1. Physical and Chemical Barriers :

   • skin , mucous membranes , lysozymes

2. Phagocytes :

   • engulf and destroy pathogens

   • Neutrophils = first responders , engulf and kill bacteria via phagocytosis and release ROS

   • Macrophages = engulf pathogens and present antigens to adaptive immune cells

   • Dendritic Cells = bridge innate and adaptive. they process and present antigen to T cells

3. Natural Killer ( NK ) Cells :

   • detect virus infected and cancerous cells

   • induce apoptosis via perforin and granzymes

4. Mast Cells :

   • release histamine and cytokines to increase vascular permeability

     • facilitates faster recruitment of other immune cells

5. Eosinophils :

   • attack helminths by releasing toxic granules

6. Basophils :

   • release histamine

• The complement system is a collection of circulating and membrane-associated proteins that are important in defense against microbes.

• The complement cascade may be initiated by any of three pathways

• Classical Pathway :

  • most often triggered by antibodies that bind to microbes or other antigens and is thus a component of the humoral arm of adaptive immunity.

  • Mechanism :

    • Triggered when C1 complex binds to antigen-antibody complexes ( IgG or IgM ) on microbial surfaces.

    • Leads to sequential activation of C2 and C4 , forming C3 convertase ( C4b2a )

    • C3 convertase cleaves C3 into C3a and C3b , initiating further immune responses.

• Alternative Pathway :

  • is triggered when some complement proteins are activated on microbial surfaces and cannot be controlled, because complement regulatory proteins are not present on microbes (but are present on host cells).

  • The alternative pathway is a component of innate immunity

  • Mechanism :

    • Activated by mannose-binding lectin (MBL) or ficolins, which bind to carbohydrate structures on microbes.

    • Activates MASPs (MBL-associated serine proteases), which cleave C2 and C4, forming C3 convertase (C4b2a), similar to the classical pathway.

• Lectin Pathway :

  • activated when carbohydratebinding plasma prteins, mannose-binding lectin (MBL) or ficolins, bind to their carbohydrate ligands on microbes.

  • These lectins activate proteins of the classical pathway

  • because it is initiated by a microbial product in the absence of antibody, it is a component of innate immunity.

  • Mechanism :

    • Continuously active at low levels due to spontaneous hydrolysis of C3.

    • When C3b binds to microbial surfaces, it stabilizes and forms C3 convertase ( C3bBb ).

    • This pathway amplifies complement activation even without antibodies.

✅ Opsonizes microbes ( C3b ) → Enhances phagocytosis ✅ Forms MAC ( C5b-C9 ) → Directly lyses pathogens ✅ Promotes inflammation ( C3a , C5a ) → Recruits immune cells ✅ Clears immune complexes → Prevents excessive immune responses

• B = bone marrow

• T = thymus

• B Cells :

  • Develop and mature entirely within the bone marrow. This is where they undergo VDJ recombination to generate a diverse antigen receptor repertoire. Once mature, they migrate to secondary lymphoid organs (e.g., spleen, lymph nodes) where they can be activated by antigens.

• T Cells :

  • Originate from the bone marrow but migrate to the thymus for maturation. In the thymus, they undergo positive and negative selection, ensuring that they recognize self-MHC molecules while eliminating strongly self-reactive clones.