Immune System Functions: Antigen Presentation and Cell Killing

c) Antigen Processing and Presentation

• Engulf protein antigens, partially digest them, and display polypeptide fragments on the surface for T cells to "see."
• Fragments are associated on the surface with Major Histocompatibility Antigens (MHC II), which are necessary to activate T cells.

d) Dendritic Cell Migration

To increase the chance of interacting with the correct T cells, dendritic cells migrate to secondary lymphoid organs (chemokines are secreted to attract T cells).

VII. Extracellular Killing by the Immune System

A. Natural Killer (NK) Cells

NK cells lyse virus-infected cells, tumor cells, and parasites. They kill cells that do not express MHC class I antigens.

B. Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

1. In ADCC, NK cells and macrophages lyse antibody-coated cells.

VIII. Immunological Memory

A. Antibody Titer

The relative amount of antibody in serum is called the antibody titer.

B. Primary Response

The response of the body to the first contact with an antigen is called the primary response. It is characterized by the appearance of IgM followed by IgG.

C. Secondary Response

Subsequent contact with the same antigen results in a very high antibody titer and is called the secondary, anamnestic, or memory response. The antibodies are primarily IgG.

IX. Types of Adaptive Immunity

A. Naturally Acquired Active Immunity

Immunity resulting from infection is called naturally acquired active immunity; this type of immunity may be long-lasting.

B. Major Histocompatibility Antigens – Human Cell Surface Molecules (MHC)

1. Location and Nomenclature

• Are on the surface of all body's cells except mature RBCs.
• Also called human leukocyte antigens (HLAs).

2. Genetic Basis

• Are coded for by a group of 4 genes on chromosome 6 called the major histocompatibility complex (MHC).
• The 4 genes have multiple alleles, creating many possible MHC types.

C. MHC Gene Products: Class-1 and Class-2 Molecules

1. MHC Class-1

Class-1s are made by all cells except RBCs.

2. MHC Class-2

Class-2s are made only by antigen-presenting cells and B cells.

• These present class-2s, together with foreign antigens, to helper T cells.
• This is the only way to activate helper T cells so they can promote B-cell activity.

3. T-Cell Activation Requirements

In order for killer and helper T cells to function, they require co-presentation of antigen with a specific MHC marker.

• Killer T cells are activated to kill a victim cell only by co-presentation of antigen and a class-1 marker.