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Macrophages are antigen-presenting cells that serve as the first line of defense against pathogens. They engulf and digest harmful bacteria through phagocytosis, then fragment the antigens and display them on their surface using MHC molecules. This presentation activates T cells to mount a specific immune response against the infection.

Helper T cells recognize antigen-MHC complexes on antigen-presenting cells and secrete chemicals that stimulate the growth and differentiation of cytotoxic T cells. Cytotoxic T cells then directly kill infected or damaged cells. This coordinated response ensures pathogens are eliminated while minimizing collateral damage to healthy tissue.

Memory T cells remain in the body after infection is cleared and provide long-lasting immunity against specific pathogens. When the same antigen is reencountered, memory T cells mount a faster and stronger immune response than the initial encounter. This is why vaccination works—it primes memory T cells before real pathogen exposure.

CD4+ T cells, or helper T cells, recognize antigens presented by MHC class II molecules on antigen-presenting cells and coordinate immune responses. CD8+ T cells recognize antigens bound to MHC class I molecules on infected or cancer cells and differentiate into cytotoxic T lymphocytes that destroy those cells. Each cell type targets different sources of infection.

Graft rejection occurs when the recipient's immune system recognizes transplanted tissue as foreign. Antigen-presenting cells either directly present graft antigens to T cells or the host's APCs recognize graft-derived antigens and activate helper and cytotoxic T cells against the tissue. This adaptive immune response damages the graft unless the host and donor are compatible or immunosuppressive medication is used.

Once the infection is under control, suppressor T cells inhibit the immune system to prevent further damage to host tissue. This regulatory mechanism stops the adaptive immune response while memory T cells remain to provide future protection. This balance prevents autoimmune damage while maintaining immunological memory.

Antigens are unique proteins on pathogen surfaces that are recognized by the immune system. After macrophages engulf pathogens, they fragment antigens and display them on their surface bound to MHC molecules. This antigen-MHC complex is detected by T cells, which then rapidly multiply and differentiate into specialized cells to combat the specific infection.