MP-Host Response to Infection

Host response to infection

Refers to the complex physiological and immunological processes the body employs to detect, combat, and eliminate pathogens.

Innate Immunity

The body’s first line of defense, which is rapid, non-specific, and present at birth.

Adaptive Immunity

A slower, specific response that develops over time and provides long-term protection.

• Physiological changes

• Fever

• Inflammation

The host response also involve _____.

Innate Immune Response

The body’s immediate defense mechanism as it responds within minutes to hours of pathogen exposure and does not require prior exposure to the pathogen.

• Skin

• Mucous Membranes

• Chemical defenses

• Normal Flora

Physical and Chemical Barriers

Skin

Acts as a physical barrier, preventing pathogen entry. Sweat and sebum contain antimicrobial substances like fatty acids.

Mucous membranes

Line the respiratory, gastrointestinal, and genitourinary tracts, trapping pathogens in mucus and expelling them via cilia or coughing.

• Saliva

• Tears

• Gastric acid

Chemical defenses in innate immune response.

Normal flora

Beneficial microorganisms in the gut, skin, and mucous membranes compete with pathogens for resources, limiting their growth.

Neutrophils

The most abundant white blood cells, rapidly recruited to infection sites engulf and destroy pathogens via phagocytosis.

Macrophages

Found in tissues, they engulf pathogens and debris, and release cytokines to signal other immune cells.

Dendritic Cells

Present pathogens to the adaptive immune system, bridging innate and adaptive immunity.

Natural Killer Cells

Attack virus-infected and cancerous cells by releasing cytotoxic granules.

Mast Cells

Release histamine, contributing to inflammation and recruiting other immune cells.

Inflammation

The hallmark of the innate response, triggered by tissue injury or pathogen invasion.

Vasodilation

Increases blood flow to the infection site, causing redness and warmth.

• Redness

• Warmth

• Swelling

• Pain

• Loss of function

Local signs of inflammation.

• Fever

• Fatigue

• Elevated WBC count (leukocytosis)

Systemic signs of inflammation.

Increased vascular permeability

Allows immune cells and plasma proteins to reach the site leading to swelling (edema).

Opsonizing pathogens

Marking them for phagocytosis.

Fever

Is a systemic response mediated by pyrogens (e.g., cytokines like IL-1, IL-6).

Adaptive Immune Response

Is slower (days to weeks) but highly specific, targeting the exact pathogen.

Humoral Immunity

Mediated by B lymphocytes (B cells), which produce antibodies (immunoglobulins).

Cellular Immunity

Mediated by T lymphocytes (T cells) and is effective against intracellular pathogens (e.g., viruses, some bacteria like Mycobacterium tuberculosis).

Immunological Memory

The adaptive immune system “remembers” pathogens, allowing a faster and stronger response upon re-exposure.

Incubation Period

Time between pathogen entry and symptom onset. The pathogen replicated but the host may be asymptomatic.

Prodromal Phase

Early, non-specific symptoms wherein the immune system begins responding, but symptoms are vague.

Acute Phase

Peak of symptoms, with full activation of innate and adaptive immunity. Local and systemic signs are prominent.

Convalescent Phase

Symptoms resolve as the immune system clears the pathogen. Tissue repair and recovery occur, but patients may feel fatigued.

Resolution

Complete elimination of the pathogen and return to baseline health.

Complications

Persistent infections, tissue damage, or systemic issues like sepsis.

Systemic Inflammatory Response Syndrome (SIRS)

A widespread inflammatory response triggered by severe infection or other insults (e.g., trauma).

Sepsis

SIRS caused by infection, leading to organ dysfunction.

quick Sequential Organ Failure Assessment

qSOFA score meaning.

Cytokine Storm

Excessive cytokine release leading to severe inflammation and tissue damage.

• Fever

• Pain

• Fatigue

• Leukocytosis/Leukopenia

• Organ-specific symptoms

Clinical manifestations of infection.

Fever

Indicates immune activation but can lead to dehydration or seizures (in children).

Pain

Often due to inflammation or tissue damage (e.g., sore throat in streptococcal pharyngitis).

Fatigue

Reflects increased metabolic demand and cytokine effects.

Neonates

Immature immune systems increase susceptibility to infections like group B Streptococcus.

Elderly

Reduced immune function and comorbidities increase infection severity (e.g., pneumonia).

Immunocompromised

Patients with HIV, cancer, or transplant recipients are prone to opportunistic infections.

Chronic Illness

Conditions like diabetes impair wound healing and immune responses.