immunity

Overview of Immunity

• Immunity is a protection state against pathogens or foreign substances.

• Key concepts involve innate and adaptive immunity, with inflammation being crucial for understanding adaptive immune responses.

Antigens and Immune Response

• An antigen is any foreign substance that triggers an immune response, e.g., pathogens, vaccines.

• The first line of defense includes physical barriers like skin and mucous membranes that prevent pathogen entry using chemicals (e.g., pH levels) and mucus production.

Innate Immunity

• Innate immunity is present at birth and involves immediate, nonspecific responses to pathogens.

• Beyond physical barriers, there are innate cellular responses that activate adaptive immunity including T and B lymphocytes.

Second Line of Defense

• If pathogens breaching the first line of defense enter tissues, the second line of defense involves cellular responses and inflammation.

• Inflammation includes increased blood flow and vascular permeability which helps to eliminate pathogens by diluting toxins.

Mechanism of Inflammation

• Inflammation arises after the entry of an infectious agent, characterized by redness, heat, swelling, and pain.

• Pain results from chemical release that stimulates nerve endings in infected tissues.

• Important cells involved in inflammation include stem cells, which differentiate into various immune cell types.

Stem Cells in Immunity

• All blood elements derive from pluripotent hematopoietic stem cells (HSCs).

• HSCs differentiate into two main lineages: myeloid and lymphoid stem cells.

Myeloid and Lymphoid Cells

• Myeloid stem cells give rise to various immune cells, including:

  • Macrophages: Can be resident (stationary) or wandering (mobile).

  • Neutrophils: Comprise around 50% of white blood cells; they phagocytose pathogens and produce chemicals for pathogen breakdown.

• Neutrophils contribute to pus formation during infections due to cellular debris.

Cytokines and Inflammation

• Cytokines and specifically chemokines play crucial roles in signaling and directing immune responses.

• Chemokines specifically recruit immune cells to sites of infection, balancing the immune response to prevent excessive inflammation.

Pathogen Recognition

• The immune system uses Pattern Recognition Receptors (PRRs) to identify Pathogen-Associated Molecular Patterns (PAMPs) and initiate immune responses.

• Major Histocompatibility Complexes (MHC) present self and foreign antigens to T cells, facilitating adaptive immune responses.

• It is essential to recognize that some pathogens can evade immune responses by downregulating MHC expression.