short spiel on immunity and immune response

Cells of the Immune System: derived from the hematopoietic stem cell

1. Lymphoid Lineage

T lymphocytes (T cells, made in the thymus)

B lymphocytes (B cells, made directly from the bone marrow)

Natural Killer cells (NK cells)

2. Myeloid lineage

Monocytes that give rise to macrophages

Langerhans cells and Dendritic cells

Megakaryocytes that give rise to Platelets

Granulocytes (eosinophils, basophils and neutrophils)

Primary lymphoid tissues: bone marrow and thymus

Secondary lymphoid tissues: spleen, lymph nodes

Leukocyte migration: T and B cells leave the thymus and bone marrow respectively as naïve lymphocytes, migrate into the blood and then into the secondary lymphoid tissue. Antigen presenting cells (APCs), such as dendritic cells, also derived from the bone marrow, migrate into tissues, take up antigen and bring it back to the secondary lymphoid tissues to present the antigen to the T and B cells. The T and B cells are now primed or activated and they migrate to the sites of infection and inflammation to mount an attack.

Immune Response

Pathogens usually have two locations: Extracellular and Intracellular

Extracellular Pathogens are targeted by antibodies by at least one of three processes: Neutralization, Opsonization and Complement Activation

Neutralization: antibody may bind to bacterial toxin and neutralize, thereby preventing the pathogen from interacting with host cells. These antibody tagged toxins are later degraded

Opsonization: Antigens are coated with antibodies and are targeted for phagocytosis.

Complement Activation: Antibodies coat bacterial cells and these antibodies act as receptors for the first protein of the complement system, eventually forming a protein complex leading usually to phagocytosis.

Antibodies in each class have different sites of action and therefore vary in their effectiveness in neutralization, opsonization and complement activation.

Intracellular Pathogens are targeted by a T-cell mediated response. There are two intracellular locations:

Cytosol (continuous with nucleus via nuclear pore): site of all viruses and some bacteria

Vesicular System (ER, Golgi, endosomes, lysosomes etc): site of some bacteria and some parasites

There are also two T cells and the intracellular location determines the type of T cell.

Cytotoxic T cells (Tc or CTL): Express CD8 and kill pathogens in cytosol

Helper T cells (Th): Express CD4 and are again of two kinds

Inflammatory Th1 that kill vesicular pathogens

Th2 (True helper cells) are involved in antibody production by B cells against T-dependent antigens on extracellular pathogens.

Both antibody (humoral) and cell-mediated responses contribute to eliminating the pathogen.