infection and response

1. Introduction to Infection

  • An infection occurs when pathogenic microorganisms invade a host, multiply, and cause damage to the host's tissues.

  • Pathogen: An agent that causes disease.

2. Types of Pathogens

2.1 Bacteria

  • Single-celled prokaryotic organisms.

  • Can cause disease by producing toxins or directly damaging cells.

  • Examples: Streptococcus pneumoniae (pneumonia), Escherichia coli (food poisoning).

2.2 Viruses

  • Non-living infectious agents that require a host cell to replicate.

  • Consist of genetic material (DNA or RNA) enclosed in a protein coat.

  • Examples: Influenza virus (flu), Human Immunodeficiency Virus (HIV).

2.3 Fungi

  • Eukaryotic organisms that can be single-celled (yeasts) or multicellular (molds).

  • Infections often referred to as mycoses.

  • Examples: Candida albicans (thrush), Tinea pedis (athlete's foot).

2.4 Parasites

  • Organisms that live on or in a host and derive nutrients from it.

  • Can be protozoa (single-celled) or helminths (multicellular worms).

  • Examples: Plasmodium falciparum (malaria), tapeworms.

3. Modes of Transmission

  • Direct Contact: Physical contact between an infected person and a susceptible person (e.g., touching, sexual contact).

  • Indirect Contact: Contact with a contaminated object (fomite) (e.g., doorknobs, shared needles).

  • Droplet Transmission: Respiratory droplets carrying pathogens travel a short distance (e.g., coughing, sneezing).

  • Airborne Transmission: Pathogens suspended in the air for longer periods and distances (e.g., tuberculosis).

  • Vector-borne Transmission: Transmitted by an intermediate organism, such as an insect (e.g., mosquitoes for malaria).

  • Vehicle Transmission: Transmitted through contaminated inanimate objects like food, water, or blood.

4. The Immune Response

4.1 Innate Immunity (Non-specific)

  • The body's first line of defense; rapid and non-specific.

  • Physical Barriers: Skin, mucous membranes, cilia.

  • Chemical Barriers: Stomach acid, tears, saliva.

  • Cellular Defenses:

    • Phagocytes: Engulf and destroy pathogens.

      • Macrophages

      • Neutrophils

    • Natural Killer (NK) Cells: Detect and destroy infected or cancerous cells.

    • Inflammatory Response: Localized response to injury or infection.

      • Signs: Redness, heat, swelling, pain.

      • Process: Increased blood flow, capillary permeability, migration of phagocytes.

4.2 Adaptive Immunity (Specific)

  • Learned response; specific to particular pathogens, with memory.

  • Key Cells:

    • Lymphocytes:

      • B Lymphocytes (B cells): Produce antibodies and mature in bone marrow.

      • T Lymphocytes (T cells): Play a central role in cell-mediated immunity and mature in the thymus.

  • Humoral Immunity (Antibody-Mediated):

    • Mediated by B cells and antibodies.

    • B cells recognize antigens, differentiate into plasma cells, and produce antibodies.

    • Antibodies: Proteins that bind specifically to antigens, neutralizing them or marking them for destruction by other immune cells. There are 5 main classes: IgG, IgA, IgM, IgE, IgD.

  • Cellular Immunity (Cell-Mediated):

    • Mediated by T cells.

    • Cytotoxic T cells (CD8+): Directly kill infected cells or cancer cells.

    • Helper T cells (CD4+): Coordinate immune responses by activating other immune cells (like B cells and cytotoxic T cells).

    • Memory T cells: Provide