, promote inflammation, and damage the membranes of some microbes. Virus-infected cells can produce interferons that help limit viral replication, while natural killer cells can kill some infected cells.

Innate defenses also help activate the targeted response: dendritic cells take up microbial material and present pieces of it to T cells.

Takeaway: Barriers and innate immune responses act quickly and broadly, and help initiate adaptive defenses.

Targeted responses and immune memory

Adaptive immunity relies mainly on B and T lymphocytes. Each lymphocyte has receptors that recognize particular antigens—molecules or molecular parts recognized by the immune system. When a lymphocyte is activated, it multiplies, creating a population suited to that target.

• B cells can develop into plasma cells that secrete antibodies. Antibodies can neutralize viruses or toxins, block microbes from attaching to host cells, and mark pathogens for destruction by phagocytes or complement.

• Helper T cells coordinate responses by signaling to other immune cells.

• Cytotoxic T cells recognize and kill host cells displaying certain microbial antigens, helping control infections inside cells.

• Some activated B and T cells become memory cells. On later exposure to the same or a sufficiently similar antigen, they can support a faster, stronger response.

These defenses cooperate: innate recognition helps activate adaptive responses, while antibodies and T cells can direct or strengthen other immune mechanisms.

Takeaway: Adaptive defenses target particular antigens, and memory can improve responses to later exposure.

How pathogens evade defenses

Pathogens can persist by avoiding recognition, resisting attack, or changing the conditions of infection. Common strategies include:

• Capsules or altered surfaces that make microbes harder for phagocytes or antibodies to recognize and capture.

• Antigenic variation, in which exposed antigens change, reducing the effectiveness of antibodies made against earlier versions.

• Intracellular survival, which can shelter a microbe from some antibodies and other defenses that act outside cells.

• Interference with host responses, such as disrupting immune signaling or resisting complement and other killing mechanisms.

These strategies do not make a pathogen invulnerable. They can delay or weaken particular defenses and contribute to persistent or recurrent infection.

Vaccines and types of immunity

A vaccine exposes the immune system to a harmless form of a pathogen, part of it, or instructions for making a selected antigen. This prompts an adaptive response without requiring the person to experience the targeted disease. The response can produce antibodies and memory B and T cells, preparing the immune system to respond more effectively if it later encounters the pathogen. Some vaccines require multiple doses to build or maintain protection; the degree and duration of protection vary by vaccine and pathogen.

Vaccination produces active immunity because the vaccinated person's immune system makes its own response. Passive immunity comes from receiving ready-made antibodies. It can provide protection more quickly, but usually does not create the same lasting immune memory.

Takeaway: Vaccination prepares adaptive defenses in advance, while passive protection supplies antibodies directly.

Antimicrobial treatments and resistance

Antimicrobial drugs act on microbes or on stages of their life cycles. The appropriate treatment depends on the suspected or confirmed cause, the infection, and the patient.

• Antibiotics treat certain bacterial infections. Depending on the drug, they can interfere with bacterial cell-wall construction, protein production, nucleic-acid synthesis, or essential metabolic pathways.

• Antivirals interfere with particular stages of viral replication. Some influenza antivirals, for example, inhibit viral proteins needed for replication or release. Antivirals are specific to particular viruses and are not interchangeable with antibiotics.

• Antifungal and antiparasitic drugs target features or processes of fungi and parasites, respectively.

Antibiotics do not treat viral infections such as colds or influenza. Unnecessary antimicrobial use exposes microbes to selection pressure: susceptible microbes may be inhibited or killed, while resistant ones may survive, multiply, and spread. Antimicrobial resistance is a property of microbes, not a person's body. Appropriate prescribing and use help protect the effectiveness of available treatments.

Takeaway: Drug choice depends on the cause of infection, and appropriate use helps limit the spread of resistance.